Chiral coupling occurs when a semiconductor quantum dot (QD) is positioned close to a C-Point (chiral point) of a nano-photonic waveguide. In these conditions, there is a strong dependence of the propagation direction on the circular polarization of the optical mode, with spin-up and spin-down exciton spins coupling to opposite propagation directions. This provides a mechanism by which a QD can impart a $\pi$ phase shift depending on the spin of a charge carrier within the dot, forming the building block for chip-based spin quantum networks [1].

Early work at Sheffield focused on InAs/GaAs QDs in GaAs nanobeam waveguides, where spin-dependent directional photoluminescence and initialization were observed [2, 3]. The spin-dependent phase shift deduced from analysis of the directional transmission data was around $0.1 \pi$ [4]. The magnitude of the phase shift is related to the coupling to the waveguide (i.e. the $\beta$-factor), and can be increased by using more sophisticated waveguide designs that combine high chirality with strong Purcell enhancement. Examples include glide-plane [5,6] and topological [7] photonic crystals designs. At Sheffield we have observed strong chirality in valley-Hall topological photonic crystals [8], and both strong chirality and Purcell enhancement in glide-plane designs [9]. Our detailed statistical analysis and simulations suggest that the glide-plane designs offer the best route towards obtaining both high chirality and strong Purcell enhancement, as required to achieve the full $\pi$ phase shift [10].

This work was supported by EPSRC Programme Grants EP/V026496/1 and the EPSRC Quantum Communications Hub Grant EP/T001011/1.

References
1. Lodahl, P. et al., Nature 541, 473 (2017).
2. Coles, R.J. et al., Nature Communications, 7, 11183 (2016).
3. Coles, R.J. et al., Phys. Rev. B, 95, 121401(R) (2017).
4. Hurst, D.L. et al., Nano Letters, 18, 5475 (2018).
5. Sollner, I. et al., Nature Nanotechnology 10, 775 (2015).
6. Mahmoodian, S. et al., Opt. Mater. Express 7, 43 (2017).
7. Barik, S. et al., Science 359, 666 (2018).
8. Jalali Mehrabad, M., et al, Optica, 7, 1690 (2020).
9. Siampour, H. et al., NPJ Quantum Information, 9, 15 (2023).
10. Martin, N. et al., Phys. Rev. Research 6, L022065 (2024).

Semiconductor quantum dots are one of the best on-demand sources of single and entangled photons to date, simultaneously merging the highest brightness and indistinguishability of the emitted photons. They are, therefore, among the strongest candidates for practical single-qubit quantum photonic devices. However, to exploit the full advantage of quantum physics, multi-qubit photonic devices are vital. This talk will present our approach to realizing practical multi-qubit photonic devices for quantum photonic networks based on novel location qubits in crystal-phase quantum dots in nanowires.

Solid-state sources of quantum light based on epitaxial quantum dots are rapidly approaching ideal performance: on-demand supply of single photons or entangled photon pairs in a pure quantum state delivered with unity efficiency into a specified quantum channel at GHz rates. Such dots are typically grown using the conventional strain-mediated Stranski–Krastanow growth or, more recently, using strain-free droplet epitaxy. Here we discuss a third growth technique based on vapour-liquid-solid epitaxy where the quantum dots are thin segments of a lower bandgap semiconductor incorporated within a larger bandgap nanowire. The bottom-up nanowire approach allows for precise control of the dot geometry, the number of dots within a nanowire, and their separation and is readily implemented with patterned-substrate epitaxy for position control. This contribution will outline the growth technique and illustrate the high quality of the sources in terms single photon collection efficiencies and multiphoton emission probabilities as well as the coherence properties of the generated photons. Approaches for coupling nanowire quantum dots with Si-based photonic integrated circuitry for on-chip operation or Si-based microcavities for high-rate operation will also be discussed.

A single ion coupled to an optical cavity is an efficient source of high-fidelity spin-photon entanglement. We will examine how to engineer this spin-photon entanglement in order to improve the efficiency and fidelity of entanglement between ions at separate nodes of a quantum network. I will present experimental results from a recent quantum-network demonstration on the University of Innsbruck campus and explain how simulations yield insight into these results. Finally, prospects for entangling a trapped ion with a solid-state emitter will be considered.

Semiconductor quantum dots (QDs) are among the most promising quantum light sources, with the potential to revolutionize quantum communication research. For instance, utilizing on-demand single photons and entangled photons in quantum key distribution (QKD) protocols can significantly enhance security and increase the maximum tolerable loss. However, several critical challenges must be addressed to bridge the gap between laboratory experiments and long-distance field tests using QDs. In this talk, I will first review our work over the past years on QD-based single-photon and entangled-photon sources. Following that, I will present our recent field tests of single photon transmissions over a 79 km link between Hannover and Braunschweig, with 25.49 dB loss — equivalent to 130 km in direct-connected optical fiber.

Non-classical light sources find applications in various fields of quantum technologies, e.g., communication, cryptography, computation and imaging. Quantum dot-based (QD) sources with performance enhanced by photonic structures have proven proof-of-principle advantage in terms of non-classical light state quality. This still requires optimization in the data transmission telecommunication spectral range, in particular in the O- and C-band. Additional advantage of QDs is their compatibility with semiconductor growth and processing technologies enabling on-chip integration of different functionalities and potential scalability.

In this work exemplary InP-based QD-photonic structures differing in growth technique and design will be discussed. They all aim at high efficiency and quality non-classical light emission at telecom wavelengths, but with different approaches.

The first group are InAs(P) QDs in InP photonic nanowires grown by vapour-liquid-solid mode of chemical beam epitaxy [1]. Optimization of the growth parameters resulted in defect-free structures in zinc blende structure with spectral range of emission determined by QD material composition and height. The extraction efficiency is targeted by geometry of the nanowire providing additionally broadband, but moderate enhancement of the emission rate.

Similar approach of broadband operation is realized in the case of molecular beam epitaxy-grown InAs QDs on distributed Bragg reflector in a photonic mesa structure. With the optimized mesa design extraction efficiency of 13% has been achieved [2]. These emitters feature low probability of multiphoton events under non-resonant pulsed excitation [3] and are potentially interesting in view of generation of pairs of entangled photons due to low exciton fine structure splitting [4].

