Biofuels derived from microalgae provide a sustainable alternative to fossil fuels, yet high production costs remain a significant barrier 1. Optimizing photobioreactors for biofuel production necessitates a detailed understanding of algal biomass, especially its organic components. Raman spectroscopy is a valuable tool for this analysis [2], however, distinguishing individual molecular conformers in complex mixtures remains challenging. This study employs Raman spectroscopy combined with statistical analysis to differentiate fatty acid conformers, using myristic acid as a model system. Density Functional Theory (DFT) calculations (B3LYP-D3/6-311++G**) with solvent effects (water) were used to simulate Raman spectra, achieving a balance between computational efficiency and accuracy. Statistical techniques enabled the classification of myristic acid conformers into chain, v-shaped, and twisted structures, with distinctive vibrational features identified at ~2900 cm$^{-1}$ (CH$_2$/CH$_3$ vibrations) and below 1200 cm$^{-1}$ (backbone motions) [3]. This approach enhances the precision of spectral analysis, offering a robust framework for the rapid identification of fatty acids in algal biomass, with implications for biofuel development.

REFERENCES
1 Y. Ye, W. Guo, H. H. Ngo, W. Wei, D. Cheng, X. T. Bui, N. B. Hoang, H. Zhang, Science of The Total Environment 935 (2024) 172863, https://doi.org/10.1016/j.scitotenv.2024.172863.
[2] K. Czamara, K. Majzner, M. Z. Pacia, K. Kochan, A. Kaczor, M. Baranska, Journal of Raman Spectroscopy 46 (1) (2015) 4–20, https://doi.org/10.1002/jrs.4607.
[3] T. Miteva, H. Friha, T. L. Hidouche, S. Suc, J. Palaudoux, M. Mogren Al-Mogren, E. Laure-Zins, M. Hochlaf, Spectrochimica Acta A (2025), accepted.

Due to its simplicity, H$_2$ constitutes a perfect tool for testing fundamental physics: testing quantum electrodynamics, determining fundamental constants, or searching for new physics beyond the Standard Model. H$_2$ has a huge advantage over the other simple calculable systems of having a set of a few hundred ultralong living rovibrational states, which implies the ultimate limit for testing fundamental physics with H$_2$ at a relative accuracy level of 10$^{-24}$. The present experiments are far from this limit. I will present our so far results of an ongoing projects aimed at spectroscopy of cold H$_2$ and trapping cold H$_2$. We develop an ultra-strong optical dipole trap. The time-dependent potential is going to recapture the coldest fraction of the cryogenic H$_2$ cloud.

[1] H Jóźwiak, P Wcisło, Scientific Reports 12, 14529 (2022)
[2] H Jóźwiak, TV Tscherbul, P Wcisło, J. Chem. Phys. 160, 094304 (2024)
[3] K Stankiewicz, et al. https://arxiv.org/abs/2502.12703

Chirality plays a fundamental role in molecular recognition processes. Molecular flexibility is also crucial in molecular recognition, allowing the interacting molecules to adjust their structures and hence optimize the interaction. Methods probing simultaneously chirality and molecular conformation are therefore crucially needed.

This is the case of a recently-introduced chiroptical effect called Photoelectron Circular Dichroism (PECD) leading to very intense (up to 40 %) forward/backward asymmetries, with respect to the photon axis, in the angular distribution of photoelectrons produced by circularly-polarized light ionization of gas phase pure enantiomers. PECD happens to be an orbital-specific, photon energy dependent chiroptical effect and is a subtle probe of the molecular potential, being very sensitive to static molecular structures such as conformers, isomers, clusters, as well as to vibrational motion, much more so than other observables in photoionization such as the cross section (Photoelectron Spectrum-PES) or the usual (achiral)  asymmetry parameter (for reviews see [1] [2]).
After an introduction to PECD, several results regarding valence-shell PECD on various floppy systems will be presented, belonging to several cases:
• No control on the conformation distribution, so that only a Boltzmann-averaged global PECD response can be measured, as in the case of the amino-acid alanine.[3,4].
• Partial control : case for which owing to a large binding energy difference between two types of conformers, we could observed directly and rationalize with the help of theoretical calculation a conformer-specific PECD as in the case of amino-acid Proline [5], or for which by changing the carrier gas of the molecular beam it was possible to control the conformer distribution [6], as it is the case of 1-Indanol
• Full conformer selection by using a two-photon ns-laser REMPI scheme as we could demonstrate on 1-indanol [7]
Such a sensitivity to conformation is both an asset and a challenge for the ongoing developments of laser-based PECD techniques as a sensitive chiral (bio)chemical analytical tool in the gas phase.

[1] L. Nahon, G. A. Garcia, and I. Powis, J. Elec. Spectro. Relat. Phen. 204, 322 (2015).
[2] R. Hadidi, D. Bozanic, G. Garcia, and L. Nahon, Adv. Physics: X 3, 1477530 (2018).
[3] M. Tia, B. Cunha de Miranda, S. Daly, F. Gaie-Levrel, G. Garcia, I. Powis, and L. Nahon, J. Phys. Chem. Lett. 4, 2698 (2013).
[4] M. Tia, B. Cunha de Miranda, S. Daly, F. Gaie-Levrel, G. A. Garcia, L. Nahon, and I. Powis, J. Phys. Chem. A 118, 2765 (2014).
[5] R. Hadidi, D. K. Božanić, H. Ganjitabar, G. A. Garcia, I. Powis, and L. Nahon, Commun. Chem. 4, 72 (2021).
[6] J. Dupont, V. Lepere, A. Zehnacker, S. Hartweg, G. A. Garcia, and L. Nahon, J. Phys. Chem. Lett. 13, 2313 (2022).
[7] E. Rouquet, J. Dupont, V. Lepere, G. A. Garcia, L. Nahon, and A. Zehnacker, Angew. Chem. Int. Ed. Engl., e202401423 (2024).

Liquid-Jet Photoelectron Spectroscopy (LJ-PES) [1] enables the direct study of the electronic structure of both solute and solvent, and has advanced the chemical analysis in aqueous solutions. The LJ facilitates in-vacuo continuous liquid replacement, and detection of photoelectrons with minimal collisions with evaporating water molecules.

Velocity Map Imaging (VMI) [2] provides optimal photoelectron collection efficiency with a full 4π steradian range, enabling the measurement of photoelectron angular distributions (PADs) in a single image. While VMI is heavily applied to both solid and gaseous phases [3], its intriguing extension to the aqueous phase remains very challenging. Major experimental and technical difficulties include the disturbance of the focusing electric fields by the presence of the dielectric liquid jet, the background resulting from scattering of the photoelectrons with the (aqueous) solution vapor, and the balance between required high electric fields in a high-vapor environment.

We have overcome the most critical technical issues, and have successfully employed our newly developed Liquid-Jet Velocity Map Imaging (LJ-VMI) setup, comprising precisely tunable high-voltage electrodes and a microchannel plate detector. This system offers a broad dynamical energy range, allowing detection of photoelectrons kinetic energies up to approximately 40 eV. Following initial lab-experiments using laser and ultraviolet light sources, we present here our recent LJ-VMI results obtained at the bending-magnet beamline PM3, at the BESSY-II synchrotron-radiation facility. Data are presented for water, aqueous solutions, as well as non-aqueous solution. We report solute and solvent core-level and valence electron binding energies, show associated PADs, and identify the principal effects of a liquid jet in VMI performance. Next steps in our continuous development of LJ-VMI will be discussed, along the perspective for future applications towards near-ionization-threshold phenomena as well as time-resolved photo-induced reactions and electron dynamics in (aqueous) solution.

[1] B. Winter, M. Faubel, Chem. Rev., 106, 4, pp. 1176–1211, (2006)
[2] A. Eppink, D. Parker, Rev. Sci. Instrum., 68(9), pp. 3477-3484, (1997)
[3] D. M. Neumark, J. Phys. Chem. A, 127, 4207−4223, (2023)

The absorption of soft X-ray photons by biological matter can lead to core-level ionization, producing excited cation radicals with the deposition of large amount of energy. The excited molecule relaxes by different competing relaxation channels, emitting a photon or release of a secondary electron called Auger electron where the core-level vacancy is filled by an outer-valence electron and the excess energy is used to emit another outer-valence electron from the same molecule. The doubly charged molecule mostly undergoes fragmentation. If the biomolecule is embedded in an environment, then apart from the local decay channels there can be different non-local decay processes involving both the biomolecule and its neighbours. One such non-local decay channel is the intermolecular Coulombic decay (ICD) [1] where the energy released from the relaxation of the initially core ionized molecule is transferred to a neighboring molecule that uses it to emit one of its electrons. Another decay channel is the electron-transfer mediated decay (ETMD) [2] where the core vacancy of the initially ionized molecule is filled by an electron from the neighboring molecule and another electron is emitted from yet another neighbor. Thus two vacancies are formed on two neighbors while the initially ionized molecule becomes neutral. These nonlocal decay mechanisms have mostly been considered to have minor contribution in case of inner-shell (core-level) vacancies, accounting for a few percent compared to the predominant Auger decay channel. However, a recent study showed experimentally that core-level ICD is an important channel for X-ray induced core-level ionization of microsolvated pyrimidine molecules [3] and theoretical calculations revealed a high branching ratio of non-local channels along with predicting a significant intensity for core-level ETMD channel. In the context of radiation damage to biological matter, such local and non-local competing decay channels can produce several low energy secondary electrons and water radicals which are the key players for causing single and/or double strand breaks in the DNA/RNA of the cells.
In the present work, we study the X-ray photoionization and fragmentation of pyrimidine embedded in water cluster to experimentally verify the core-level ETMD channel using the photoelectron-photoion-photoion coincidence (PEPIPICO) spectrometer connected at the gas phase endstation of the Finnish-Estonian beamline (FinEstBeAMS) [4] at the MAX IV synchrotron radiation facility. The PEPIPICO coincidence maps measured in coincidence with the C 1s photoelectron of pyrimidine shows signature of non-local processes especially the ETMD channel causing ionization of the neighboring water molecules to distribute the internal energy to the environment when the pyrimidine is ionized by the initial irradiation.
References
[1] T. Jahnke et. al., Chem. Rev. 120, 11295 (2020)
[2] M. Förstel et. al., Phys. Rev. Lett. 106, 033402 (2011)
[3] A. Hans et. al., J Phys. Chem. Lett. 12, 7146 (2021)
[4] K. Kooser et. al., J. Synchotron Rad., 27, 1080 (2020)

We report on experiments with highly intense femtosecond laser pulses with tailored polarization to study entanglement of spatially separated atoms on femtosecond time scales.

Previously, it has been shown that circularly polarized light favors electrons with a certain magnetic quantum number in strong field ionization [1]. This preference was used to prepare and detect ring currents in single argon ions [2,3].

Building on these insights, we use a pump-probe scheme to prepare and detect ring currents in dissociating oxygen molecules. The laser pulses have intensities on the order of $10^{14}$ W/cm$^2$.

The pump pulse excites a ring current in molecular oxygen and simultaneously triggers the dissociation of the molecule into two spatially separated entangled oxygen atoms. The probe pulse allows us to investigate this pair of atoms on femtosecond time scales. We find that the valence electrons of the two atoms are entangled in their magnetic quantum number [5].

The momenta of the liberated electrons and the ions are measured in coincidence using cold-target recoil-ion momentum spectroscopy (COLTRIMS) reaction microscopes [4].

[1] I. Barth, O. Smirnova, Phys. Rev. A 84, 063415, (2012)
[2] T. Herath et al., Phys. Rev. Lett. 109, 043004, (2012)
[3] S. Eckart et al., Nat. Phys. 14, 701, (2018)
[4] J. Ullrich et al., Rep. Prog. Phys. 66, 1463, (2003)
[5] S. Eckart et al., Science Advances 9, eabq8227, (2023)

X-ray free-electron lasers (XFELs) open new avenues towards studying collective x-ray emission and nonlinear x-ray matter interaction. In this talk I will present recent advancements on the experimental and theoretical exploration of collective spontaneous x-ray emission (x-ray superfluorescence) following ultrafast inner-shell photoinization. X-ray superfluorescence has been demonstrated in atomic gases in the soft x-ray range [1], in rare-gases [2] and clusters [3] in the XUV, and in solids and liquids in the hard x-ray range [4,5]. As opposed to the XFEL pulses that are based on the process of self-amplified spontaneous emission and have limited temporal coherence, x-ray superfluorescence produces phase-stable, ultrabright x-ray pulses of fs and sub-fs duration [6,7]. A quantitative theoretical prediction of this effect is intricate and computationally demanding, since it involves incoherent pumping of a large ensemble of atoms of several electronic states that undergo strong decoherence through electronic decay channels and pulse propagation effects, along with the need of a quantum-electrodynamical description of the field modes. I will present a theoretical framework [8,9] strongly linked to stochastic sampling of the time-dependent positive-P distribution of the multi-dimensional Liouville space. In this novel method, we extend a previous phenomenological treatment and treat quantum fluctuations of the electromagnetic field by appropriate stochastic contributions. The resulting set of coupled stochastic partial differential equations resemble the generalized Maxwell-Bloch equations to follow the evolution of the electromagnetic fields and the density matrix of the emitters [10]. The stability of the equations will be discussed and I address potential applications of the method in cavity and nonlinear quantum optics.

[1] N. Rohringer et al., Nature 481, 488 (2012).
[2] L. Mercadier et al., Physical Review Letters 123, 023201 (2019).
[3] A Benediktovitch et al. Physical Review A 101, 063412 (2020).
[4] T. Kroll et al., Physical Review Letters 120, 133203 (2018).
[5] T. Kroll et al., Physical Review Letters 125 (3), 037404 (2020).
[6] M. D. Doyle et al., Optica 10, 1602 (2023).
[7] T. M. Linker et al., Nature, accepted (2025). https://arxiv.org/abs/2409.06914
[8] S. Chuchurka, V. Sukharnikov, A. Benediktovitch and N. Rohringer, Phys. Rev. A 110, 053703 (2024).
[9] S. Chuchurka, V. Sukharnikov and N. Rohringer, Phys. Rev. A 109, 063705 (2024).
[10] S. Chuchurka, A. Benediktovitch, Š. Krušič, A. Halavanau, and N. Rohringer, Phys. Rev. A 109, 03375 (2024).

Chirality—the property of an object that cannot be superimposed on its mirror image—is ubiquitous in nature. Like our hands, opposite versions of the same chiral molecule (R and S enantiomers) behave identically unless they interact with another chiral object. Molecular chirality is rapidly becoming essential in nanotechnology [1], e.g. for developing molecular motors and spintronic devices. The unbalance between R and S biomolecules on Earth (amino acids, sugars, DNA, etc.) supports life. This homochirality gives different biological activities to opposite versions of a chiral drug or pesticide, with profound implications for pharmaceuticals and agriculture. Moreover, abnormal enantiomeric ratios of chiral biomarkers have recently been linked to cancer, Alzheimer’s, diabetes, and other diseases [2].