The most advanced sources are based on InAs/InP QDs grown by metalorganic chemical vapour deposition [5] in hybrid circular Bragg grating cavities. These are not only deterministically fabricated using microphotoluminescence mapping technique, but also feature Purcell enhancement of emission by a factor of 4 and emit indistinguishable photons [6]. Cavity-based approaches are typically narrow-band, but in the case of circular Bragg grating cavity design high Purcell factor and broadband operation are combined owing to small mode volume.

[1] G. Bucci et al., ACS Applied Materials & Interfaces 16, 26491 (2024).
[2] A. Musiał et al., Applied Physics Letters 118, 221101 (2021).
[3] A. Musiał et al., Advanced Quantum Technologies 3, 1900082 (2020).
[4] A. Kors et al., Applied Physics Letters 112, 172102 (2018).
[5] D. A. Vajner et al., ACS Photonics 11, 339 (2024).
[6] P. Holewa et al., Nature Communications 15, 3358 (2024).

Towards quantum information processing in silicon-on-insulator (SOI), single photons play an important role as carriers of information. Thereby, a key element is an integrated, efficient single-photon source (SPS) with high purity and fidelity operating on demand. Although sources of entangled photon pairs and heralded single photons have been successfully developed and integrated on SOI photonic chips, such sources are based on non-linear effects, and do not ensure “on demand” emission.
The observation of photon antibunching for the spontaneous emission of isolated color centers in SOI [1] has opened novel opportunities in this context. Among a dozen of other point defects, the W center looks as an attractive candidate because of its zero-phonon line at 1218 nm, which permits low-loss propagation through SOI waveguides, its large Debye-Waller factor (ca. 40%), its radiative quantum efficiency around 0.65 (in SOI membranes) and relative easy selective fabrication process [2]. By integrating an ensemble of W centers inside a SOI “Bullseye” cavity, clear signatures of Purcell enhancement of their emission has been recently reported [3].
Building upon these results, we have recently developed a method to integrate single W centers at the core of SOI bullseye cavities in a deterministic way. Our approach, inspired by ref 4, is based on the implantation of Si ions through a PMMA mask comprising nanoholes defined by electron-beam lithography. The nanoholes have a diameter ranging from 30 nm to 2 µm in small steps, allowing to finely tune the average number of implanted ions per hole. After mask removal and annealing at 250°C, we witness the formation of single W centers with microphotoluminescence spectroscopy at well-defined locations. Then, we fabricate Bullseye cavities, spectrally resonant with the zero-phonon line of the W center, on the same axis as the implantation spots. The observation of photon antibunching with $g^{(2)}(0)<0.05$ shows that the cavity fabrication step does not generate additional color centers. Furthermore, Purcell enhancement is obtained for several cavity-coupled emitters, showing that the W centers are located in the vicinity of the antinode of the resonant mode.
While this contribution focuses on fabrication-related issues, another presentation at EQEP will focus on the advanced characterization of the properties of this on-demand SPS.

[1] Redjem, W. et al. Nat Electron 3, 738–743 (2020); Durand, A. et al, PRL 126, 083602 (2021)
[2] Y. Baron et al., ACS Photonics 9 (7), 2337-2345 (2022).
[3] B. Lefaucher et al., ACS Photonics 11 (1), 24-32 (2024)
[4] M. Hollenbach et al., Nat Commun. 13, 7683 (2022).

From atomic physics, it is well known that an ensemble of indistinguishable quantum emitters can show intriguing cooperative emission effects like superradiance, which is due to coherent coupling of the emitters to their common electromagnetic environment. Entanglement between the emitters leads to the formation of a giant dipole and hence to a superextensive enhancement of light absorption and emission.

Harnessing such cooperative effects [1] for quantum technology with solid-state nanodevices has been challenging due to fabricational challenges to produce nearly indistinguishable emitters in the right spatial arrangements, and because the solid-state environment in the form of phonons affects inter-emitter entanglement [2,3].

Here, I summarize the state of art in cooperative emission for solid-state devices based on quantum dots. In particular, I present examples for experimental realizations [4], review recent theoretical [2,3] and methodological [5,6] insights on the impact of the phonon environment, and discuss ambiguities in the interpretation of g^(2) photon coincidence signals [1].

[1] Z. X. Koong, M. Cygorek, et al., Sci. Adv. 8, abm8171 (2022)
[2] J. Wiercinski, E. M. Gauger, and M. Cygorek, Phys. Rev. Research 5, 013176 (2023)
[3] J. Wiercinski, M. Cygorek, E. M. Gauger, Phys. Rev. Research 6, 033231 (2024)
[4] D. Hallett, et al., to be published
[5] M. Cygorek, et al., Nat. Phys. 18, 662 (2022)
[6] M. Cygorek, E. M. Gauger, Chem. Phys. 161, 074111 (2024)

Advances in silicon photonics have so far largely focused on the development of classical optoelectronic devices. While there has been great success in this classical field of photonics, the development of silicon-compatible quantum photonics components, which can be combined with cost-efficient silicon electronics, has faced significant challenges, particularly regarding the growth of high-quality single quantum dots (QDs) on silicon substrates.

Here, we demonstrate the direct growth of InGaAs QDs emitting at 940 nm with excellent quantum optical properties on a silicon platform \cite{1}. Through the heteroepitaxy of GaAs structures on a silicon substrate using an intermediate GaP buffer layer, we observe high multi-photon suppression ($g^{(2)}(0) = (3.7 \pm 0.2) \times 10^{-2}$) and good photon indistinguishability ($V_{\text{HOM}} = (66 \pm 19)\%$) under non-resonant excitation. Moreover, we achieve a photon extraction efficiency (PEE) of up to $(18 \pm 2)\%$ in a simple planar device structure with a backside DBR, indicating a high internal quantum efficiency of the QDs.