Having efficient tools for rapid chiral discrimination is therefore vital. However, current optical methods are inefficient because they rely on the (chiral) helix that circularly polarised light draws in space. The pitch of this helix—determined by light’s wavelength—is ~10,000 times larger than the molecules. Consequently, the molecules perceive the helix as a flat circle, hardly feeling its chirality. This results in weak chiral sensitivity, typically <0.1%, which presents major limitations [3]. We can overcome these limitations by creating synthetic chiral light [4-6], where the tip of the electric-field vector traces a 3D chiral trajectory in time. This new type of chiral light can drive ultrafast chiral currents inside the molecules, which interact with the chiral molecular skeleton in a highly enantiosensitive manner, leading to 100% chiral sensitivity.

In this presentation, I will show how we can shape light’s polarisation in 3D to achieve highly efficient chiral sensing [4-10] and manipulation [11], together with theoretical and computational results that support the feasibility of our approaches. Current optical instrumentation enables several strategies for 3D shaping, such as using several laser beams that propagate non-collinearly [4-6,11], only one beam but tightly focused [7,8], vortex beams to create topological chiral light [9], or ultrafast TACOS [10]. I will discuss how these ideas enable highly efficient chiroptical sensing via high harmonic generation using strong fields, as well as gentle chiroptical imaging in the low-order perturbative regime.

[1] J. Brandt et al, Nature Reviews Chemistry 1, 0045 (2017)
[2] Y. Liu et al, Nature Reviews Chemistry 7, 355 (2023)
[3] D. Ayuso et al, Phys Chem Chem Phys 24, 26962 (2022)
[4] D. Ayuso et al, Nature Photonics 13, 866 (2019)
[5] D. Ayuso et al, Nature Communications 12, 3951 (2021)
[6] J. Vogwell et al, Science Advances 9, eadj1429 (2023)
[7] D. Ayuso et al, Optica 8, 1243 (2021)
[8] L. Rego et al, Nanophotonics 12, 14, 2873 (2023)
[9] N. Mayer et al, Nature Photonics 18, 1155 (2024)
[10] J. Terentjevas et al, ArXiv:2406.14258v1 (2024)
[11] A. Ordóñez et al, ArXiv:2309.02392 (2023)

We report precise measurements of inter-species interactions in a bosonic optical lattice clock based on $^{88}$Sr atoms [1]. We observe a nonlinear behavior of the clock density shift, showing features deriving from many-body physics beyond the mean-field theory,even without reaching the quantum degeneracy. This findings enable a cancellation of the density shift systematic effect through a careful choice of a "magic" density and excitation fraction. We discuss the implications of these findings in many-body physics, quantum simulation [2], and precision isotope shift measurements [3], which provide a powerful probe for new physics beyond the Standard Model [4].

[1] J.P. Salvatierra, et al, in preparation (2025).
[2] T. Comparin, et al., Phys. Rev. Lett. 129, 113201 (2022)
[3] H. Miyake, et al., Phys. Rev. Research 1, 033113 (2019)
[4] J. Berengut, et al., Phys. Rev. Lett. 120, 091801 (2018)

Interferometers based on ultracold atoms enable an absolute measurement of inertial forces with unprecedented precision. However, their resolution is fundamentally restricted by quantum fluctuations. Improved resolutions with entangled or squeezed atoms were demonstrated in internal-state measurements for thermal and quantum-degenerate atoms and, recently, for momentum-state interferometers with laser-cooled atoms. Here, we present a gravimeter based on Bose-Einstein condensates with a sensitivity of $−1.7^{+0.4}_{−0.5}$ dB beyond the standard quantum limit. Interferometry with Bose-Einstein condensates combined with delta-kick collimation minimizes atom loss in and improves scalability of the interferometer to very-long-baseline atom interferometers.

Recent advances in the control of the energy, quantum state and even shape of neutral molecules and molecular ions in the gas phase have opened up new possibilities for detailed studies of molecular interactions, collisions and chemical reactions. In the talk, we will present a range of recent results and new experimental developments which are all joined by the common theme of using controlled molecules for studies of collisional and chemical dynamics. First, we will discuss experimental techniques for the preparation of individual conformations of both neutral molecules and molecular ions and their use in the investigation of conformational effects in chemi-ionisation and ion-molecule reactions. Second, we will highlight how recently developed quantum-logic assisted protocols for molecular state readout can be employed for exploring the state-to-state dynamics of molecular collisions with high sensitivity on the single-molecule level. Third, we will present a new approach for the simultaneous (“hybrid”) trapping of cold neutral molecules and ions for studies of ion-molecule processes in the millikelvin regime. The talk will conclude with an outlook on future developments.

In our laboratory, we have started a systematic investigation of the reactions involving oxygen atoms and aromatic compounds under single collision conditions using the crossed molecular beam technique with mass spectrometric detection. The first systems we have looked at are O(³P) + benzene [1], pyridine [2], and toluene [3]. More recent results are on O(³P) + ethylbenzene and O(³P) + thiophene, while we plan to investigate the reactions with styrene, anisole, and other functionalized aromatics.

A detailed understanding of these reactive systems has practical implications ranging from biomass gasification to the design of novel space-technology aromatic polymers resistant to the attack by the oxygen atoms in the Low Earth Orbit, where most satellites reside. From a fundamental point of view, these studies aim to derive the structure dependency of the reactivity of aromatics.

As in the case of the O(³P) reactions with unsaturated organic compounds, the investigated reactions are strongly affected by intersystem crossing (ISC) to the underlying singlet potential energy surface with a significant alteration of the reaction mechanism and product branching fractions. ISC can occur in the entrance channel, as observed for the O + pyridine reaction [2], or at a later stage. In all cases, upon ISC, either ring-contraction or ring-opening (in the case of O + thiophene) mechanisms with CO-elimination were observed to be significant or dominant (in the case of the reaction with pyridine and thiophene). The experimental results are interpreted with the help of ad hoc theoretical simulations.

[1] G. Vanuzzo et al., J. Phys. Chem. A 125, 8434 (2021)
[2] P. Recio et al., Nature Chemistry 14, 1405 (2022)
[3] N. Balucani et al., Faraday Disc. 251, 523 (2024)

We acknowledge financial support under the National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 1.1, Call for tender No. 104 published on 02.02.2022 by the Italian Ministry of University and Research (MUR), funded by the European Union – NextGenerationEU– Project Title 20227W5CLJ Biomass gasification for hydrogen production (Bio4H2) – CUP J53D23001970006 - Grant Assignment Decree No. 961 adopted on 30.06.2023 by the Italian Ministry of Ministry of University and Research (MUR).

The feasibility of performing merged beam experiments with trapped fast ion beams of molecular cations and anions at the double electrostatic storage ring (DESIREE) and the hybrid electrostatic ion beam trap (HEIBT),[1,2] opens new opportunities to study mutual neutralization reactions. Here, I will present our findings from merged beam experiments performed at DESIREE on the mutual neutralization of hydronium (H3O+) and hydroxide (OH¯),[3] and their isotopomers.[4] 3D coincidence imaging of the neutral products allowed us to disentangle the different competing proton-transfer and electron-transfer mechanisms. We identified a predominant e-transfer mechanism that forms either one or two OH radicals in a single neutralization reaction. By analyzing measured 3-body momentum correlations, we found that the distance at which the electron transfer occurs determines the final product channel. Figure 1 illustrates the two competing non-adiabatic electron transfer pathways. Electron transfer at a distance of ~4Å (left panel) forms the neutral H3O radical intermediate ground state, which then dissociates into H2O and H. In contrast, electron transfer at ~10Å presented in the right panel forms an electronically excited H3O intermediate that dissociates into H2 and a 2nd OH radical. These mutual neutralization dynamics of the isolated water ions can be related to ion-ion reactions on the liquid water surface and offer an explanation for the recent observations of spontaneous hydrogen peroxide formation in pure water microdroplets.[5-7]

Fig 1: Schematic representation of the competing non-adiabatic pathways that dominate the mutual neutralization of hydronium and hydroxide.

References
1 A. Bogot et al, Phys. Chem. Chem. Phys., 25, 25701-25710 (2023)
[2] H.T. Schmidt et al, Rev. Sci. Instrum., 84, 055115 (2013)
[3] A. Bogot et al, Science, 383, 285-289 (2024)
[4] A. Bogot et al, preprint available at Research Square https://doi.org/10.21203/rs.3.rs-4777257/v1
[5] J.K. Lee et al, PNAS, 116 (39), 19294-19298 (2019)
[6] P. Skurski and J. Simons, J. Chem. Phys., 160, 034708 (2024)
[7] J.P. Heindel et al, Nat Commun, 15, 3670 (2024)

Quantum state-selected scattering at low temperatures is essential for understanding molecular reaction dynamics and the chemistry of astrophysical processes. A key aspect of these studies is the use of techniques that cool reactants and precisely select their quantum states. We have recently enhanced our research capabilities by integrating a laser-cooled ion trap with a high-resolution time-of-flight mass spectrometer (TOF-MS) and fluorescence imaging. Additionally, we have developed a cooling apparatus for Be⁺ and C⁺ ions that achieves sub-kelvin temperatures using laser and sympathetic cooling. This setup provides meticulous control over ion micromotion, significantly improving our ability to explore ion-molecule reactions across diverse collision energies. Furthermore, by leveraging ion motional heating, we have achieved two-dimensional cooling of Be⁺ ions without the need for repump laser beams.
Moreover, our setup incorporates a stimulated Raman pumping (SRP) system paired with a fast chopper to achieve high-efficiency molecular state selection for H₂ and N₂. Furthermore, by integrating a cavity-enhanced infrared excitation scheme with a milliwatt laser, we achieved over 30% efficiency in exciting the overtone transition of CO (v=3), enabling detailed studies of state-selected reaction dynamics. In this talk, I will discuss recent advancements in this project, as well as ongoing ion-molecule reaction studies utilizing these systems.

Resonance fluorescence—the light emitted by a coherently driven two-level quantum emitter—has long served as a paradigm in quantum optics. In this talk, I will present two recent experimental investigations that reveal both the fundamental richness and the technological potential of this seemingly simple system (1,2). In the first part, I revisit the textbook notion that a single atom cannot scatter two photons simultaneously. Our results provide direct experimental evidence for an alternative quantum interference-based explanation, in which antibunching emerges from the coherent superposition of distinct two-photon scattering amplitudes. By selectively suppressing the coherently scattered component of the fluorescence spectrum, we isolate photon pairs that are simultaneously scattered by the atom, thereby validating a decades-old theoretical prediction. In the second part, I will show how resonance fluorescence can be harnessed as a highly efficient source of time-bin entangled photon pairs. Using beam splitters, delay lines, and post-selection only, we transform the emission from a single atom into a stream of maximally entangled photon pairs, achieving a strong violation of a Bell inequality. Together, these experiments illustrate how resonance fluorescence—traditionally viewed as a fundamental textbook example—can be reimagined as a powerful resource for quantum information science.
1. L. Masters et al., Nature Photonics 17, 972 (2023)
2. X.-X. Hu et al., arXiv:2504.11294

Collective atomic or solid-state excitations present important advantages for encoding qubits, such as strong directional coupling to light. Unfortunately, they are plagued by inhomogeneities between the emitters, which make the qubit decay into a quasi-continuum of dark states. In most cases, this process is non-Markovian. Through a simple and resource-efficient formalism, we unveil a regime where the decay is suppressed by a combination of driving and non-Markovianity. We experimentally demonstrate this "driving protection" using a Rydberg superatom, extending its coherent dynamics beyond the characteristic free decay time by an order of magnitude [1].

[1] A. Covolo et al, arXiv:2501.07232.

Study of information scrambling in a quantum many-body system is key to understanding the dynamics of thermalization and the evolution towards equilibrium. This work reports our experimental investigation into this topical subject by directly observing the out-of-time-order correlation (OTOC) function in a Rydberg atom array.

A key challenge in measuring the OTOC in an analog-digital hybrid circuit is the difficulty of implementing time-reversed evolution. We address this by leveraging the inherent constraints imposed by the strong van der Waals interactions in the Rydberg atom array system.

Our observations show that the scrambling dynamics for quantum many-body scar in a Rydberg atom array is anomalous, accompanied by a linear lightcone with a smaller valued butterfly velocity and persistent periodic oscillations inside, which differs from both thermal and many-body localized systems, and signifies an unusual breakdown of thermalization.

If time permits, I will briefly mention our another experiment on disorder-induced topology in a Rydberg atom array.

Astrophysical ices, mainly composed of simple molecules such as H2O, CO, CO2, NH3, CH3OH and others are ubiquitous in space: they are present in comets, satellites of planets (e.g Jovian moons) and on the grains of the dense molecular clouds in the interstellar medium. They are constantly exposed to complex and diverse radiation fields and interact with photons, electrons and ions (solar/stellar winds, magnetospheres or/and cosmic rays). This induces several physico-chemical processes such as radiolysis and subsequent formation of new molecules, release of molecules to the gas phase (sputtering, desorption) and structural ice modifications.

During the talk, I will present an overview of results obtained for ion irradiation of ices containing small molecules as well as films of complex organic molecules (e.g. PAHs, nucleobases) performed with the "space simulator" device IGLIAS at CIMAP using GANIL (Caen, France) ion beams. Additional experiments were performed at ATOMKI (Debrecen, Hungary) and GSI (Darmstadt, Germany). Infrared absorption spectroscopy is used to in situ observation of physico-chemical modifications of the icy layers during energetic processing. Ex-situ analysis of residues is performed by e.g. high resolution mass spectrometric methods. Astrophysical application of obtained results will be discussed. Moroeover, the MIRRPLA platform will be also presented. MIRRPLA is a unique multibeam irradiation platform (UV photons, keV electrons and keV-GeV ions), which is under construction at CIMAP laboratory, to investigate the origin and evolution of organic matter of the Solar System. This project is financed by ANR PEPR ("Programmes et équipements prioritaires de recherche") "Origins ».

Collisions of molecular ions with electrons, such as dissociative recombination (DR) and inelastic electron collisions, play a key role in shaping the charge density and composition of cold plasmas, including the interstellar medium (ISM). Accurate rate coefficients are essential for modeling these astrophysical environments, yet theoretical calculations remain challenging due to the complexity of the underlying quantum dynamics. Experimentally, obtaining data at ISM-relevant conditions—collision temperatures of 10–100 K and low internal excitation—has been difficult. While previous storage ring studies reached the required collision temperatures, they struggled to achieve internal excitation temperatures below 300 K. This limitation has been overcome with the development of electrostatic cryogenic storage rings.

The Cryogenic Storage Ring (CSR) at the Max Planck Institute for Nuclear Physics in Heidelberg is a state-of-the-art electrostatic facility designed for studies with stored atomic, molecular, and cluster ion beams. Its cryogenic chamber operates at temperatures below 6 K, ensuring exceptionally low residual gas densities and enabling beam lifetimes of hundreds to thousands of seconds. For many molecular ions, this is sufficient time to relax into their lowest ro-vibrational states through spontaneous photon emission.

By replicating the extreme conditions of the cold ISM, CSR provides a unique platform for laboratory astrochemistry and quantum dynamics studies with well-defined molecular states. This talk will present recent CSR measurements on electron recombination of molecular ions in their lowest rotational states, with a focus on the unusual recombination behavior observed for TiO⁺, ArH⁺, and fullerenes.

[1] R. von Hahn et al., Rev. Sci. Instr. 87, 063115 (2016)
[2] O. Novotný et al., Science, 365, 676 (2019)
[3] N. Jain et al., J. Chem. Phys. 158, 144305 (2023)
[4] Á. Kálosi et al., Phys. Rev. A 110, 022816 (2024)

The cryogenically cooled ion beam storage ring facility DESIREE (Double ElectroStatic Ion Ring ExpEriment) is uniquely designed for studying mutual neutralization (MN) reactions in collisions between oppositely charged ions that are prepared in well-defined or narrow ranges of quantum states and with fine-control of the collision energy down to the sub-electronvolt regime [1,2]. In recent years, this has allowed a range of studies in which the final-state excitation energy distributions in atomic MN have been measured [3] and for the first ever MN studies with cooled molecular ions [4-6]. Such reactions are expected to be important for the ionization balance in any dilute environment where atomic or molecular anions are prominent negative charge carriers. Furthermore, the excellent vacuum conditions in DESIREE allows studying the cooling dynamics and stabilities of molecular ions on timescales exceeding minutes and in unprecedented detail [7-9].

This presentation will highlight recent DESIREE studies with a focus on complex molecular ions such as fullerenes, polycyclic aromatic hydrocarbons (PAHs), and biomolecules. These are important to benchmark theory and models that may be used for determining survival probabilities when molecules are exposed to different types of excitation agents in e.g. various interstellar environments, and for reliable predictions of MN rates that are expected to strongly influence the charge balance and hence the chemistry in dark interstellar clouds [10,11]. In addition, the MN studies reveal the importance of energy transfer, bond-breaking and bond-forming reactions in head-on collisions, which are driven by the Coulomb force between the oppositely charged ions. Here, support from quantum chemistry calculations is key to advance the understanding of such reactive charge transfer processes and to obtain accurate values of their rates.

References
[1] R. D. Thomas et al, Review Scientific Instruments 82, 0655112 (2011).
[2] H. T. Schmidt et al, Review Scientific Instruments 84, 055115 (2013).
[3] See e.g. J. Grumer et al, Physical Review Letters 128, 033401 (2022) and references therein.
[4] M. Poline et al, Physical Review Letters 132, 023001 (2024).
[5] A. Bogot et al, Science 383, 285-289 (2024).
[6] M. Gatchell et al, Astronomy & Astrophysics 694, A43 (2025).
[7] M. Gatchell et al, Nature Communications 12, 6646 (2021).
[8] P K Najeeb et al. Physical Review Letters 131, 113003 (2023).
[9] M. Stockett et al, Nature Communications. 14 395 (2023).
[10] S. Lepp and A Dalgarno, ApJ. 324, 553 (1988).
[11] V. Wakelam and E. Herbst, ApJ. 680, 371 (2008).

In a molecular system, the correlation-driven charge migration [1] (CDCM) is a purely electronic process that involves the ultrafast dynamics of electrons originating from coherent superposition of eigenstates followed by the ionization of a single molecular orbital [2]. The possibility of observing CDCM has been a driving force behind theoretical and experimental developments in the field of attosecond molecular science since its inception. Although X-ray free-electron lasers (XFELs) have recently emerged as a promising tool for experimentally observing CDCM, the unambiguous observation of CDCM, or more generally, charge migration dynamics triggered by ionization, remains elusive. In this work [3], we present a method to selectively trigger such dynamics using molecules predicted to exhibit long-lived electron coherence. We show that these dynamics can be selectively triggered using infrared multi-photon ionization and probed using the spacial resolution of X-ray free-electron laser, proposing a promising experimental scheme to study these pivotal dynamics. Additionally, we demonstrate that real-time time-dependent density-functional theory can describe correlation-driven charge migration resulting from a hole-mixing structure involving the HOMO of a molecule.

References
[1] A. I. Kuleff, L. S. Cederbaum, J. Phys. B: At. Mol. Opt. Phys., 47(12), 124002, (2014).
[2] J. Breidbach, L. S. Cederbaum, J. Chem. Phys., 118(9), 3983, (2003)
[3] C. Guiot du Doignon, R. Sinha-Roy, F. Rabilloud, V. Despré, arXiv:2410.04978, (2024)

We study the role of electronic correlations during high harmonic generation (HHG) in multi-electron atoms. Originally viewed as a process involving one single active electron, the influence of multi-electron effects on the HHG spectrum has lately been extensively studied (see e.g. [1, 2]). We quantify the time-dependence of electron-electron correlations on ultrafast time scales using correlation measures from quantum information theory.
By explicitly solving the time-dependent Schrödinger equation with the multi-configurational time-dependent Hartee-Fock (MCTDHF) method [3], we obtain fully correlated results for He, Ne, Be and Mg. Using driving fields adjusted to the ionization potential of each atom, such that the driving occurs in the strong-field regime and double-ionization is negligible, we compare the HHG yields for the different atomic species.
We find prominent features of the influence of correlations on both the tunneling as well as the recombination step. While during tunneling the correlations systematically increase for noble gase atoms (He, Ne), they decrease for alkaline earth atoms (Be, Mg). During recombination we find that the correlated electrons oscillate out of phase relative to each other. Both processes imprint distinct signatures of correlations on the HHG spectrum.

[1] A. D. Shiner et al., Nature Phys 7, 464–467 (2011).
[2] Y. Li et al., Phys. Rev. A 99, 043401 (2019).
[3] T. Sato et al., Phys. Rev. A 94, 023405 (2016).

Ultracold atomic gases offer a versatile platform for exploring rich phenomena in quantum matter. In particular, topological states akin to those found in the quantum Hall effect can be engineered by simulating orbital magnetic fields—an approach greatly facilitated by the use of synthetic dimensions.

In this talk, I will present our experimental realization of a quantum Hall system using ultracold gases of dysprosium atoms. By leveraging the atom’s large internal spin (J = 8), we encode a synthetic dimension and couple it to atomic motion via two-photon optical transitions, which generates an effective magnetic field. We observe hallmark signatures of quantum Hall physics, including a quantized Hall response and gapless, chiral edge modes.

I will then describe a more intricate experiment designed to probe spatial entanglement by simulating the so-called entanglement Hamiltonian. Using the Bisognano-Wichmann theorem—which relates the entanglement Hamiltonian to a spatially deformed version of the original system—we implement this deformation along the synthetic dimension.

Lastly, I will discuss our recent investigation into a topological phase transition, induced by introducing an additional lattice potential. I will highlight the system’s behavior in the critical regime and explore the emergent features associated with the transition.

Single-Shot Coherent Diffractive Imaging (CDI) has become a mature tool to capture the structure and dynamics of nanoscale systems such as viruses [1], nanoparticles [2] and helium droplets in free flight [3]. The conventional application of the underlying single-shot imaging implies that the imaging pulse interacts instantly and perturbatively with the target such that the diffraction image reflects the target shape and (unperturbed) optical properties via the linear-response field propagation through (and around) the target. Especially the advancing capabilities of intense XUV light sources render the question important, at which point non-linear quantum state dynamics driven by the imaging pulse become significant [4]. In this talk I will discuss an attempt to tackle this question theoretically by means of scattering simulations that include field propagation and local quantum state dynamics for the example of Helium droplets [5]. It will be shown for the example of resonantly driven Helium droplets that the departure from the strictly linear regime may open up a wide range of opportunities to track and drive quantum state population dynamics [6]. Emerging routes for associated new metrologies in the field of CDI will be discussed.

[1] M. Seibert et al., Nature 470, 78 (2011)
[2] C. Peltz et al., New J. Phys. 24, 043024 (2022)
[3] Gomez et al., Science 345, 907 (2014)
[4] D. Rupp et al., Nat. Commun. 8, 493 (2017)
[5] B. Kruse et al., J. Phys. Photonics 2, 024007 (2020)
[6] B. Kruse, PhD thesis, University of Rostock (2024)

Helium nanodroplets are quantum fluid clusters that feature extraordinary properties such as ultralow temperature and superfluidity. They have mostly been used as inert nanometer-sized cryo-matrices for isolating molecules and for aggregating molecular complexes and nanostructures that are hard or impossible to form by other means. However, when helium nanodroplets are resonantly excited or photoionized, they turn into a highly reactive species that feature a complex reaction dynamics.

In this contribution I’ll present experiments probing the ultrafast dynamics of pure and doped helium nanodroplets initiated by resonant photoexcitation and ionization of the helium droplets. Using synchrotron radiation, free-electron lasers and high-harmonic generation sources, we track ultrafast relaxation processes such as internal conversion, bubble formation, as well as energy- and charge-transfer ionization between (super-)excited helium atoms and between helium atoms and attached dopant atoms and molecules [1-5]. In particular, interatomic Coulombic decay (ICD) processes prevail nearly in the entire extreme-ultraviolet range of the spectrum [6-12]. ICD is initiated by resonant single or double excitation [6,7], simultaneous ionization and excitation [8], and indirectly by photoelectron impact excitation [9,10]. By forming tailored complexes in helium nanodroplets, their ionization and fragmentation dynamics can be studied under controlled conditions [12,13].

References
[1] C. Medina et al., New Journal of Physics 25, 053030 (2023).
[2] A. C. LaForge et al., PCCP 24, 28844-28852 (2022).
[3] J. D. Asmussen et al., J. Phys. Chem. Lett. 13, 4470–4478 (2022).
[4] A. C. LaForge et al., Phys. Rev. X 11, 021011 (2021).
[5] J. D. Asmussen et al., J Chem. Phys. 159, 034301 (2023).
[6] L. Ben Ltaief et al., Phys. Rev. Research 6, 013019 (2024).
[7] B. Bastian et al., Phys. Rev. Lett. 132, 233001 (2024).
[8] M. Shcherbinin et al., Phys. Rev. A 96, 013407 (2017).
[9] L. Ben Ltaief et al., Phys. Rev. Lett. 131, 023001 (2023).
[10] L. Ben Ltaief et al., Rep. Prog. Phys. 88, 037901 (2025).
[11] A. C. LaForge et al., Rep. Prog. Phys. 87, 126402 (2024).
[12] J. D. Asmussen et al., PCCP 25, 24819 – 24828 (2023).
[13] S. De et al., J. Chem. Phys. 160, 094308 (2024).

In this communication we present how ionizing radiation influences the formation of peptide bonds in clusters of amino acids in the gas phase. In the past, simulations and experiments were carried out in parallel to understand the possible mechanisms involved[1,2]. Due to these promising previous results, we have expanded the study, including clusters of other amino acids. The main objective of this study is to evaluate the conditions of peptide bond formation in pure clusters of glycine, threonine, valine, serine and cysteine in the gas phase induced by collisions with alpha particles.

Experimentally, mass spectrometry is used to analyze the charged species formed after the collision with the highly charged ions. From the theoretical point of view, first the search for conformers is carried out using the CREST program[3], and then the structures obtained are further reoptimized with the Gaussian16 software[4] using the density functional theory DFT. The second stage of the work has been to make an analogous study, but on protonated clusters, since in these species believed to play the key role in the experiments. Finally, we have explored the potential energy surfaces to locate the transition states that explains the mechanism for peptide bond formation from the protonated clusters, also using DFT.

Nanoparticle (NP) mass spectrometry in the gas phase is a unique way
to characterize individual isolated particles and thus assess their
intrinsic properties, NP-to-NP variability and structural evolution,
e.g. in studies on charging mechanisms [1], photophysics [2] or high
temperature reaction kinetics [3]. Our group focuses on cryogenic
experiments to employ absorption spectroscopy, based on adsorption of
messenger atoms or molecules on the NP surface and their desorption
driven by laser heating with rates that are proportional to the
absorption cross section [4]. An overview is given over the related
goals, challenges and progress in charge state control, fluorescence
thermometry [5] with the aim for temperature controlled experiments,
and quantitative characterization of the adsorption on a NP surface.

[1] M. Grimm et al., Phys. Rev. Lett. 96, 066801 (2006)
[2] V. Dryza et al., Phys. Chem. Chem. Phys. 15, 20326 (2013)
[3] C. Y. Lau et al., J. Phys. Chem. C 127 (31), 15157 (2023)
[4] B. Hoffmann et al., J. Phys. Chem. Lett. 11, 6051 (2020)
[5] S. C. Leippe et al., J. Phys. Chem. C 128 (50), 21472 (2024)

The generation and distribution of entanglement as a resource is one of the big challenges for the field of quantum communication. We discuss some of our work on single photon entanglement and building up complex quantum states from individual photons for use in quantum networks. In particular, some of our recent work looks at going beyond entanglement swapping to heralding entanglement at a distance, which is of relevance for quantum repeaters as well as device-independent QKD. We finish with a quick look at how we’re developing photonic sources compatible with quantum memories and some recent results on photon number resolving detectors

We predict that ultracold bosonic dipolar gases, confined within a multilayer geometry, may undergo self-assembling processes, leading to the formation of chain gases and solids. These dipolar chains, with dipoles aligned across different layers, emerge at low densities and resemble phases observed in liquid crystals, such as nematic and smectic phases. We calculate the phase diagram using quantum Monte Carlo methods, introducing a newly devised trial wave function designed for describing the chain gas, where dipoles from different layers form chains without in-plane long-range order. We find gas, solid, and chain phases, along with quantum phase transitions between these states. Specifically, we predict the existence of quantum phase transitions from gaseous to self-ordered phases, as the interlayer distance is decreased. Remarkably, in the self-organized phases, the mean interparticle distance can significantly exceed the characteristic length of the interaction potential, yielding solids and chain gases with densities several orders of magnitude lower than those of conventional quantum solids.

[1] G. Guijarro, G. E. Astrakharchik, G. Morigi, and J. Boronat, Phys. Rev. Lett. 133, 233402 (2024).

Mixtures of ultracold gases with long-range interactions are expected to open new avenues in the study of quantum matter. Natural candidates for this research are spin mixtures of atomic species with large magnetic moments. However, the lifetime of such assemblies can be strongly affected by the dipolar relaxation that occurs in spin-flip collisions. Here we present experimental results for a mixture composed of the two lowest Zeeman states of 162Dy atoms, that act as dark states with respect to a light-induced quadratic Zeeman effect. We show that, due to an interference phenomenon, the rate for such inelastic processes is dramatically reduced with respect to the Wigner threshold law [1]. Additionally, we determine the scattering lengths characterizing the 𝑠-wave interaction between these states, providing all necessary data to predict the miscibility range of the mixture, depending on its dimensionality. Looking ahead, our setup is a promising platform for studying supersolidity, including the formation of the so-called double supersolid [2].