Additionally, we shift the emission wavelength of the Si-compatible InGaAs QDs into the telecom O-band using a strain-reducing layer (SRL) approach. The emitters are subsequently integrated into bullseye cavities through in-situ electron beam lithography (EBL). The resulting structure demonstrates a PEE of $(40 \pm 1)\%$ and a single-photon purity of $(99.9 \pm 0.1)\%$ at 4 K, decreasing to $(85 \pm 1)\%$ at 40 K. These results indicate the emitter's potential for operation with Stirling cryocoolers, maintaining high performance at elevated cryogenic temperatures.

High quality single photon sources (SPSs) are extremely desirable in quantum information processing. In order to facilitate integration with existing silica fiber based photonic infrastructure, the SPS should emit photons in telecom spectral range where dispersion and propagation losses are the lowest. Such SPS can be realized in InAsP/InP quantum dot (QD) nanowires (NWs). Tailoring the NW geometry to couple QD spontaneous emission (SE) to a NW single optical mode results in lower in-plane losses and better upward collection of the SE into limited numerical aperture (NA) available experimentally [1]. In this material system, defect-free zinc blende structures can be grown using chemical beam epitaxy with great reproducibility, precision and control over both QD and NW parameters [2]. The aim of this work is to provide optimal NW design taking into account technological limitations and tolerances.
In our study, we model the emission of an electric dipole (simulating the QD) embedded in a single InP NW vertically grown on InP (100) substrate. The simulations were performed in commercial Ansys Lumerical software, using finite-difference time-domain method (FDTD). In the first step, the emission of the dipole, placed in an infinite square-based InP NW, was modeled as a function of NW shell thickness to find the estimated value for optimal optical confinement providing single mode propagation along the NW. In the next step, a realistic geometry of a square-based tapered NW was implemented. The simulations covered the optimization of the NW length and taper angle, QD position along the NW axis as well as fine-tuning of the NW shell thickness. The calculations focused on understanding the photon extraction efficiency upward within the most commonly used NA of 0.4 and 0.65 and the corresponding far-field pattern evaluated for the dipole emission using the reasonable parameter space achievable in this fabrication approach for NW growth. Next, by using a metallic mirror beneath the NW we provide optimized structure design with extraction efficiency as high as 39% for 1500 nm (NA of 0.65). The influence of a gold mirror under the NW was modelled using the PMC boundary condition. This contribution gives valuable feedback for guiding the growth of optimized structures and provides theoretical emission extraction efficiency, which could be compared afterwards to the experimental results.
We acknowledge financial support from the National Science Centre (Poland) within Project No. 2020/39/D/ST5/02952

[1] J. Claudon, N. Gregersen, P. Lalanne, J.M. Gérard, ChemPhysChem 14, 2393 (2013)
[2] G. Bucci, V. Zannier, F. Rossi, A. Musiał, J. Boniecki, G. Sęk, L. Sorba, ACS Appl. Mater. Interfaces 16, 26491 (2024)

Quantum key distribution (QKD) enables the transmission of information that is secure against general attacks by eavesdroppers. The use of on-demand quantum light sources in QKD protocols is expected to help improve security and maximum tolerable loss. Semiconductor quantum dots (QDs) are a promising building block for quantum communication applications because of the deterministic emission of single photons with high brightness and low multiphoton contribution.

In a recent work we have reported on the first intercity QKD experiment using a bright deterministic single photon source. A BB84 protocol based on static polarisation encoding has been realised using the high-rate single photons in the telecommunication C-band emitted from a semiconductor QD embedded in a circular Bragg grating structure [1]. Here we present the high-speed modulation of the polarisation states of telecom C-band single photons. A sequence of 32-bit digital pseudo-random numbers is repeatedly encoded into the polarisation via the interferometer for proof-of-concept purposes. An ultra-low quantum bit error rate compared to existing actively modulated single-photon based QKD is identified using such a configuration.

[1] J. Yang et al., Light Sci. Appl. 13, 150 (2024)

Quantum information distribution over quantum networks is essential for the advancement of secure communication and distributed quantum computing. To achieve this, reliable and efficient sources of flying qubits are crucial. GaAs quantum dots are promising candidates due to their outstanding features [1, 2, 3]. However, these quantum dots often exhibit dissimilar emission properties, so that interfacing two distinct emitters is generally a significant challenge. We address this issue through state-of-the-art methods for the engineering of the emitters electronic structure and a high time-resolution detection system, that allow us to reach high indistinguishability and high entanglement at the same time. Thanks to these techniques, we successfully demonstrated the first all-photonic quantum teleportation between two separate quantum dots over a fiber and free-space quantum network laid over the Sapienza University campus. This achievement illustrates the potential of engineered quantum dots for the realization of quantum relays and quantum repeaters and paves the way for the implementation of practical quantum networks.

[1] Huber, D. et al. Highly indistinguishable and strongly entangled photons from symmetric GaAs quantum dots. Nature communications 8, 15506 (2017)
[2] Schöll, E. et al. Resonance fluorescence of GaAs quantum dots with near-unity photon indistinguishability. Nano Letters 19, 2404–2410 (2019)
[3] Schweickert, L. et al. On-demand generation of background-free single photons from a solid-state source. Applied Physics Letters 112 (2018)

Epitaxially grown semiconductor quantum dots (QDs) have demonstrated to be excellent sources of single photons and entangled photon pairs for quantum information and computational applications. With demonstrated on-demand single photon purities [1] and indistinguishabilities [2] exceeding 99%, QDs are a natural inclusion in quantum integrated photonics, allowing for future device scalability. QDs integrated into epitaxially grown nanowires provide a platform that combines consistent emission properties [3] and efficient photon collection [4]. A limitation of this approach is the difficulty of incorporating the quantum dot into a resonant cavity. Operating in the Purcell regime has shown to improve single photon indistinguishability—effectively operating faster than typical dephasing processes—by reducing exciton lifetimes [5].

Here we show our progress in achieving Purcell enhancement by combining silicon CBR structures with InP nanowires. We discuss design and process considerations, numerical results, and measurements of fabricated devices using photoluminescence spectroscopy. Due to the need to grow the nanowires in a controlled environment, the current implementation of this technology involves a pick-and-place technique to create the hybrid device. A major challenge in this process is matching the cavity resonance to the emission energies of a given QD. We compensate for resonance locations by studying the response of devices to incident broadband and tunable light sources. Characterized QDs can be measured and paired with suitable CBR cavities.

References
[1] P. Laferriére et al., Nano Lett 23, 3, 962-968 (2023)
[2] Y. Wei et al., Nano Lett 14, 6515-6519 (2014)
[3] P. Laferriére et al., Scientific Reports 12, 6376 (2022)
[4] E. Yeung et al., Phys Rev B 108, 195417 (2023)
[5] J. Liu et al., Nature Nanotechnology 14, 586-593 (2019)

As quantum technology gains more importance, single photon sources such as semiconductor quantum dots become more relevant. One of the advantages of using single photons as qubit is the ability of photons to bridge long distances, hence the reason they are also referred to as flying qubits. Bridging long distances can be achieved by utilizing silica fibers, a mature technology which constitutes the backbone of our telecommunication infrastructure. However, the single photons have to be coupled into an optical fiber first, which in turn may also lead to unwanted losses.

A new method to avoid these losses is to place optical fiber structures directly above the quantum dots for efficient coupling. In this work, we use a single mode fiber with a 3D printed lens at the fiber tip to investigate the coupling efficiency and sensitivity of the coupling in the spatial lateral and vertical direction. The fiber coupling is performed onto quantum dots embedded in circular Bragg gratings operating in the telecom C-Band. Moreover, these results are then compared to a bare single mode fiber without the 3D printed lens and a commercially available microscope objective. In terms of the overall fiber coupling efficiency, the lensed and bare fiber outperform the microscope objective by up to a factor of 2.9 corresponding to a measured count rate at the detectors of 0.44MHz and 1.11MHz, respectively. For the spatial sensitivity, the lensed (bare) fiber exhibit their maximum FWHMs of 1.25µm (4.95µm) and 3.56µm (37µm) in the lateral and vertical direction, respectively.
These results will play a key role in the future development of fiber-coupled sources of quantum light.

Applications in quantum computing and communication require entangled photon pairs featuring simultaneously a high degree of entanglement as well as a high single photon indistinguishability. Epitaxially grown quantum dots (QDs), especially gated GaAs QDs have shown to both be able to emit highly indistinguishable single photons as well as highly polarization entangled photon pairs from the biexciton (XX) - exciton (X) decay cascade. However, the state purity of the individual photons from a cascade is limited since the states are not entirely separable due to an entanglement in time between them. The reduction of the state purity $\mathbb{P}$ and thereby the indistinguishability of the photons is given by the lifetime ratio of the cascade $r = \tau_{XX}/\tau_X$ due to an unwanted temporal coupling with $ \mathbb{P} =1/(r+1)$. In GaAs QDs, this puts an upper bound on the reachable indistinguishability with typical lifetime ratios of $r=0.5$ at about $0.66$ which is too low for many applications. The ability to independently tune the lifetimes of the two decays therefore is required to increase this theoretical limit. Here we present a novel approach, achieving this by manipulating the vertical electric field.

Instead of working in forward bias, here we apply negative a gate voltage, which will increase the total field and thereby increase the band bending. The resulting decrease in overlap between the electron and hole wave function facilitates a change in lifetimes. While the exciton lifetime increases towards higher fields, the lifetime of the biexciton seems largely unaffected. This leads to an improvement of the raw measured indistinguishability from $0.60(1)$ to $0.74(1)$.

Site-controlled Pyramidal QDs are a class of solid state quantum emitters that are photo-lithographically defined on (111)B GaAs, grown through MOVPE inside selectively etched tetrahedral recesses. This system has the unique ability to finely control the nominal thickness of the dots, the barriers and their composition. However, pre-processed QDs rarely satisfy the requirements that are vital for quantum computation applications. That is why post-processing techniques are required. Here we show the strategies our group is currently adopting to increase extraction efficiency, reduce transitions lifetimes and tune FSS of excitons.

Quantum cryptography and optical quantum computing require well-performing single and entangled photon sources, placing stringent demands on semiconductor quantum dots (QDs) embedded in microcavities such as circular Bragg grating (CBG) resonators [1,2]. Their low optical mode volume facilitates enhanced coupling between quantum dot and resonator. At the same time, it necessitates accurate placement of the QD with respect to the cavity center of Δs < 20 nm. A larger displacement will result not only in a reduced Purcell factor but also produce a pronounced polarization bias on the emitted single photons [3].
We present recent advancements in the deterministic placement of CBG around pre-characterized self-assembled semiconductor QDs. Using hyperspectral imaging, we achieve high spectral and spatial accuracy. The introduction of optical markers with periodic features significantly enhances the reliability of position determination. To further improve accuracy, we apply post-processing image correction algorithms that reduce distortions.
Our results demonstrate that the accuracy achieved meets the demanding requirements of QD-cavity integration. We assess accuracy and device yield through imaging test fields and micro-photoluminescence measurements on fabricated devices. We evaluate the resonator-induced Purcell enhancement by measuring the excitonic lifetimes before and after integration of the QD into CBG resonators. The observed polarization of emitted light is correlated with the spatial displacement of QDs relative to the CBG center, providing key insights into optimizing QD placement for quantum applications.

[1] M. Davanço et al., A circular dielectric grating for vertical extraction of single quantum dot emission. Appl. Phys. Lett. 99, 041102 (2011).
[2] H. Wang et al., On-demand semiconductor source of entangled photons which simultaneously has high fidelity, efficiency, and indistinguishability. Phys. Rev. Lett. 122, 113602 (2019).
[3] G. Peniakov et al., Polarized and Unpolarized Emission from a Single Emitter in a Bullseye Resonator. Laser Photonics Rev. 18, 2300835 (2024).