Moreover, by exploiting spin-dependent light shifts, we probe several dimer bound states with binding energies as low as a few MHz across a magnetic field range of 0–20  G. These preliminary results reveal intriguing features in the collisional properties of dipolar gases.

[1] M. Lecomte, A. Journeaux, J. Veschambre, J. Dalibard and R. Lopes PRL 134, 013402, 2025
[2] D. Scheiermann, A. Gallemí, and L. Santos, “Excitation spectrum of a double supersolid in a trapped dipolar Bose mixture,” arXiv:2412.05215, 2024.

Trapped and laser-cooled ions allow for a high degree of control of atomic quantum systems. They are the basis for modern atomic clocks, quantum computers and quantum simulators. In our research we use ion Coulomb crystals, i.e. many-body systems with complex dynamics, for precision spectroscopy. This paves the way to novel optical ion frequency standards with ultra-high stability and accuracy for applications such as relativistic geodesy and quantum simulators in which complex dynamics become accessible with atomic resolution.
On the other hand, the high precision obtained in the spectroscopy of trapped cold ions enables sensitive tests of the Standard Model and the search for new physics. The long-lived F-state of the Yb+-ion has a high sensitivity to both relativistic and nuclear effects. We use isotope-shift spectroscopy as a sensitive probe for nuclear structure and fifth forces mediated by a new boson that couples to electrons and neutrons . Deviations from a linear relation in the King-plot analysis can indicate new physics or higher-order SM effects. This powerful technique revealed large King-plot nonlinearities in Yb . We present two-orders-of-magnitude improved spectroscopic measurements in all five stable spinless isotopes of this element. The transition frequency of the forbidden 2S1/2 to 2D5/2 and 2S1/2 to 2F7/2 transitions are determined with an accuracy of 6 and 16 Hz, respectively, yielding isotope shifts with a relative precision as low as 10−9. We combine these spectroscopic results with new mass measurements with a relative precision of a few 10−12. With this, we can extract a new bound on the mass and coupling strength of the potential new bosons. The results are also used to investigate higher-order nuclear structure effects along a chain of Yb isotopes. In combination with ab initio nuclear structure calculations, this provides a window to nuclear deformation and nuclear charge distributions along isotopic chains towards exotic, neutron-rich nuclei.

In the latest adjustment of fundamental constants [1], spectroscopy of ro-vibrational transitions in the molecular hydrogen ion HD$^+$ contributed for the first time to the determination of particle masses, in particular the proton-electron mass ratio $m_p/m_e$, the precision of which was improved by a factor of 3.5. It was also used to constrain hypothetical beyond-standard-model interactions [2,3].
After reviewing the recent experimental and theoretical advances that led to these results, I will describe ongoing work towards improving further the theoretical accuracy by calculating certain QED corrections in a fully relativistic framework, relying on a high-precision numerical resolution of the Dirac equation for the bound electron [4]. Finally, perspectives for improved determination of fundamental constants offered by extending spectroscopic measurements to other isotopologues of the molecular hydrogen ion family (H$_2^+$, D$_2^+$, …) will be discussed [5].
[1] P. J. Mohr, D. B. Newell, B. N. Taylor, and E. Tiesinga, arXiv:2409.03787
[2] M. Germann et al., Phys. Rev. Research 3, L022028 (2021).
[3] C. Delaunay et al., Phys. Rev. Lett. 130, 121801 (2023).
[4] H.D. Nogueira and J.-Ph. Karr, Phys. Rev. A 107, 042817 (2023).
[5] S. Schiller and J.-Ph. Karr, Phys. Rev. A 109, 042825 (2024).

Optical tweezers offer new opportunities to control and manipulate trapped ions with applications in quantum information processing, metrology and precision spectroscopy. The tweezers may be used to modify the local confinement of the ions, thereby modifying the soundwave spectrum of the entire crystal. In this way, the soundwave mediated spin-spin interactions between the ions may be programmed with applications in quantum computing and simulation 1. I will highlight our experimental progress towards implementing this system in the lab with emphasis on optimizing tweezer delivery and eliminating aberrations by using single ions as probes 2. In the tightly focused tweezers, the paraxial approximation breaks down, leading to unexpected state-dependent forces on the ions in the direction perpendicular to the tweezers. I will explain how these may cause small, but avoidable errors in existing trapped ion quantum computing platforms and, conversely, how these may be used to our advantage for implementing novel quantum gates [3,4]. Finally, I will discuss our ideas of using the optical tweezers in precision spectroscopy.

1 J. D. Arias Espinoza et al., Phys. Rev. A 104, 013302 (2021).
2 M. Mazzanti et al., Phys. Rev. A 110, 043105 (2024).
3 M. Mazzanti et al., Phys. Rev. Research 5, 033036 (2023).
4 L.P.H. Gallagher et al., arXiv:2502.19345 (2025).

Coherence and entanglement are key features of quantum mechanics, although they are susceptible to environmental perturbations. A conventional strategy to entangle qubits with high fidelity is to leverage precisely controlled interactions while keeping qubits from decohering. By leveraging electric dipolar interactions, I will report entangling individual trapped NaCs molecules in optical tweezers and realizing a quantum logic gate with trapped molecules[1]. The bulk of the talk will try to go beyond this paradigm to explore and ask: Can coherence be preserved during chemical reactions and subsequently harnessed to produce entangled products? To address this question, we conduct investigations within the context of an atom-exchange chemical reaction (2KRb -> K2 + Rb2) at a temperature of 500nK[2]. I will share our research findings including surprises and puzzles.

[1] Entanglement and iSWAP gate between molecular qubits, Nature 637, 821 (2025)

[2] Quantum interference in atom-exchange reactions, Science 384, 1117 (2024)

Ultracold mixtures involving highly magnetic atoms such as Er+Li, Dy+K, Cr+Li, and Er+Yb have already been realized for studying exotic quantum many-body phases. Such mixtures also open the way for the formation of ultracold diatomic molecules having both significant magnetic and electric dipole moments. Molecules involving atoms with large orbital angular momenta, such as Dy and Er, have a prohibitively complex internal structure with chaotic rovibrational spectra. In contrast, molecules involving highly magnetic symmetric atoms such as Cr or Eu may form exotic doubly polar molecules with easier-to-predict structures. On the other hand, molecules involving electronegative coinage metals such as Ag and alkali-metal atoms should possess very large permanent electric dipole moments.

In my talk, I will present several classes of highly polar and paramagnetic diatomic molecules, which can be produced at ultralow temperatures from laser-cooled ultracold atoms. I will show a new mechanism of useful Feshbach resonances in Cr+Yb mixtures [1] and the application of ground-state YbCr molecules for new physics searches [2], a recent experimental realization of weakly-bound LiCr molecules and our theoretical prediction of transferring them to the absolute ground state [3], as well as all-optical formation schemes of highly polar KAg and CsAg molecules [4]. Finally, I will discuss the possible application of ultracold highly polar and paramagnetic diatomic molecules in studying controlled collisions and chemical reactions and using them for precision measurements, quantum simulations, and quantum computing.

References:
[1] M. D. Frye, P. S. Żuchowski, M. Tomza, Phys. Rev. Research 6, 023254 (2024)
[2] A. Ciamei, A. Koza, M. Gronowski, M. Tomza, arxiv (2025)
[3] S. Finelli, A. Ciamei, B. Restivo, M. Schemmer, M. Inguscio, A. Trenkwalder, K. Zaremba-Kopczyk, M. Gronowski, M. Tomza, M. Zaccanti, PRX Quantum 5, 020358 (2024)
[4] M. Śmiałkowski, M. Tomza, Phys. Rev. A 103, 022802 (2021)

The most precise measurements of the electron’s electric dipole moment (eEDM) all use molecules [1,2]. The molecules are spin polarized, and the eEDM determined by measuring the spin precession frequency in an applied electric field. The precession is due to the interaction of the eEDM with an effective electric field which can be exceptionally large for heavy polar molecules. To reach high precision we need long spin precession times, which is only possible with neutral molecules if they are cooled to low temperatures. I will report progress towards an eEDM measurement using laser-cooled YbF molecules [3]. In one experiment, we produce a beam of molecules cooled to sub-Doppler temperatures in the two transverse directions and measure the spin precession frequency as the molecules fly. This experiment is operational, and I will present the sensitivity that we reach and our efforts to control systematic errors. I will also present our progress in producing very slow YbF molecules and trapping them, with the longer-term aim of making an eEDM measurement using molecules trapped in an optical lattice.
[1] V. Andreev et al., Nature 562, 355 (2018)
[2] T. S. Roussy et al., Science 381, 46 (2023)
[3] N. J. Fitch, J. Lim, E. A. Hinds, B. E. Sauer and M. R. Tarbutt., Quantum Sci. Technol. 6, 014006 (2021)

Light induced charge transfer in molecular complexes containing electron donor and acceptor groups is at the basis of organic photovoltaic devices. To capture the time evolution of this process at the early stages, ideally attosecond time-resolution is required. With the help of elaborate theoretical methods, attosecond pump-probe experiments allow one to image the motion of the “fast” electronic motion in these molecules, mostly in the gas phase, and understand how this motion affects the “slower” motion of atomic nuclei and vice versa. Currently, attosecond pulses produced by high harmonic generation inevitably lead to ionization, so that most of the reported studies concern electron dynamics generated in molecular cations [1-7]. But very recently, UV pulses of few-fs duration have become available [8,9], which combined with attosecond probe pulses, have opened the way to investigations of charge transfer dynamics in neutral molecules with attosecond resolution.

In this talk, I will present the results of theoretical simulations of attosecond pump-probe experiments to investigate the early stages of charge transfer in the donor-acceptor para-nitroaniline (PNA) and meta-nitroaniline (MNA) molecules, both in the neutral and cationic forms. The theoretical approach describes (i) the coherent excitation or ionization of the molecules by the pump pulse, (ii) the ensuing coupled electron and nuclear dynamics, and, in some cases, (iii) the time-resolved photoelectron spectra that should ideally be measured. The results show the strong interdependence between electronic and nuclear motions even during the first few femtoseconds of the charge dynamics.

[1] G. Sansone at al, Nature 465 763 (2010).
[2] F. Calegari et al, Science 346, 336 (2014).
[3] M. Nisoli, P. Decleva, F. Callegari, A. Palacios, and F. Martín, Chem. Rev. 117, 10760 (2017).
[4] F. Calegari and F. Martín, Commun. Chem. 6, 184 (2023).
[5] A. Palacios and F. Martín, WIREs Comput. Mol. Sci. e1430 (2020).
[6] G. Grell et al, Phys. Rev. Res. 5, 023092 (2023).
[7] F. Vismarra et al, Nature Chemistry 16, 2017 (2024).
[8] M. Galli et al, Optics Letters 44, 1308 (2019).
[9] M. Reduzzi et al, Optics Express 31, 26854 (2023).

The broad energy bandwidth of ultrashort pulses enables building a coherent superposition of elec-tronic states. As a result, the electronic density is out of equilibrium and its localization between nuclei can be controlled on a purely electronic time scale.[1] As the nuclei begin to move, the electronic and nuclear motions are entangled.[2] This entanglement can be usefully exploited for controlling the force exerted by the vibronic wave packet on the nuclei [3] and steering chemical reactivity for bond making and fragmentation.[4,5] The dynamics of entanglement will be discussed for oriented molecules: The photoexcited bound N2 molecule and the LiH molecule whose excited states are dissociating. The role of entanglement will be then discussed in polyatomic molecules undergoing structural rearrangements: Bond making in the photoinduced isomerization of norbornadiene into quadricyclane(4), the structural Jahn-Teller rearrangement of the methane cation upon sudden photoionization(5). Control schemes exploiting electronic coherences are not limited to oriented the molecules. We will show that it can be implemented in ensemble of molecules with initial random orinentations with respect to the pulse po-larization. The principal orientations of the ensemble are characterized using Singular Value Decom-position (SVD).[6] SVD provides insights on the stereodynamics. The approach will be illustrated for the case of an ensemble of initially randomly oriented LiH molecules photoexcited by a short CEP controlled IR pulse and for the forces driving the ultrafast Jahn-Teller (JT) structural rearrangement of the methane cation induced by XUV photoionization.

[1]. F. Remacle et al Proc. Natl. Acad. Sci. USA 103, 6793 (2006).
[2]. M. Blavier et al Phys. Chem. Chem. Phys. 24, 17516 (2022).
[3]. M. Cardosa et al J. Phys. B 57, 133501(2024).
[4]. A. Valentini et al Phys. Chem. Chem. Phys. 22, 22302 (2020).
[5]. C.E.M. Gonçalves, et al Phys. Chem. Chem. Phys. 23 12051 (2021).
[6]. M. Cardosa et al J. Phys. Chem. A. 128, 2937 (2024).

The coupled motion of electrons and nuclei is central to understanding fundamental processes in molecules. Here, we investigate the ultrafast photodissociation of bromine (Br$_2$) using a femtosecond pump-probe scheme [1]. In our experiment a weak 400 nm pulse initiates dissociation along the neutral C-state, followed by an 800 nm probe pulse that ionizes the evolving fragments at variable delay. We detect the three-dimensional momenta of both photoions and photoelectrons in coincidence using COld Target Recoil Ion Momentum Spectroscopy (COLTRIMS) [2], in order to correlate the internuclear distance with the transition from a molecule to two separate atoms.

Positive delays mean that the intense, 800 nm probe pulse arrives after the pump pulse. Coulomb explosion is used to infer the internuclear distance in the single ionization channel which is correlated to the photoelectron momentum distributions.

Our measurements reveal a clear transition from molecular-like ionization channels at short pump-probe delays to atom-like channels at longer delays. The extracted photoelectron distributions show the evolution of valence orbitals from Br$_2$-centered wavefunctions to Br atomic orbitals as the internuclear distance increases. Numerical simulations using semiclassical two-step (SCTS) modeling and time-dependent density functional theory corroborate that a redistribution of ion core charge drives these characteristic momentum features [3].

By comparing ion kinetic energy release with the molecular-frame photoelectron momentum distributions, we find that the electronic evolution precedes the full separation by about 50 fs, emphasizing the significance of electron-nuclear coupling [1]. These findings highlight the need for multi-observable approaches to disentangle parallel electronic and structural rearrangements, thereby clarifying benchmark timescales for chemical bond cleavage.