We report on an experiment in which orbital-angular-momentum (OAM)-entangled photon pairs generated by the spontaneous parametric down-conversion process can be engineered to have particular symmetry properties. Our method is based on the use of a Dove-prism pair in conjunction with Hong-Ou-Mandel (HOM) interferometry resolved in transverse position and OAM. The latter allows us to engineer the postselected two-photon state to exhibit a specific type of symmetry. By selecting particular topological charge values for the pump and for the postselected two-photon state, we can transition from a symmetric two-photon state and a HOM dip to an antisymmetric state and a HOM peak. Spatial resolution allows us to obtain the HOM interferogram both at the single-pixel level and globally by summing over all sensor pixels. Furthermore, through spatially selective OAM projection of the detected photon pairs, we can define multiple transverse regions with different symmetry properties, as verified by our spatially resolved HOM apparatus. Although we used two transverse regions for this proof-of-concept demonstration, this method could in principle be scaled to a larger number of regions, leading to a new technique to be added to the existing toolbox for quantum technologies in the photonic domain.

Integrated quantum photonic circuits (IQPCs) are very promising candidates for scalable and flexible on-chip quantum computation and quantum communication hardware. One critical requirement for their realization is the scalable integration of quantum emitters delivering single indistinguishable photons on-demand. This is potentially possible through the resonant excitation of an integrated QD in a waveguide by means of an on-chip integrated microlaser [1]. Towards this goal, we investigate the coupling and lasing properties of laterally emitting whispering gallery mode (WGM) type micropillar resonators evanescently coupled to single mode ridge waveguides, following numerically optimized design rules [2]. The IQPCs are based on III-V QD-heterostructures composed of a GaAs cavity sandwitched between two, distributed Bragg Reflectors, and an activle layer consisting of self-assembled InGaAs QDs, and are processed using high-resolution electron beam lithography and plasma etching. Various geometries of micropillars coupled to waveguides with grating outcouplers are processed and subsequently investigated systematically via micro-photoluminescence spectroscopy for a systematic study.

[1] S. Rodt, S. Reitzenstein, APLPhotonics2021, 6, 1.

[2] L. J. Roche and al., Numerical Investigation of a Coupled Micropillar - Waveguide System for Integrated Quantum Photonic Circuits. Adv Quantum Technol. 2024, 2400195.

The interaction between the charge carrier spin of a quantum dot (QD) exciton and photons enables the transfer of quantum information between separate quantum emitters, which lays the foundation of a scalable, photon-mediated quantum spin network[1]. With a specially designed spin-photon interface, the exciton spin could be coupled to the propagation direction of the photons scattered from the emitter via the chiral light-matter interaction[2]. Following up the work in ref.[3], here we demonstrate strong Purcell enhancement and high directionality for individual QDs embedded in a glide-plane photonic crystal waveguide (GPW). Additionally, by leveraging the spin preservation of p-shell quasi-resonant excitation, we successfully initialise an exciton spin on-chip. We have recorded a dramatic increase in chirality when a QD is driven in-plane and achieved a near-unity chiral contrast in the transmission geometry. This indicates a polarisation-dependent spin transfer between the QD and the waveguide mode. Besides, the directional coupling also gives rise to a non-linear single-photon phase shift, forming the basis for scalable quantum phase gates and other on-chip spin-photonics applications in chiral quantum optics[4].

References :
[1] Warburton R. J. et al. Single spins in self-assembled quantum dots. Nat. Mater. 12, 483–493 (2013).
[2] Lodahl P. et al. Chiral quantum optics. Nature 541, 473–480 (2017).
[3] Siampour H. et al. Observation of large spontaneous emission rate enhancement of quantum dots in a bro-ken-symmetry slow-light waveguide. npj Quantum Information, 9, 15 (2023).
[4] Dietrich, C. P. et al. GaAs integrated quantum photonics: Towards compact and multi-functional quantum photonic integrated circuits. Laser Photonics Rev. 10, 870–894 (2016).

Solid-state quantum emitters, particularly semiconductor quantum dots, are promising photon sources for quantum technologies. However, their interaction with phonons dampen the Rabi oscillations under resonant driving, leading to reduced state preparation fidelities. The phonon spectral density, a quantity for the carrier-phonon interaction strength, is a non-monotonic function in energy. This leads to the reappearance of Rabi rotation for high pulse powers, which was theoretically predicted in PRL 98, 227403 (2007).
Here, we experimentally demonstrate the reappearance of Rabi rotations by resonantly driving the negative trion transition in a GaAs quantum dot, obtained by local droplet-etching-epitaxy, using a few-picosecond laser pulse.

The promising application of single-photon sources (SPS) in quantum communication and cryptography has driven significant advances in this field and the search for new active materials. Among various emitters, semiconductor quantum dots (QDs) are of particular interest due to their high-quality single-photon emission, high generation rates, low multi-photon emission probabilities, and compatibility with semiconductor technologies [1]. However, only QDs emitting below 1 µm have reached near-ideal SPS status. GaAs/AlGaAs QDs grown via nanohole droplet etching epitaxy are notable examples, allowing precise control over size, resulting in high uniformity [2]. For fiber-optic communication, emission in the 3rd telecommunication window is essential, which drives interest in antimony-based and InP-based material systems.
This work explores (In)GaSb QDs epitaxially grown by filling nanoholes etched with GaSb droplets in an AlGaSb matrix [3]. Unlike other antimony-based QDs, such as InSb/InAs and GaSb/GaAs, InGaSb/AlGaSb QDs exhibit a type-I band alignment and narrow emission near 1.55 μm, making them promising telecom SPS [4,5]. We performed optical characterization on samples with varying In compositions, tuning emission between the S, C, and L bands of the 3rd telecommunication window. Techniques such as photoluminescence excitation, power- and temperature-dependent photoluminescence were used to determine emission wavelengths, uniformity, and excitonic properties, providing insight into optimizing QD performance and understanding photoluminescence quenching and spectral tunability.
Additionally, we conducted theoretical modeling using the multi-band k·p method, which includes strain, piezoelectric fields, and spin-orbit coupling [6]. We developed a realistic 3D model of the QDs based on structural data (3D atomic force microscopy scans of surface nanoholes after etching and after filling with QD material) to predict electronic and optical properties. Excitonic effects are included using the configuration interaction method. These theoretical predictions are crucial for interpreting optical spectra, guiding further quantum dot optimization.
This work was financed by FiGAnti project funded within the QuantERA II Programme that has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No 101017733 and National Science Centre, Poland- project 2023/05/Y/ST3/00125.

[1] X. Zhou, L. Zhai, and J. Liu, Photonics Insights 1, R07 (2022).
[2] S. Covre da Silva et al., Appl. Phys. Lett. 119, 120502 (2021).
[3] J. Hilska, A. Chellu, and T. Hakkarainen, Crystal Growth & Design 21, 1917 (2021).
[4] A. Chellu et al., Appl. Phys. Lett. Mater. 9, 051116 (2021).
[5] J. Michl et al., Adv. Quantum Technol. 6, 2300180 (2023).
[6] K. Gawarecki, Phys. Rev. B 97, 235408 (2018).

A single quantum emitter coupled to an optical waveguide is a promising platform for realizing highly efficient non-linear light-matter interactions. By employing a tapered geometry, both the injection and collection efficiency of light to such systems can be enhanced to near-unity. In this study, we investigate light injection into tapered InP nanowires with embedded InAsP quantum dots (QDs) using k-space extinction microscopy. Numerical simulations estimate that > 80% extinction of reflected light under ideal mode matching conditions corresponds to ≈ 90% in-coupling of incident light into the fundamental optical mode of the nanowire. We experimentally verify this claim by utilizing the QD emission under two-photon resonant excitation as a sensor to quantify the proportion of light coupled into the fundamental mode of the nanowire waveguide.

Photonic integrated circuits are applied on a broad range of technologies, from communication to computing and sensing. With the increasing interest on quantum technologies, quantum photonic integrated circuits became subjects of additional attention.
Several material platforms have been investigated, keeping in mind that the following elements are strictly necessary [1]: efficient sources of quantum light, single-mode low-loss waveguiding elements, beamsplitters, phase-shifters and single-photon detectors. So far, while several systems showed interesting perspectives, no platform has clearly emerged as winner of this race. This stimulated the use of hybrid approaches [2], where different complementary platforms are brought together in order to benefit from their respective strengths, furthermore foregoing the weaknesses.

In this work we report on the efficient interface of telecom quantum dots to low-loss silicon-nitride circuitry. In(Ga)As QDs emitting at telecom wavelength thanks to strain engineering, are embedded into etched microlenses. The use of laser-written photonic wire bonding enables the funnelling of single photons into a Si3N4 single-mode waveguide (average efficiency of ~ 29%) being this the input of a 50:50 beamsplitter. The light is then out-coupled with additional laser-written elements to increase the coupling into single-mode fibers connected to single-photon detectors. The on-chip element is used as beamsplitter into a Hanbury-Brown and Twiss experiment which results in a g^{(2)}(0) = 0.11 ± 0.2, clearly showing injection of single photons in the silicon-nitride chip. Several devices are fabricated to verify the scalability of the process as well as to quantify the efficiency of the photonic wire bonding interface.
These results set the basis for the up-scaling of photonic integrated circuits complexity.

[1] E. Knill, R. Laflamme G. J. Milburn, Nature 2001, 409, 46–52.
[2] A. W. Elshaari, W. Pernice, K. Srinivasan, O. Benson V. Zwiller, Nature Photonics 2020, 14(5), 285–298.

The investigation of semiconductor quantum dots (QDs) that can emit single photons directly in the wavelength range of existing fiber communication networks is of key interest in order to harness the quantum properties of photonic qubits for everyday applications. While the first big achievements in fabricating highly efficient quantum light sources and the generation of spin-photon or multi-photon entangled states utilizing QDs were performed in a wavelength range around 900 nm, the past few years have shown a rapid technological evolution that enables the transfer to the telecom range.
Here, we report on our progress studying photoluminescence from QDs in the telecom C-Band. In detail, we present the quantum-optical properties of InAs QDs grown by molecular beam epitaxy on an InP substrate around which circular Bragg gratings are fabricated. The observed lines exhibit Purcell enhanced emission featuring high raw Hong-Ou-Mandel visibility and low multi-photon probability. Furthermore, we investigate excitonic complexes in QDs grown by metal-organic vapor-phase epitaxy on a metamorphic buffer layer on a GaAs substrate, the spectral properties of which are enhanced by a hybrid cavity consisting of a semiconductor mirror on the bottom and a dielectric mirror on top. Full polarization tomography capability and picosecond-time resolution, together with an in-plane magnetic field enables us to study spin dynamics and to analyze and control the interplay between electron/hole spins and the polarization of the emitted photons.

A single photon source is a key technology for realizing quantum information processing tasks such as quantum key distribution, linear optical quantum computing and measurement-based quantum computing (MBQC). Many of these tasks - including MBQC – require deterministic generation of identical single photons [1]. Quantum dots (QD) embedded within nanowires are promising candidates for the on-demand generation of indistinguishable single photons [2]. What sets them apart from other QD based emitters is their scalability and ease of on-chip integration. It is widely known that a strictly resonant excitation scheme is necessary for the generation of transform-limited single photons. However, two main indicators of single photon resonance fluorescence, i.e., sub-poissonian statistics and Rabi-oscillations have not been demonstrated in nanowire QDs yet.
In this work [3], we implement a dark-field microscope setup [4] to demonstrate the resonant excitation of the neutral exciton (X) in an InAsP QD embedded within an InP nanowire at 4K. The QD is excited by 20 ps long laser pulses at a repetition rate of 80 MHz. To observe resonance fluorescence with a high signal to background ratio, the back-reflected laser must be suppressed in the detection path. We achieve this by using orthogonal excitation and detection polarization states. The confocal configuration of the microscope enhances the laser suppression by spatially rejecting the clover-leaf shaped profile of the reflected laser beam – resulting in laser suppression by a factor of $10^6$. We record excitation laser power-dependent count rates on superconducting nanowire single-photon detectors to demonstrate Rabi oscillations—a clear signature of coherent driving. Our second-order correlation measurements on resonance fluorescence reveal a $g^{(2)}(0)≈0.06$ at an excitation power corresponding to a π/2 pulse. Ways of further reducing the non-zero coincidences at zero delay are discussed, as are the implications of resonance fluorescence on linewidths and two-photon interference visibilities. This important milestone will enable us to take another step towards transform-limited, indistinguishable single photon generation and coherent control of excitons in nanowire quantum dots.