References
[1] W. Li et al. 2010 PNAS 107 20219
[2] J. Ullrich et al. 2003 Rep. Prog. Phys. 66 1463
[3] T. Wang et al. 2025 in preparation

Single molecular ions present a highly attractive platform for high resolution and highly sensitive spectroscopy. These molecules can be held for many hours in a pristine environment, and can be motionally laser cooled into the millikelvin regime and below. Prior to this work, methods to study "generic" single molecular ions have not been demonstrated. Here, we demonstrate a novel single molecule action-spectroscopy technique that is compatible with high precision measurement, and present rotationally resolved spectra of single polyatomic ions. The method is generally applicable to a wide range of polyatomic molecular ions, and promises spectral resolution comparable to state of the art quantum logic methods, with significantly less stringent experimental overhead. Progress towards extending this technique to include chiral recognition of single molecules will be discussed. Adaptations of this technique will prove useful in a wide range of precision spectroscopy arenas including the search for parity violating effects in chiral molecules and searches for biological signatures in samples from beyond Earth.

We report the first experimental realization of cold atoms interacting with an optical frequency comb (OFC) inside a high-finesse Fabry-Perot cavity in the dispersive regime. This is achieved by leveraging the narrow optical transition of the cold atoms, with a linewidth much smaller than the free spectral range of the cavity, allowing all cavity modes to be detuned far from atomic resonance and simultaneously excited by a multiple of frequency comb modes.

In the linear dispersive regime of light-matter interaction, we observe a collective frequency shift of multiple cavity modes in the spectrum of the transmitted OFC. In the nonlinear regime, we study bistability in the transmission of a single comb mode, induced by optical pumping in Zeeman ground states.

These results introduce a new approach to controlling light-matter interactions with multiple OFC modes in cavity-enhanced setups, paving the way for the implementation of recent proposals in cavity cooling [1], quantum annealing [2, 3], and quantum simulations [4].

References:
[1] Torggler, I. Krešić, T. Ban, and H. Ritsch, New Journal of Physics 22, 063003 (2020).
[2] V. Torggler, S. Krämer, and H. Ritsch, Phys. Rev. A 95, 032310 (2017).
[3] Torggler, P. Aumann, H. Ritsch, and W. Lechner, Quantum 3, 149 (2019).
[4] N. Masalaeva, H. Ritsch, and F. Mivehvar, Phys. Rev. Lett. 131, 173401 (2023).

The development of structured ultrafast laser sources has been crucial for advancing our understanding of the fundamental dynamics of electronic and spin processes in matter. In particular, ultrafast laser pulses structured in their spin angular momentum (linked to light polarization) and orbital angular momentum (related to the transverse phase profile or vorticity of a light beam) play a relevant role in studying chiral systems and magnetic materials at their intrinsic temporal and spatial scales.
Recent advancements have enabled the generation of structured ultrafast laser pulses on attosecond timescales, leading to significant progress in nonlinear optics. Specifically, the highly nonlinear process of high harmonic generation (HHG)—which up-converts an intense infrared driving beam into the extreme-ultraviolet (EUV) or soft X-ray region—now allows the creation of structured light pulses at the attosecond timescale.
This talk will review key developments in the field of attosecond structured pulses over the past decade [1-4], with a special focus on the up-conversion of spatiospectral and spatiotemporal optical vortices [5]. These beams are particularly well-suited for probing ultrafast electronic dynamics in systems with coupled spatial and temporal responses. We shall explore potential applications of these structured attosecond pulses in investigating localized ultrafast phenomena across fundamental and applied science.

[1] C. Hernández-García, A. Picón, J. San Román, and L. Plaja “Attosecond extreme ultraviolet vortices from high-order harmonic generation” Phys. Rev. Lett.111, 083602, (2013).
[2] A. K. Pandey, et al., ACS Photonics 9 (3), 944-951 (2022)
[3] A. de las Heras, D. Schmidt, J. San Román, J. Serrano, J. Barolak, B. Ivanic, C. Clarke, N. Westlake, D. Adams, L. Plaja, C. G. Durfee, C. Hernández-García, “Attosecond vortex pulse trains”, Optica 11, 1085 (2024).
[4] 4 N J. Brooks, A. de las Heras, B. Wang, I. Binnie, J. Serrano, J. San Román, L. Plaja, H. C. Kapteyn, C. Hernández-García, M. Murnane, “Circularly Polarized Attosecond Pulses enabled by an Azimuthal Phase and Polarization Grating”, ACS Photonics 12, 1, 495–504 (2025).
[5] R. Martín-Hernández, G. Gui, L. Plaja, H. K. Kapteyn, M. M. Murnane, C.-T. Liao, M. A. Porras and C. Hernández-García. “Extreme-ultraviolet spatiotemporal vortices via high harmonic generation” Nature Photonics, in press (2025).

1. Introduction

The X-ray spectral range can address atomic scale (nm) spatial resolution at ultrafast time (fs) scales, with element specificity and site-selective excitation. Non-linear wave mixing techniques in this range, in particular four-wave mixing (FWM) methods, can thus provide information on the structural and electronic dynamics of atomic and molecular systems with unprecedented resolution. X-ray FWM brings the capability to study the electronic states coupling between spatially localized inner and/or core transitions among different sites of a quantum system or to study transport phenomena at the nanoscale. Whereas mixed XUV/X-ray - optical four-wave mixing and all-EUV have been successfully demonstrated in a transient grating (TG) configuration (see refs. [1-3] and refs. therein), non-linear all-X-ray four-wave mixing spectroscopy has been envisioned and theoretically described [4] but not yet realized experimentally, remaining as a long-awaited goal until now [5].

We demonstrate nonlinear X-ray four-wave mixing (XFWM) will all photons in the soft X-ray range (850-870 eV) using a non-collinear folded ‘Box’ or BoxCARS configuration [6]. In this robust configuration and obeying phase-matching, the X-ray photons generated by three interacting soft X-ray beams are emitted towards the fourth corner of a square, allowing for background-free detection. The signal could thus be clearly discriminated from the incoming beams and detected either by an in-line X-ray grating spectrometer (ΔE≈0.4 eV), allowing for its spectral characterization or by recording the fluorescence from a YAG screen moved into the signal beam path, for its spatial characterization.

2. Results

SASE pulses from the Swiss Free Electron Laser (SwissFEL) are focused by a pair of KB mirrors into an in-vacuum gas-cell filled with a few hundreds mbar of Ne producing a coherent response from core-shell electrons. When scanning the FEL pink beam available at the Maloja end-station around the Ne K-absorption edge at ~870 eV, the YAG fluorescence shows a laser-like signal beam, well isolated from the incoming beams, which shows the maximum signal strength when approaching the pre-edge resonances of Ne. The signal generation efficiency, as defined by the ratio of signal photons that are scattered into the phase-matched direction and the incoming photons is in the order of 0.15 %, demonstrating an efficient signal generation.

The spectrally dispersed XFWM signals provide a rich map, comprising multiple contributions from neutral neon and its ions. The large set of parameters explored, including FEL intensity, gas pressure and pulse duration ranging from 30 fs down to 3 fs (rms), provides key information that allow us to disentangle the different origin of the XFWM signals, including resonantly-enhanced X-ray two-color processes. The measured results are compared to the available literature on stimulated Raman X-ray scattering in Neon [7] and with the results of a dedicated model that accounts for the beam propagation and population changes during the X-ray pulse duration. We discuss in detail the measured and calculated 2D spectral maps, and how the correlation plots reflect the coupling between electronic states of neon, allowing us to distinguish these signals from X-ray lasing processes originated in the ions.

The general feasibility of non-collinear four-wave mixing in the X-ray range is demonstrated. The robustness of the setup, the strength of the signals and the spectral and spatial information achieved from the experiment implies a major breakthrough for the application of nonlinear X-ray wave mixing as a spectroscopic tool in general, and as basis for 2D X-ray correlation spectroscopy. In addition, preliminary time resolved signals and approaches to extend the proposed methodology to the time domain, based on two-colour mode available at the Athos branch of SwissFEL are introduced.

3. References

[1] F. Bencivenga et al., “Four-wave mixing experiments with extreme ultraviolet transient gratings” Nature 520, 205 (2015).
[2] F. Bencivenga, et al. "Nanoscale transient gratings excited and probed by extreme ultraviolet femtosecond pulses." Sci. Adv. 5(7) eaaw5805 (2019).
[3] J.R. Rouxel, D. Fainozzi, R. Mankowsky, B. Rösner, G. Seniutinas, et al. “Hard x-ray transient grating spectroscopy on bismuth germinate” Nat. Photon 15(7), 499–503 (2021)
[4] S. Tanaka and S. Mukamel “X-ray four-wave mixing in molecules” J. Chem. Phys. 116(5), 1877–1891 (2002)
[5] A.S. Morillo-Candas, et al. "Coherent all X-ray four-wave mixing at core shell resonances." arXiv preprint arXiv:2408.11881 (2024), https://doi.org/10.48550/arXiv.2408.11881
[6] Y. Prior, “Three-dimensional phase matching in four-wave mixing” Appl. Opt. 19(11), 1741–11743 (1980)
[7] C. Weninger, M. Purvis, D. Ryan, R.A. London, J.D. Bozek, C. Bostedt, A. Graf, G. Brown, J.J. Rocca, and N. Rohringer, "Stimulated Electronic X-Ray Raman Scattering" PRL 111, 233902 (2013).

Investigations aiming at the determination of molecular chirality by chiroptical techniques continue to attract a significant amount of interest in chemistry, physics biology and pharmacology. Some focus has been put on the measurement and analysis of the photo ion circular dichroism (PICD) – a total ion yield effect – and the Photoelectron Circular Dichroism (PECD) – a molecular frame angular distribution effect. Most efforts have put immense work into establishing “perfect” circular polarization.
In recent years it has been demonstrated, that Photoelectron Elliptical Dichroism (PEELD) – which considers a systematic variation of the elliptical polarization of light – is also a powerful approach for studying chirality. Basically, PEELD is the extension of PECD to a variation of elliptical polarization. So far, PEELD has been reported for a number of neutral analytes using multi photon ionization, demonstrating advantages over plain PECD experiments.[1,2,3] PEELD measurements with single photon ionization have not been reported yet, due to the comparably high ionization energy of neutral analytes.
In this work we demonstrate the measurement of PEELD in single photon photodetachment of electrons from electro-sprayed anions. Photodetachment of electrons from anions has recently been developed as a new technique for chirality analysis.[4,5,6,7] When combined with electro-spraying the anions of interest, the technique, ESI-PECD, allows the study of proteins containing many amino acids.[5]
As analytes we chose, amino acids, for which the quantification of enantiomeric excess has already been demonstrated by ESI-PECD in our group, i.e. phenylalanine and tryptophan.[7] In this work, we show that the PEELD of the phenylalanine-anion scales linearly with the STOKES parameter S3 for small values of S3. There is a distinct extremum of the PEELD in the region of [S3 / So ] = 0.95 with very good symmetry between D- and L-phenylalanine. We note, that there are literature reports indicating non-linear variation of the PEELD with S3, however, in the case of multi photon ionization.[1,2,3]

The study of PEELD in photodetachment is expected to shed new light on the fundamental question of asymmetry in the photoelectron angular distribution comparing multiphoton and single photon processes. It is argued that a well-defined elliptic polarization has to be preferred over an ill-defined circular polarization.

[1] A. Comby, D. Descamps, S. Petit, E. Valzer, M. Wloch, L. Pouységu,
S. Quideau, J. Bocková, C. Meinert, V. Blanchet, B. Fabre, Y. Mairesse,
Phys. Chem. Chem. Phys. 2023, 25, 16246 – 16263.
[2] A. Comby, E. Bloch, C. M. M. Bond, D. Descamps, J. Miles, S. Petit, S. Rozen, J. B. Greenwood, V. Blanchet, B. Fabre, Y. Mairesse, Nat. Commun. 2018, 9, 5212.
[3] J. B. Greenwood, I. D. Williams, Phys. Chem. Chem. Phys. 2023, 25, 16238 – 16245.
[4] P. Krüger, K.-M. Weitzel, Angew. Chem. Int. Ed., 2021, 60, 17861 – 17865.
[5] P. Krüger, J. H. Both, U. Linne, F. Chirot, K.-M. Weitzel, J. Phys. Chem. Lett., 2022, 13, 6110 – 6116.
[6] J. Triptow, A. Fielicke, G. Meijer, M. Green, Angew. Chem. Int. Ed., 2022, 62, e202212020.
[7] J. H. Both, A. Beliakouskaya, K.-M. Weitzel, Analytical Chemistry, 2025, in press, (https://doi.org/10.1021/acs.analchem.4c05964).

Optical trapping has transformed our ability to manipulate and control atoms and micro/nanoparticles, offering numerous applications and research directions across a wide range of scientific fields. From biological cells and colloidal microparticles to nanoparticles and cold atoms, the manipulation of single particles using optical tweezers has provided us with invaluable insights. One promising alternative to the conventional optical tweezers lies in the use of optical nanofibres - ultrathin fibres with a diameter smaller than the wavelength of light they are designed to guide. Such nanofibres confine light very tightly in the radially direction, creating an high intensity evanescent field beyond their glass boundary.

Advantageously, setups involving optical nanofibres tend to be simple and have a very small footprint, leading to “lab-on-chip” type applications. Within cold atom setups, they minimally disturb the magneto-optical trapping fields and provide an efficient data communication channel for directly sending or collecting light even down to the single photon level. With their strongly confined light fields, long interaction lengths, and low loss, nanofibres are excellent platforms for exploring phenomena like chiral atom-light interactions and waveguide quantum electrodynamics.

In our work, we have shown how Rydberg atom excitation can be mediated via an optical nanofibre, significantly reducing the power needed for the excitation to occur. We have also proposed trapping schemes for cold ground state and Rydberg state atoms by exploiting the evanescent field from optical nanofibres and combining it with a holographic tweezers configuration to create a hybrid atom trap platform. We will discuss our recent progresses, and also the limitations of this platform, during the talk.

Ultracold gases of paramagnetic atoms or dipolar molecules are fascinating in the quantum degenerate regime, to be sure. However, even a thermal gas consisting of these entities may have unusual features. Specifically, polar molecules with sufficiently large dipole moment can enter the hydrodynamic regime, where the mean-free path for collisions is far smaller than the scale of structures in the gas, for example, the wavelength of sound. In this circumstance it is plausible to consider the gas as a fluid, governed by Navier-Stokes type equations, but with the novelty that transport coefficients, such as heat conduction and viscosity, are anisotropic. I present a formulation of these equations and some preliminary results on sound modes as well as “weltering” dynamics that occurs in a trapped gaseous sample.

Cavity quantum electrodynamics studies the strong interaction between matter and the electromagnetic field of an optical cavity: the enhanced interaction is useful both for reading the properties of the atoms with a fast, sensitive and weakly destructive measurement and for quantum simulation where atoms interact by exchanging photons with each other at a distance. One of the drawbacks of these systems is the loss of spatial information that cavity-based measurement implies: the result of these measurements is an average of the properties of the atoms over the entire cavity field volume.