References
[1] E. Cogan et al., Nature Photonics, 17, 324–329.
[2] E. Yeung et al., Physical Review B, 108, 195417.
[3] J. Gao et al., https://arxiv.org/pdf/2409.14964
[4] A. V. Kuhlmann et al., Review of Scientific Instruments, 84(7), 073905.

Quantum emitters in transition metal dichalcogenides (TMDs), such as WSe2, are promising sources of single photons for quantum technologies, where one qubit is encoded in the quantum state of a single photon. The layered nature of TMDs allows straightforward fabrication and seamless integration of the source with other photonic structures. However, both the total efficiency (defined as the number of collected photons per excitation trigger) and the indistinguishability of emitted photons must be increased close to unity to scale up the number of available qubits.
High collection efficiency from WSe2 emitters coupled to a photonic cavity was recently demonstrated thanks to an excellent control of the emission dynamics [1]. On the other hand, a significant challenge is to find efficient and scalable solutions to control the excitation dynamics, namely, to excite the emitter on-demand and with near unity fidelity. It is widely accepted that resonant excitation poses a fundamental limitation to the efficiency due to polarization filtering of the outgoing single photons, while pumping into a higher energy state limits the indistinguishability significantly. Thus, identifying suitable non-resonant excitation schemes and assessing their performance is of the utmost importance.
In this work [2], we investigate the excitation dynamics of WSe2 emitters under different schemes, unveiling the critical role of phonons on the state preparation fidelity. We develop a quantitative theory of WSe2 emitters interacting with 2D acoustic phonons, and fit the parameters to our own experimental results. Crucially, the model shows that phonon coupling in this platform is an order of magnitude stronger compared to established InAs/GaAs quantum dot emitters, in agreement with recent experiments [3].
Using our detailed phonon theory, we assess the performance of different state-preparation protocols. We show, for example, that the well-known Rabi oscillations are strongly suppressed by phonon scattering in WSe2. On the other hand, near-resonant phonon-assisted excitation [4] allows to prepare the exciton state with fidelity larger than 99\%. We also discuss how to excite WSe2 emitters with the so-called SUPER scheme [5], where the impact of phonon coupling is crucial but often under-estimated [6]. Finally, we provide a quantitative description of the spectral broadening induced by phonon coupling, which sets fundamental limitations on the single-photon indistinguishability.
This work provides fundamental insights into the physics of phonon coupling in WSe2 emitters, offering an avenue to control the excitation process.

[1] J.-C. Drawer et al., Nano Lett. 23, 8683 (2023).
[2] L. Vannucci et al., Phys. Rev. B 109, 245304 (2024).
[3] V. N. Mitryakhin et al., Phys. Rev. Lett. 132, 206903 (2024).
[4] S. E. Thomas et al., Phys. Rev. Lett. 126, 233601 (2021).
[5] T. Bracht et al, PRX Quantum 2, 040354 (2021).
[6] L. Vannucci and N. Gregersen, Phys. Rev. B 107, 195306 (2023).

We investigate trivalent thulium ions embedded in a lithium niobate crystal (Tm$^{3+}$:LiNbO$_3$) as a platform for quantum information applications, including single and entangled photon sources, quantum memories, and quantum gates.

For this, we consider three infrared zero-phonon lines in (Tm$^{3+}$:LiNbO$_3$), corresponding to wavelengths of 795 nm, 1450 nm, and 1765 nm. We first describe how to realize a quantum repeater (including source and memory) either using 795 nm photons or using energy-time entangled 1450 nm and 1765 nm photons. All photon sources rely on individual Tm ions and nanophotonic cavities that allow increasing the light-matter interaction by means of the Purcell effect, and the required quantum memories are based on large ensembles of Tm ions and mm-sized cavities. In addition, we also discuss the realization of qubits using Zeeman levels of a single Tm ion, and single and two-qubit gates that exploit the 795 and 1765 nm transitions.

Long-distance quantum communication can potentially suffer under decoherence of the photonic state in optical fibers. In comparison to polarization entanglement, time-bin entanglement is remarkably robust in fibers, which is an essential property for the implementation and usage of long-distance quantum information protocols such as quantum key distribution. In time-bin entanglement, the photons are entangled with respect to the time-bin in which they are created.

Generating these photon pairs requires precise temporal control of a photon pair source. Several proposals exist on how time-bin entangled photon pairs can be created from quantum emitters. For the generation of the entangled state, semiconductor quantum dots are our system of choice, as they provide on-demand generation of photon pairs with a high yield. Additionally, the presence of optically dark exciton states in quantum dots allow for the deterministic preparation of time-bin entangled states [1]. These dark states that can be addressed using tilted magnetic fields and strongly chirped pulses [2]. An experimentally simpler approach relies on probabilistic generation, using sequential excitation of the biexciton state, addressing the biexciton in a two-photon resonant process [3].

On this poster, we introduce the theoretical background for the numerical simulation of time-bin entanglement. This includes the multi-time correlation functions calculated from the full time dynamics of the quantum dot excitation [4]. We apply these techniques to simulate the degree of time-bin entanglement achieved by a scheme relying on dark excitons and compare it to the previously used probabilitstic generation scheme. Our results are another crucial step to bright time-bin entanglement from quantum dots.