I will explain how we built and operated a cavity-microscope device that overcomes this problem: it realizes both a cavity and a pair of high numerical-aperture lenses in a single device and can be used to couple a microscopic part of the atomic cloud to the cavity field. We produce a cavity-based image of the atomic density by scanning the position of the microscope focus [1].

This technology opens the doors to analog quantum simulations of programmable, all-to-all interacting systems. I will report about the self-organization phase transition of a Fermi gas in a high-finesse cavity in the presence of tight confinement and the development of optical techniques to randomize cavity-mediated interactions. These interactions can drastically change the behavior of the system, and open the door to the exploration of models of holographic quantum matter such as the Sachdev-Ye-Kitaev model [2][3].

Dissipative quantum many-body problems, such as those arising in collective light-matter interactions, present theoretical challenges. To explore these phenomena experimentally, we have developed an experimental setup that studies collective light scattering from an ordered ensemble of atoms. Recently, we achieved the first trapping and imaging of single dysprosium atoms in optical tweezers [1], extending the single-atom toolbox to lanthanides. Leveraging the rich internal structure of dysprosium, we can measure the atoms' internal states which we use to investigate collective dissipation both in the linear optics regime and at high saturation [2]. To further enhance the collective behavior of the atoms, we have two approaches. First, we cool the atoms close to their ground state using a 2 kHz transition [3]. Additionally, we are working to bring the atoms to a distance comparable to the wavelength of the transition used for light scattering by implementing a hybrid tweezer and accordion lattice setup.

[1] D. Bloch, B. Hofer, S. R. Cohen, A. Browaeys, and I. Ferrier-Barbut, Trapping and imaging single dysprosium atoms in optical tweezer arrays, Phys. Rev. Lett. 131, 203401 (2023)
[2] B. Hofer, D. Bloch, G. Biagioni, N. Bonvalet, A. Browaeys and I. Ferrier-Barbut, Single-atom resolved collective spectroscopy of a one-dimensional atomic array, arXiv:2412.02541 (2024)
[3] G. Biagioni et al. Narrow line cooling of single dysprosium atoms, in preparation

Attosecond science addresses the dynamics of electrons on their natural time scale, and much progress has been made in the understanding of processes such as attosecond charge migration; attosecond photoionization times; and the driven attosecond dynamics that takes place during high-harmonic generation (HHG). The majority of attosecond studies have been performed on atoms and molecules in the gas phase, with recent extensions of HHG into crystalline solids. However, many organic molecules of interest exist most naturally in the liquid phase, and only a few studies have emerged on the influence of a liquid environment on the initiation, evolution, and probing of attosecond dynamics.

In this talk I will report on two recent efforts studying attosecond dynamics in solutions, in which a low concentration of a molecule of interest is mixed into a liquid solvent: (i) We have compared core-hole-initiated charge migration in fluoroaniline in different solutions, using time-dependent density functional theory calculations. We find that the charge migration period, which is less than 1 fs, stays relatively constant independent of the solvent. However, in the solutions we find finite dephasing times for the dynamics that depend strongly on the solvent, suggesting that solute-solvent interactions indeed have an influence on the attosecond electron dynamics. (ii) In a joint experiment-theory work, we have studied HHG in different solutions of halobenzenes (PhX) and methanol (MeOH). Our experimental results show evidence of local order in the solvation of fluorobenzene (PhF) in MeOH. This manifests as a near-complete suppression of a single harmonic in the solution, which is absent in each of the pure liquids and absent in other PhX-MeOH solutions. We interpret the results in terms of a solvation shell that is formed in the PhF-MeOH solution and acts like a local barrier in the HHG process.

Coulomb explosion imaging (CEI) of polyatomic molecules induced by intense, femtosecond pulses at the European X-ray free-electron laser (EuXFEL) has allowed several new insights. In recent years, CEI has resolved asymmetric deformation and bond-angle opening in fragmenting water molecules [Jahnke2021], reconstructed the three-dimensional geometry of a five-atom molecule using a charge buildup model [Li2021, Li2022], and imaged a complex eleven-atom molecule—including all hydrogen atoms—while tracing intramolecular electron transfer [Boll2022]. More recently, CEI has captured the time-resolved deplanarization reaction following UV excitation of a heterocycle [Jahnke2025].

The multidimensional nature of CEI allows selective investigation of complex structural dynamics. Here, we demonstrate how X-ray induced CEI can be used to trace the correlated dynamics of a molecular elimination reaction—a minor reaction pathway involving the cleavage of two bonds and the formation of a vibrationally excited di-halogen fragment [Li2025]. In parallel, we map the light-induced bending vibration of the bound molecular wave packet and disentangle competing dissociation channels.

As CEI advances toward more quantitative structural analysis, interpretation becomes increasingly complex. Previous studies have relied on molecule-specific data processing tailored to each system and science case. To establish CEI as a more general and broadly applicable method, systematic approaches for analyzing high-dimensional ion-coincidence data are needed, as some quantitative insights can only be obtained through comparison with advanced modelling. Recent efforts in this direction include the application of principal component analysis (PCA) to simulated CEI data [Richard2021]. Building on this, we have now applied PCA to experimental data for the first time, overcoming significant technical challenges. This analysis enabled the extraction of collective ground-state structural fluctuations (zero-point motion) in a complex molecule [Richard2025]. To our knowledge, this represents the first direct experimental observation of this fundamental quantum phenomenon in a system of comparable complexity. The zero-point motion manifests as correlated variations in ion momenta, establishing CEI as a powerful tool for accessing high-dimensional quantum dynamics.

References

[Boll2022] R. Boll et al., Nat. Phys. 18, 423 (2022)
[Jahnke2021] T. Jahnke et al., Phys. Rev. X 11, 041044 (2021)
[Jahnke2025] T. Jahnke et al., Nat. Commun. 16, 2074 (2025)
[Li2021] X. Li, R. Boll, D. Rolles and A. Rudenko, Phys. Rev. A 104, 033115 (2021)
[Li2022] X. Li et al., Phys. Rev. Res. 4, 013029 (2022)
[Li2025] X. Li et al., Imaging a Light-Induced Molecular Elimination Reaction with an X-ray Free-Electron Laser, under review (2025)
[Richard2021] B. Richard et al., Journal of Physics B 54, 194001 (2021)
[Richard2025] B. Richard et al., Imaging collective quantum fluctuations of the structure of a complex molecule, under review (2025)

When two or more quantum particles in a many-body system are entangled, its wavefunction cannot be factorized as a product of the wavefunctions of its constituents. It has been central to the ongoing second quantum revolution, as seen in the rapid development of the quantum information science. Initially, the idea of entanglement was dismissed by Albert Einstein himself as the 'spooky' action-at-a-distance. However, as it turned out, the photoelectric effect itself presents a unique opportunity to study the quantum entanglement between the emitted photoelectron and the residual ion – measurement of the kinetic energy of the former determines the exact quantum state of the later. Here, using intense extreme ultraviolet (XUV) pulses from a seeded free-electron laser, FERMI [1], we generate quantum entanglement between a photoelectron wave-packet rapidly expanding in space and a hybrid light-matter system, namely, a He+ ion dressed with an XUV photon. In the presence of the entanglement between these two massive particles, the measured photoelectron spectra clearly show an avoided crossing reminiscent of the Rabi cycling in the XUV-dressed residual ion [2]. In the absence of entanglement, however, the measured kinetic energies of the photoelectrons increase monotonically with increase in the photon energy, as predicted by Einstein’s photoelectric equation. We show that even if the photoelectron wave-packet expands up to a mesoscopic distance of almost 200 nm [3], it still remains entangled to the residual ion. Our results offer opportunities to explore quantum entanglement across ultrafast timescales over macroscopic distances.

References:
[1] E. Allaria et al., Highly coherent and stable pulses from the FERMI seeded free-electron laser in the extreme ultraviolet. Nature Photonics 6, 699 (2012).
[2] S. Nandi et al., Observation of Rabi dynamics with a short-wavelength free-electron laser. Nature 608, 488 (2022).
[3] S. Nandi et al., Generation of entanglement using a short-wavelength seeded free-electron. Science Advances 10, eado0668 (2024).

Highly excited (Rydberg) atoms have exaggerated properties making them extremely sensitive to external electromagnetic fields and interacting strongly with their environment. Atomic vapor cells represent an attractive platform for studying Rydberg atoms and fabricating quantum devices. For example, Rydberg atoms in vapor cells have been used as sensitive detectors of electric fields of frequencies ranging from DC up to the THz range but also as single photon sources for quantum technology applications exploiting collective phenomena due to the Rydberg blockade effect [1].

Rydberg atoms also find applications in fundamental physics, in particular for the measurement of dispersive interactions of the Casimir-Polder type (atom-surface interactions) [2] or of the van der Waals type (atom-atom interactions). One major advantage of Rydberg atoms is that they expose limitations in the traditional perturbative approach of Casimir-Polder (CP) theory [3, 4]. Indeed, in the extreme near-field (i.e. when the atomic radius is no longer negligible compared to the atom-surface distance), the dipole approximation breaks down and higher-order terms need to be considered. Our recent theoretical study on Rydberg-surface interactions has provided calculations of the dipole-dipole terms that scale as −$C_3$/$z^3$ as well as quadrupole-quadrupole and dipole-octupole terms that scale as −$C_5$/$z^5$, clearly demonstrating that higher order terms could be experimentally relevant in vapor nanocell spectroscopy [4].

We report on extensive experimental measurements of the Rydberg-surface interaction using spectroscopy in cesium vapor nanocells of a thickness ranging roughly from 200-700nm, as well as selective reflection spectroscopy on a macroscopic cesium all-sapphire cell. Atoms are first excited to the Cs(6$P_{1/2}$) level with a 894nm pump laser and subsequently a green laser ≈ 510nm probes Rydberg n$D_{3/2}$ or n$S_{1/2}$ states, where the principal quantum number n ranges between 15-17. Our experiments evidence the dipole-dipole term of the Casimir-Polder interaction providing a measurement of the $C_3$ coefficient for cesium Rydberg states. Furthermore, our experiment clearly evidences an additional interaction that induces shifts and broadens the linewidth of the probed transitions in the vicinity of the dielectric windows of our cells. We believe that this interaction is due to electric fields that are either generated by patch charges (trapped on the surface or induced by the excitation lasers), or by cesium adsorbants. We show that the different polarizability (of opposing sign) between S and D Rydberg states can be exploited to extract quantitative measurements of the strength and distance scaling (z-dependence) of such parasitic electrostatic interactions.

Our experiments suggest that the sensitivity of Rydberg atoms to external electric fields could provide a unique tool for probing electrostatic interactions in the vicinity of surfaces. This could allow systematic error corrections in Casimir-Polder experiments with excited or even ground state atoms that aim at putting bounds on the existence of non-Newtonian gravity [5]. We are currently exploring the possibility of coating the internal window interfaces with conducting 2D material such as graphene to reduce the influence of parasitic charges. This could allow us to probe atoms closer to the surface, at distances around 100nm, where quadrupole interactions ($C_5$ coefficient) could be experimentally attainable for the first time.

References
[1] H. Kubler, J. P. Shaffer, T. Baluktsian, R. Loew, T. Pfau, Nat. Photon., 4, 112–116 (2013).
[2] V. Sandoghdar et al., Phys. Rev. Lett 68, 3432–3435 (1993).
[3] J. A. Crosse et al., Phys. Rev. A 82, 3010901 (2010).
[4] B. Dutta et al., Phys. Rev. Res. 6, L022035 (2024).
[5] A. Laliotis, B-S. Lu, M. Ducloy, D. Wilkowski, AVS Quantum Sci. 3, 043501 (2021).

Matter-wave interferometry serves as a fundamental test of quantum mechanics, directly probing the superposition principle and constraining potential modifications to the theory.
Recent experiments have demonstrated quantum interference of nanoparticles exceeding 25,000 Da [1], and the newest generation of interferometers aims to extend this mass limit by one to two orders of magnitude [2]. We present preliminary data from our new experimental setup, designed to observe interference of metal clusters at unprecedented mass scales of 100.000 Da and beyond.
Additionally, we explore applications of matter-wave interferometry in molecular metrology and discuss ongoing efforts to extend these techniques to biomolecules, including proteins.
[1] Y. Fein et al., Nature Phys. 15, 1242, (2019)
[2] F. Kialka et al., AVS Quantum Sci. 4, 020502 (2022)

Quantum resonances in low-energy collisions are a sensitive probe of the intermolecular forces. They dominate the final quantum state distribution even for strong and highly anisotropic interactions, as recently observed for Feshbach resonances populated by Penning ionization of dihydrogen colliding with a metastable rare gas atom [1]. For such a small collision complex, full quantum scattering calculations can be carried out [1]. The theoretical predictions for the cross section involve then only the approximations made when constructing the potential energy surface (PES). Changes in the shape of the PES thus translate directly into modifications of the cross sections. This can be used to to improve calculated PES, starting from the experimental data [2]. Conversely, one can also ask by h
ow much the experimental resolution of measured cross sections must improve in order to unambiguously discriminate predictions derived from different levels of advanced ab initio electronic structure theory [3]. Such discrimination is equivalent to resolving the intermediate resonances governing the reaction dynamics, on top of the initial and final states.

[1] Margulis et al., Science 380, 77 (2023)
[2] Horn et al., Science Advances 10, eadi6462 (2024)
[3] Horn et al., arXiv:2408.13197

The study of molecular collisions with the highest possible detail has been an important research theme in physical chemistry for decades. Experimentally, the level of detail obtained in these studies depends on the quality of preparation of the collision partners before the collision, and on how accurately the products are analyzed afterward.

Over the last years, methods have been developed to get improved control over molecules in a molecular beam. With the Stark decelerator, a part of a molecular beam can be selected to produce bunches of molecules with a computer-controlled velocity and with longitudinal temperatures as low as a few mK. The molecular packets that emerge from the decelerator have small spatial and angular spreads, and have almost perfect quantum state purity. These tamed molecular beams are excellent starting points for high-resolution crossed beam scattering experiments.

I will illustrate the possibilities this new technology offers to study molecular collisions with unprecedented precision and at low collision energies. I will discuss our most recent results on the combination of Stark deceleration and velocity map imaging. The narrow velocity spread of Stark-decelerated beams results in scattering images with an unprecedented sharpness and angular resolution. This has facilitated the observation several quantum effects in state-to-state cross sections, such as diffraction, scattering resonances and product pair correlations in bimolecular collisions. Finally, I will present recent results on bimolecular collisions at collision energies down to 0.1 cm-1 obtained by merged beam configurations, featuring novel effects induced by the dipole-dipole interaction.

Compton scattering is the fundamental light-matter interaction process discussed in the textbooks as a billiard-type collision, in which a photon (as a particle) is deflected and transfers parts of its energy and momentum to an electron initially at rest. If electron is bound in an atom or molecule, its momentum distribution contributes to the balance, which is known as the impulse approximation [1]. As a consequence, the momentum distribution of direct Compton electrons is given by the Fourier transform of the initial orbital displaced by the photon momentum transfer. Recent experimental and theoretical studies [2,3] highlighted the need to go beyond the this approximation. In particular, Ref. [2] reported a backward scattering of the direct Compton electrons with respect to the photon momentum transfer and a simultaneous forward kick of the parent nucleus, while Ref. [3] demonstrated a scattering of the direct Compton electrons to all angles and a Coulomb focusing of the electrons by the ionization potential of the ion.

In the present work, we consider theoretically and experimentally the K-shell ionization of C and O atoms in carbon monoxide molecules by Compton scattering of 20 keV photons and report differential electron momentum distributions [4]. We observe diffraction patterns in the momentum distributions, which persist after integration over magnitudes and orientations of the photon momentum transfer in the frame of molecular reference. This phenomenon relies on the interference of the direct Compton electrons and those which are scattered on the parent and neighboring nuclei. The double-slit interference patterns in the electron momentum distribution can directly be related to the molecular orientation and the internuclear distance. The present results suggest that the imaging techniques, widely employed in the optical regime via laser-induced diffraction and soft x-ray domain via one-photon inner-shell photoionization, can be extended to the hard X-ray domain, where the photoionization is strongly suppressed, and the ionization by Compton scattering became dominant.

[1] J. W. M. Du Mond, Phys. Rev. 33, 643 (1929).
[2] M. Kircher et al., Nat. Phys. 16, 756 (2020).
[3] N. Melzer et al., Phys. Rev. Lett. 133, 183002 (2024).
[4] D. M. Haubenreißer et al, (2025) submitted

The results of a newly developed version of the Molecular Convergent Close-Coupling (MCCC) method [1,2] of calculating cross sections for electron scattering on the H$_3^+$ molecule are reported. Integrated cross sections for dissociative electronic excitation and ionisation are presented, yielding good agreement with the experiment [3,4]. The causes of previous disagreements between theory and experiment are identified. The method is presented in both the fixed-nuclei and adiabatic nuclei formulations, with optional point-group symmetry adaptation. The results of the first-ever calculation of fragment kinetic energy release distributions in electron impact dissociation of H$_3^+$ are also reported, yielding good agreement with the strong textexperiment at high energies. The new method opens the door to the modelling of electron and positron scattering on polyatomic molecules using CCC techniques.

Fig 1: Comparison of the total dissociative excitation cross section for electron scattering on H$_3^+$ and D$_3^+$ in several MCCC models [1,2] with the experiments of Lecointre et al [3], Jensen et al [4], the R-matrix calculation of Gorfinkiel and Tennyson [5] and the complex-Kohn calculation of Orel [6].

[ 1] Horton et al, Phys. Rev. Lett. 134, 063001 (2025).
[2] Horton et al, Phys. Rev. A. 111, 022802 (2025).
[3] Lecointre et al, J. Phys. B. 42, 075201 (2009).
[4] Jensen et al, Phys. Rev. A. 63, 052701 (2001).
[5] Gorfinkiel and Tennyson, J. Phys. B 38, 1607 (2005).
[6] Orel, Phys. Rev. A. 46, 1333 (1992).

In recent years, it has become possible to control and the measure the quantum states of the motion of macroscopic mechanical objects. Such efforts are motivated by both the study of fundamental science and the promise of new quantum technologies. I will give a brief overview of how recent advances in the fields of optomechanics and electromechanics have allowed us to explore the quantum behavior of solid-state mechanical objects and use them as new quantum resources. This will be followed by a few examples from my group’s work on quantum control of bulk acoustic resonators and using them as sensors for new physics.

In this talk I will describe our work on quantum networks of quantum emitters [1]. From the point of view of Quantum Optics, these are setups of quantum emitters (i.e. qubits or cavities that talk to qubits) connected by waveguides that transport photons among those nodes. This leads to a paradigm of quantum systems interacting through the exchange of retarded photons, creating what we call a non-local quantum many-body system. I will discuss the theoretical tools we have developed and how these tools can already provide insight in topics ranging from quantum state transfer among quantum processors to a new type of collective emission, called cascaded superradiance, that takes place in these networks.

[1] Time-delayed collective dynamics in waveguide QED and bosonic quantum networks, Carlos Barahona-Pascual, Hong Jiang, Alan C. Santos, Juan José García-Ripoll, arXiv:2505.02642

I present the recent concept of first-order dissipative phase transitions, that can occur in meso- and even microscopic quantum systems.

One of the first examples of this phenomenon was the photon-blockade breakdown (PBB) effect, occurring most simply in the driven-dissipative Jaynes-Cummings model that contains only a single qubit. For PBB, an abstract thermodynamic limit has been identified [1] where the coupling between the qubit and the mode goes to infinity without affecting the system size (a.k.a. zero-dimensional or finite-component phase transition). This limit was studied in a finite-size scaling approach [2], with scaling exponents determined numerically. PBB and its thermodynamic limit were observed in circuit QED systems [3,4]. For the highest realized coupling strength, the system alternates with a characteristic timescale as long as 6s – exceeding the microscopic timescales by 6 orders of magnitude – between a bright coherent state with approximately 10000 intracavity photons and the vacuum state.

In the second half of the talk, I describe analogous effects observed in a rubidium cold-atom cavity-QED setup [5]. Atoms in an optical cavity can manifest a first-order dissipative phase transition dubbed transmission-blockade breakdown. The stable coexisting phases are quantum states with high quantum purity, that include atomic hyperfine ground states and coherent states of electromagnetic field modes [6]. Similarly to PBB, the phases become perfectly pure states in the thermodynamic limit, making the phenomenon relevant for quantum technologies.

[1] Carmichael, Howard J. "Breakdown of photon blockade: A dissipative quantum phase transition in zero dimensions." Physical Review X 5, no. 3 (2015): 031028.

[2] Vukics, András, András Dombi, Johannes M. Fink, and Péter Domokos. "Finite-size scaling of the photon-blockade breakdown dissipative quantum phase transition." Quantum 3 (2019): 150.

[3] Fink, Johannes M., András Dombi, András Vukics, Andreas Wallraff, and Peter Domokos. "Observation of the photon-blockade breakdown phase transition." Physical Review X 7, no. 1 (2017): 011012.

[4] Sett, Riya, Farid Hassani, Duc Phan, Shabir Barzanjeh, Andras Vukics, and Johannes M. Fink. "Emergent macroscopic bistability induced by a single superconducting qubit." PRX Quantum 5, no. 1 (2024): 010327.

[5] Dombi, András et al. "Collective self-trapping of atoms in a cavity." New J. Phys. 23 (2021): 083036

[6] Gábor, Bence et al. "Ground-state bistability of cold atoms in a cavity." Physical Review A 107.2 (2023): 023713; Gábor, Bence et al. "Quantum bistability in the hyperfine ground state of atoms." Physical Review Research 5.4 (2023): L042038.

Detecting and manipulating individual atoms with high fidelity is essential for quantum simulation, metrology and, with even more stringent requirements, for quantum computing. I will present our recent results on ultrafast single-atom imaging, based on alternated pulses of highly saturated light addressing the broad 1S0-1P1 transition in ytterbium. With this scheme, we achieve in-trap single-atom detection fidelity and survival probability exceeding 99.8% within just few microseconds.
Beyond single-atom detection, we extend this ultrafast imaging scheme to the detection of multiple atoms in free space with single-particle resolution. By preparing and releasing multiply filled traps, we demonstrate single-atom-resolved detection without parity projection. This capability will enable new explorations of correlations and many-body dynamics in tweezer-trapped atomic ensembles.
Finally, I will also mention the newest tweezer-based quantum platform under construction at the University of Trieste. This system integrates three key components: a quantum computing unit for executing arbitrary operations, a tweezer-cavity interface for cavity quantum electrodynamics and quantum networking experiments, and an atom reservoir to enable fast experimental cycle times and uninterrupted operations. This architecture will serve as a versatile testbed for advancing neutral atom quantum technologies.

Exciton-polaritons in 2D semiconductor heterostructures provide an ideal platform for exploring novel properties of strongly coupled light-matter quantum fluids. Recently, significant efforts have been dedicated to investigating beyond mean-field effects, with the long-term goal of entering the regime of quantum polaritonics. This presentation focuses on spinor polariton condensates, where, analogous to binary Bose mixtures in ultracold atomic systems, a bipolariton Feshbach resonance enables the realisation and study of beyond mean-field effects across various regimes—from the Bose polaron regime to the regime of self-bound polariton droplets, stabilized by a balance between mean-field interactions and quantum fluctuations.

Several recent experiments performing VUV laser nuclear spectroscopy of Thorium-229 doped into calcium fluoride single crystals allow to probe nuclear properties and host material parameters with unprecedented accuracy. In this talk I will resume our current understanding of the microscopic doping structure, combining theoretical modelling, solid-state techniques like XAFS and RBS, and precision laser spectroscopy. We also report quenching of the thorium-229 isomer population under X-ray and laser illumination in the VUV, UV and optical range, indicating a strong and controllable coupling of nuclear and solid-state degrees of freedom.

I will describe experiments for precision tests of gravitational physics using quantum devices based on ultracold atoms, namely, atom interferometers. I will report on the measurement of the gravitational constant G, on experiments for gravity measurements at small spatial scales, and on new tests of the Einstein equivalence principle.

Transition energy measurements in heavy, few-electron ions are unique tools to test bound-state quantum electrodynamics (QED) in extremely high Coulomb fields, where perturbative methods cannot be implemented. In such fields, the effects of the quantum vacuum fluctuations on the atomic energies are enhanced by several orders of magnitude with respect to light atoms. However, up to now, experiments have been unable to achieve sensitivity to higher-order (two-loop) QED effects in this strong regime. Here we present a novel multi-reference method based on Doppler-tuned x-ray emission from fast uranium ions stored in the ESR ring of the GSI/FAIR facility. By accurately measuring the relative energies between $2p_{3/2} \to 2s_{1/2}$ transitions in two-, three-, and four-electron uranium ions, we were able, for the first time in this regime, to disentangle and test separately high-order (two-loop) one-electron and two-electron QED effects, and set a new important benchmark for this theory in the strong field domain [1]. The achieved accuracy of 37 parts per million allows us to discriminate between different theoretical approaches developed throughout the last decades for describing He-like systems. Experimental outlooks will be presented, as new calibration schemes and the implementation of a new time- and position-sensitive detector, based on Timepix3 technology. These considered improvements will allow for a reduction of the uncertainties by another order of magnitude, lower than the present theoretical prediction uncertainties and nuclear deformation effects.

[1] R. Loetzsch et al., Nature 625, 673-678 (2024).

Supersolids are exotic states of matter that spontaneously break two symmetries: gauge invariance through the phase-locking of the wavefunction, and translational symmetry owing to the emergence of a crystalline structure. In a first part, we report on the theoretical study and experimental observation of vortices in a dipolar supersolid of Dysprosium [1]. When rotated, the supersolid phase shows a mixture of rigid-body and irrotational behavior, highlighting a fundamental difference between modulated and unmodulated superfluids.
Neutral atoms in optical tweezers are one of the most promising platforms for quantum simulation and computation as they offer the implementation of arbitrary geometries, dynamical reconfiguration, generation of free-defects arrays and controllable long-range coupling via Rydberg-mediated interactions. In the second part, we will present our latest results on the successful loading and detection of single erbium atoms in a linear array of optical tweezers [2]. By implementing two complementary techniques for single atoms detection - narrow-linewidth non-destructive and broad-linewidth ultrafast imaging - we characterized the differential light shift for the intercombination line of erbium, and we investigated light-assisted collisions (LAC) and heating-induced losses.

[1] Observation of vortices in a dipolar supersolid, E. Casotti, E. Poli, L. Klaus, A. Litvinov, C. Ulm, C. Politi, M. J. Mark, T. Bland, F. Ferlaino, Nature, 635, 327–331, 2024
[2] Optical Tweezer Arrays of Erbium Atoms, D. S. Grun, S. J. M. White, A. Ortu, A. Di Carli, H. Edri, M. Lepers, M. J. Mark, F. Ferlaino, Phys. Rev. Lett., 133, 223402, 2024

Crystals of cold trapped ions are a promising platform for quantum technology and for studying the quantum many-body problem as a well-controlled toy many-body system. In modern state-of-the-art experiments, managing the entropy of large Coulomb crystals becomes challenging due to the exponential scaling of the Hilbert space with the number of trapped ions. In particular, as we demonstrate, collective effects must be taken into account and play an important role in both motional temperature measurement and the cooling process. Regarding the latter, the possible influence of collective effects has been debated in the literature in recent years. In my talk, I will present a thermometry protocol for large ion crystals that accounts for the emerging collective dynamics [1] and describe a mechanism that enhances the cooling of collective modes as more ions are added to the crystal.

[1] I.Vybornyi et. al, PRX Quantum 4, 040346 (2023)

Quantum optics with Rydberg atoms and superconducting resonators is well-established, with attention now towards interfaces with on-chip quantum computing devices [1,2]. We are developing a similar platform using Rydberg excitons – bound states of excitons and holes in a semiconductor. We have measured the largest microwave-optical Kerr coefficient to date [3], and our experiments are able to reach the ultra-strong driving limit where the coupling strength exceeds the splitting between between nearby Rydberg states [4]. These results are in near-quantitative agreement with an atomic-physics inspired theoretical models [3,4]. Rydberg excitons may also exhibit optical strong coupling [5], and I will describe our progress towards a hybrid quantum system of Rydberg excitons strongly coupled to on-chip optical and microwave resonators at T<300 mK.

[1] A. A. Morgan and S. Hogan, Phys. Rev. Lett. 124, 193604 (2020)
[2] M. Kaiser et al., Phys. Rev. Res. 4, 013207 (2022)
[3] J. D. Pritchett et al. APL Photonics 9, 031303 (2024)
[4] A. Brewin et al., New J. Phys. 26, 113018
[5] K. Orfanakis et al., Nature Materials 21, 767 (2022).

Heat engines convert thermal energy into mechanical work and have been extensively studied in the classical and quantum regimes. In the quantum domain, however, nonclassical forms of energy exist, which are distinct from traditional heat and which can also be harnessed to generate work in cyclic engine protocols.

We introduce the concept of the Pauli engine: a novel quantum many-body engine powered by the energy difference between fermionic and bosonic ultracold particle ensembles, arising from the Pauli exclusion principle. The distinct quantum statistics lead to a redistribution of population across energy levels, enabling engine cycles that replace traditional heat strokes in the quantum Otto cycle. This concept has recently been realized experimentally in the BEC-BCS crossover regime [1].

Building on this idea, we also present several concepts for hybrid quantum-classical engines, where a change in quantum statistics is implemented either during the adiabatic work strokes or the isochoric heat strokes. While the Pauli engine alone demonstrated high efficiency, we show that combining quantum and classical effects can further enhance both efficiency and work output. All cycles are discussed in the context of ultracold atomic gases, which are well suited for their experimental realisation.

[1] J. Koch, K. Menon, E. Cuestas, S. Barbosa, E. Lutz, T. Fogarty, Th. Busch, A. Widera, Nature 621, 723 (2023).

Rydberg atoms efficiently links photons between the radio-frequency (RF) and optical domains. They provide a medium in which the presence of an RF field imprints on the transmission of a probe laser beam by altering the coherent coupling between atomic quantum states. The immutable quantized energy structure of the atoms underpins quantum-metrological RF field measurements and has driven intensive efforts to realize inherently self-calibrated sensing devices. Here we investigate spectroscopic signatures owing to angular momentum quantisation of the atomic states utilized in an electromagnetically-induced transparency (EIT) sensing scheme. Specific combinations of atomic terms are shown to give rise to distinctive fingerprints in the detected optical fields upon rotating the RF field polarization.

The characteristic angular variation for a term combination is universal and independent of the atomic species involved. By examining two ostensibly similar angular momentum ladders, we unveil a striking complementarity in their spectroscopic responses. Our study adds an important new building block for quantum metrological electric field characterisation via Rydberg atomic polarimetry.

Detailed information about atomic excitation and decay processes are urgently required in many research fields in physics, science and elsewhere, from astro physics to (atomic) spectroscopy and metrology, to the development of light sources, and up to material science, to recall just a few of them [1]. More often than not, this information need to be obtained from atomic computations that are able to provide reliable data on demand. With the Jena Atomic Calculator (JAC), I have developed a relativistic structure code for the computation of atomic amplitudes, properties processed which is suitable for (most) open-shell atoms and ions across the periodic table [2,3].

In this talk, I explain a "pedestrian" route to modeling many, if not most, of these processes [4]. Apart from the frequently employed excitation and (auto-) ionization of atoms and ions, this includes rare processes, such as the collective Auger decay [5] or hyperfine-induced transitions. This route has been found helpful and equally accessible to working spectroscopists, theoreticians as well as code developers.

[1] S. Fritzsche, P. Palmeri and S. Schippers, Symmetry 13, 520 (2021)
[2] S. Fritzsche, Comp. Phys. Commun. 240, 1 (2019).
[3] https://github.com/OpenJAC/JAC.jl
[4] S. Fritzsche et al., Eur. J. Phys. D 78, 75 (2024).
[5] Y. Hikosaka and S. Fritzsche, PRL, in print (2025).

The electronic structure of liquid water and aqueous solutions is directly accessible by liquid-jet photoelectron spectroscopy (LJ-PES). Energies of photoelectron spectral features have so far typically been referenced to the vacuum-energy level, i.e., in relation to the gas phase, which per definition disregards explicit surface properties such as the work function of aqueous solutions. We discuss how the solution work function changes, as a function of solute type and concentration, can be inferred from a LJ-PES measurement, based on an explicit consideration of a solution’s Fermi energy. Competing surface-charging effects contributing to energy shifts of solute and solvent spectral features are important to be quantified and will be explored. This inherently connects to the ability to extract accurate electron binding energies from aqueous solution via LJ-PES experiments.
This talk further discusses the application of LJ-PES to provide structural information of biomolecules in a complex aqueous environment, such as adenosine triphosphate interacting with metal cations. This is achieved by the simultaneous analysis of valence, core-level, and non-local autoionization electron signals. We conclude with a consideration on applying electron Velocity Map Imaging (VMI) to liquid jets, with a focus on electron scattering in solution. VMI promises to vastly increase photoelectron collection efficiency and angular range, and in particular to enable the detection of photoelectron angular distributions in a single measurement. This includes the challenging detection of photoelectron circular dichroism (PECD) from aqueous-phase chiral (bio)molecules.

When two electrons are excited far away from the nucleus, their motion results from the subtle balance between electronic repulsion and the Coulomb attraction of the residual ion. The strong electron correlations give rise to complex and fascinating two-electron dynamics that range from chaotic motion to quasi-stable orbits associated with sharp resonances. We report the experimental and theoretical observation of resonances belonging to a new and general type of doubly-excited states with a distinctive two-electron motion. Such states form a regular Rydberg-like series converging to the double-ionization threshold.

Experimentally, ground-state Sr atoms are excited by sequential multiphoton resonant excitation to doubly-excited states of high angular momentum ($L\sim 15$) lying below the Sr$^+$($N=9-15$) ionization thresholds $-$ less than 0.5 eV below the double-ionization threshold. The excitation spectra reveal the presence of a regular series of lines that persists even at high energy, where most other spectral features merge into an unstructured continuum. To identify the origin of these lines, we combine large-scale theoretical calculations from first principles with simple modelling. The analysis reveals that the states of the series emerge from a surprising binding mechanism: the electronic repulsion creates, through a Stark-like avoided crossing, an effective, adiabatic potential-energy well in which the outermost electron is localized. Such wells appear at each principal quantum number $N$ and give rise to the series, whose properties scale with powers of $N$ that are different from usual Rydberg-state scaling laws. We also discuss the existence of such series in systems other than Sr.

Recent astronomical observations [1,2] have shown that chloronium (H$_2$Cl$^+$) is an important intermediate in Cl-chemistry of the interstellar medium (ISM) due to its specific thermodynamic properties. For a proper interpretation of the astronomical observations, radiative transfer modelling is required that takes into account the non-local thermodynamic equilibrium (non-LTE) effects of the environment through reliable collisional rate coefficients, which have been lacking in the literature so far for chloronium.

We have recently calculated the first accurate 5-dimensional interaction potential between H$_2$Cl$^+$ and molecular hydrogen (H$_2$), which allowed their rotational excitation to be studied from accurate quantum scattering theories [3]. State-to-state rotational (de-)excitation cross sections have been calculated using the numerically exact close-coupling (CC) scattering method for collision energies from the first inelastic threshold up to 1500 cm$^{-1}$, involving the lowest 21 rotational states for both the ortho and para nuclear spin species. This allowed the thermal rate coefficients to be derived up to kinetic temperatures of 200 K. Finally, the importance of the collisional rate coefficients is demonstrated by non-LTE radiative transfer modelling of some H$_2$Cl$^+$ rotational transitions that have been observed in different interstellar sources.

[1] D. A. Neufeld et al. Astrophys. J. 2015, 807, 54.
[2] S. H. J. Wallström et al. Astron. Astrophys. 2019, 629, A128.
[3] S. Demes et al. J. Phys. Chem. A 2025, 129, 253.

Intense laser-matter interaction leads to high harmonic generation (HHG), where the low frequency photons of a driving laser field are converted into photons of higher frequencies. This process has enabled breakthroughs in AMO physics and attosecond science [1]. Until recently, it was described by classical or semi-classical approaches [2], ignoring the quantum nature of light.
Recent investigations conducted using fully quantized approaches have shown how the HHG process can be used for the generation of optical Schrödinger "cat" and entangled light states from the infrared to the extreme-ultraviolet spectral range [3-10].
Here, after a brief introduction, I will focus my talk on the most recent studies concerning the generation of optical "cat" states and their role in nonlinear optics [11]. I will also emphasize the potential of using fully quantum approaches in laser-driven semiconductor crystals to develop non-classical and entangled light states in the VUV spectral region with attosecond pulse duration [12]. Finally, I will briefly discuss some additional representative key approaches developed in the recent past by other groups in this research field [13-19].

References
[1] P. Agostini, F. Krausz and A. L’Huillier Nobel prize in physics 2023
[2] K. Amini, et al., Rep. Prog. Phys. 82, 116001 (2019) (and references herein).
[3] M. Lewenstein, et al., Nat. Phys. 17, 1104 (2021).
[4] J. Rivera-Dean, et al., Phys. Rev. A 105, 033714 (2022).
[5] P. Stammer, et al., PRL 128, 123603 (2022).
[6] J. Rivera-Dean arXiv:2409.02016
[7] P. Stammer arXiv:2410.15503
[8] P. Stammer, et al., PRX Quantum 4, 010201 (2023).
[9] U. Bhattacharya, et al., Rep. Prog. Phys. 86, 094401 (2023).
[10] J. Rivera-Dean et al., PRB 109, 035203 (2024).
[11] Th. Lamprou, et al., PRL 134, 013601 (2025).
[12] A. Nayak et al., Nat. Commun. 16,1428 (2025)
[13] M. E. Tzur, et al., Nat. Photon. 17, 501 (2023).
[14] A. Gorlach, et al., Nat. Phys. 19, 1689 (2023)
[15] J. Heimerl, et al., Nat. Phys. 20, 945 (2024).
[16] D. Theidel, et al., PRX Quantum 5, 040319 (2024).
[17] S. Lemieux, et al., arXiv:2404.05474 (2024).
[18] A. Rasputnyi, et al., Nat. Phys. 20, 1960 (2024).
[19] S. Yi, et al., Phys. Rev. X 15, 011023 (2025).

Ultracold atoms represent a flexible platform for simulating topological and many-body phenomena of condensed matter and high-energy physics. The use of atomic dark states (long-lived superpositions of atomic internal ground states immune to atom-light coupling) offers new possibilities for such simulations. Making the dark states position-dependent, one can generate a synthetic magnetic field for ultracold atoms adiabatically following the dark states [1]. Recently, two-dimensional (2D) dark-state lattices were considered [ 2,3].
Here we present a general description of 2D topological dark state lattices elucidating an interplay with the sub-wavelength lattices [4]. In particular, we demonstrate that one can create a 2D Kronig-Penney lattice representing a periodic set of 2D subwavelength potential peaks affected by a non-staggered magnetic flux. Away from these patches of the strong magnetic field, there is a smooth magnetic flux of the opposite sign, compensating for the former peaks. While the total magnetic flux is zero, the system supports topological phases due to the flux variation over a unit cell, akin to Haldane-type lattice models with zero net flux over an elementary cell, but non-trivial topology due to non-zero fluxes over the plaquettes constituting the elementary cell. This work paves the way for experimental exploration of topological phases in dark-state optical lattices, offering new possibilities for simulating quantum Hall systems, fractional Chern insulators and related strongly correlated phases.
[1] N. Goldman, G. Juzeliūnas, P. Öhberg, and I. B. Spielman, Rep. Prog. Phys., 77,126401 (2014).
[2] E. Gvozdiovas, I. B. Spielman, and G. Juzeliūnas, Phys. Rev.. A, 107, 033328 (2023).
[3] S. Nascimbene and J. Dalibard, arXiv:2412.15038 (2024).
[4] D. Burba and G. Juzeliūnas, to be published.

Chiral molecules play a vital role in pharmaceuticals, materials science, and agrochemicals, as their mirror-image forms (enantiomers) can exhibit different biological and chemical properties. Distinguishing between these enantiomers is challenging, driving research into advanced light-matter interaction techniques, including nonlinear methods based on electric dipole interactions, which can enhance chiroptical sensitivity by orders of magnitude and introduce concepts like local chirality [1]. For instance, locally chiral Lissajous figures of synthetic chiral light can generate strong enantio-sensitive signals in high-harmonic generation [2].

Inspired by this, we examine the interplay between two locally chiral structures: the induced polarization vector of chiral electron currents, and the rotational trajectory of a molecule, both driven by locally chiral fields. Using a pump-probe setup, we analyse how the chiral rotational dynamics affect photoionization and photoexcitation, finding signatures of the interferences between chiral motions evolving on different time scales. This analysis has been conducted for both an achiral molecule and a chiral molecule using chiral hydrogenic electronic states [3] as a model of chiral electronic density. We demonstrate that even achiral spherical top molecules can exhibit chiral rotational trajectories in space (analogous to Lissajous figures of synthetic chiral light) under controlled excitation. Furthermore, by exploring the ultrafast enantio-sensitive electronic response in rotating chiral molecules, we identify new efficient enantio-sensitive observables, such as transient absorption and photoelectron circular dichroism of chiral rotors.

FIG. 1: Excitation scheme induced by the probe pulse, acting on molecular state as product of chiral rotational wavepacket (left ket in the product) and chiral electronic state (right ket in the product).

Acknowledgement
We gratefully acknowledge ERC-2021-AdG project ULISSES, grant agreement No.101054696

References
[1] D. Ayuso, A. F. Ordonez, and O. Smirnova, Phys. Chem. Chem. Phys. 24, 26962 (2022).
[2] D. Ayuso, et al, Nat. Photonics 13, 866 (2019).
[3] A. F. Ordonez, and O. Smirnova, Phys. Rev. A 99, 043416 (2019).

Alkali dimers, Ak$_2$, residing on the surface of helium nanodroplets, are set into rotation and vibration, through the dynamic Stark effect, by a moderately intense 50-fs pump pulse. Coulomb explosion of dimers in the singlet X $^1\Sigma_g^+$ and triplet a $^3\Sigma_u^+$ state [1, 2], induced by an intense, delayed femtosecond probe pulse, is used to record the time-dependent nuclear motion.

Concerning rotation, the measured alignment traces for Na$_2$, K$_2$, and Rb$_2$ show distinct periodic features that differs qualitatively from the well-known alignment dynamics of linear molecules in either the gas phase or dissolved in liquid helium [3]. Instead, the observed alignment dynamics of Na$_2$ and K$_2$ in the a $^3\Sigma_u^+$ state and of K$_2$ and Rb$_2$ in the X $^1\Sigma_g^+$ state agree with that obtained from a 2D rigid rotor model, strongly indicating that the rotation of each dimer occurs in a plane - defined by the He droplet surface [4, 5].

Concerning vibration, the Coulomb explosion probe method enables us to measure the distribution of internuclear distances as a function of time. For K$_2$ in the a $^3\Sigma_u^+$ state, we observe a distinct oscillatory pattern caused by a two-state vibrational wave packet in the initial electronic state of the dimer. The wave packet is imaged for more than 250 vibrational periods with a precision better than 0.1 Å on its central position. Unlike the rotational motion, the vibration of the dimer is essentially unaffected by the presence of the He droplet [6].

References:
[1] H. H. Kristensen, et al. Phys. Rev. Lett. 128 (2022), 093201
[2] H. H. Kristensen, et al. Phys. Rev. A 107 (2023), 023104
[3] A. S. Chatterley, et al. Phys. Rev. Lett. 125 (2020), 013001
[4] L. Kranabetter, et al. Phys. Rev. Lett. 131 (2023), 053201
[5] H. H. Kristensen, et al., In preparation. (Available at arXiv:2502.14521 [physics.atm-clus])
[6] N. K. Jyde, et al. J. Chem. Phys. 161 (2024), 224301