References
[1] C. Simon and J.-P. Poizat, Phys. Rev. Lett. 94, 030502 (2005)
[2] F. Kappe, R. Schwarz, Y. Karli, T. Bracht et al., arXiv:2404.10708 (2024)
[3] H. Jayakumar et al., Nat. Communications, 5, 4251, (2014)
[4] T. Bracht, F. Kappe, M. Cygorek et al., arXiv:2404.08348 (2024)

Frequency and time-resolved photon correlations [1] involve studying the photon statistics of a quantum source while considering both the detection times and the frequencies of emitted photons. In the case of a continuously driven two-level system, it is well-known that including the frequency of photons reveals a rich landscape of correlations beyond the expected antibunching behavior [2]. Furthermore, in the time-dependent case, where a two-level system is excited by finite pulses, the photon statistics oscillate between antibunching and bunching, depending on whether the pulse area is odd or even. However, this model does not account for correlations between photons of different frequencies [3].
In this work, we explore the time-dependent, frequency-filtered photon statistics of a two-level system under pulsed excitation. Our results show that the photon statistics for a given pulse are not fixed but can range from bunching to uncorrelated emission to antibunching, depending on the frequency filters applied. This reveals that, much like the continuous wave (cw) case, time-dependent photon statistics offer a rich variety of features, highlighting the intricate interplay between pulse area, photon frequencies, and correlations in the time-dependent regime.
This approach could therefore contribute to simplify the current pulsed driven schemes for generating entanglement, where subsequential excitation of emitters leads to photon number entanglement [4]. Instead, just by adequately selecting the relevant frequency windows, one could potentially select the number of photons in early and late time bins that lead to the desired entanglement of the state.

[1] E. del Valle, A. Gonzalez-Tudela, F. P. Laussy, C. Tejedor, and M. J. Hartmann. Theory of Frequency-Filtered and Time-Resolved N -Photon Correlations. Physical Review Letters, 109(18):183601, October 2012.
[2] Juan Camilo Lopez Carreno, Elena del Valle, and Fabrice P. Laussy. Photon Correlations from the Mollow Triplet: Photon Correlations from the Mollow Triplet. Laser & Photonics Reviews, 11(5):1700090, September 2017.
[3] Fischer, K., Hanschke, L., Wierzbowski, J. et al. Signatures of two-photon pulses from a quantum two-level system. Nature Phys 13, 649–654 (2017).
[4] Wein, S.C., Loredo, J.C., Maffei, M. et al. Photon-number entanglement generated by sequential excitation of a two-level atom. Nat. Photon. 16, 374–379 (2022).

Strain-free GaAs/AlGaAs semiconductor quantum dots (QDs) grown by droplet etching and nanohole infilling (DENI) are highly promising candidates for the on-demand generation of indistinguishable and entangled photon sources. The spectroscopic fingerprint and quantum optical properties of QDs are significantly influenced by their morphology. The effects of nanohole geometry and infilled material on the exciton binding energies and fine structure splitting are well-understood. However, a comprehensive understanding of GaAs/AlGaAs QD morphology remains elusive. To address this, we employ high-resolution scanning transmission electron microscopy (STEM) and reverse engineering through selective chemical etching and atomic force microscopy (AFM). Cross-sectional STEM of uncapped QDs reveals an inverted conical nanohole with Al-rich sidewalls and defect-free interfaces. Subsequent selective chemical etching and AFM measurements further reveal asymmetries in element distribution. This study enhances the understanding of DENI QD morphology and provides a fundamental three-dimensional structural model for simulating and optimizing their optoelectronic properties.

Novel concepts aiming at efficient processing of information require a strong and controlled coupling of single photons with single atomic quantum systems. In this talk, I will first give an introduction to the efficient generation of single photons using planar dielectric antennas. These antennas allow us to collect the emission from an arbitrarily oriented single quantum emitter with >99% efficiency. By using a planar metallo-dielectric antenna applied to an organic molecule, we demonstrate the most regular stream of single photons reported to date. The measured intensity fluctuations were well below the shot-noise limit and amounted to 2.2 dB squeezing.
In the second part of the talk, I will discuss our efforts toward the realization of quantum networks and present experiments where photons and single solid-state emitters strongly interact. A single molecule can amplify a weak laser beam and generate nonlinear effects like three-photon amplification and four-wave mixing. In order to achieve an even stronger interaction, we have coupled a single molecule to a tunable Fabry-Perot microcavity. The system is operated in the strong coupling regime of cavity quantum electrodynamics, where a strong Purcell factor effectively turns the molecule into a two-level quantum system. We observe 99% extinction of a laser beam, which means that our molecule in the cavity acts almost as a perfect scatterer of photons.

The Swing-UP of quantum EmitteR (SUPER) scheme utilizes two red-detuned pulses to excite a quantum system. After its theoretical prediction for a two-level system [1], the SUPER scheme was experimentally demonstrated in quantum dots [2]. To explore the full possibilities of the SUPER scheme, a two-level emitter can be embedded in a photonic cavity, which introduces new phenomena.
In this talk, we will discuss the implications of applying SUPER to a quantum emitter in a photonic cavity. This allows us to consider SUPER in the few-photon limit, where we can link SUPER to few-photon scattering processes, as known from quantum optics. While transferring photons from one mode to the other, the two-level quantum emitter can be fully excited [3].
During the off-resonant excitation with strong pulses, the induced Stark effect shifts the quantum emitter out of resonance. As a results, the application of the SUPER scheme to a quantum dot in a cavity results in the creation of perfectly entangled photon pairs, even up to temperatures of 80K [4]. Additionally, the Stark effect influences the photon number coherence (PNC) of single photons generated via SUPER. While for resonant excitation, the PNC follows mostly the excitation probability and is strongly influenced by phonons, preliminary calculations indicate that SUPER significantly reduces PNC compared to the resonant case.
Unlocking the full potential of off-resonant excitation schemes, such as SUPER, promises to reveal even more intriguing phenomena in the future.
References:
[1] Bracht et al., PRX Quantum 2, 040354 (2021)
[2] Karli et al., Nano Lett. 22, 6567 (2022)
[2] Bracht et al,. Optica Quantum 1, 103 (2023)
[4] Richter et al., arXiv:2405.20095 (2024)

Since 2020, photon antibunching [1-3] and Purcell enhancement [4-5] in cavities have been observed for a variety of point defects acting as color centers in silicon. These results open the way to the development of a first deterministic source of single photons in Si (SPS), with a huge potential impact for the upscaling of integrated quantum photonics on SOI chips.
Building upon previous results on W centers [3,5], we have recently developed the integration of a single W center at the core of SOI bullseye cavities in a deterministic way, with a positioning accuracy on the order of +/- 50 nm and nearly perfect spectral matching between the W center zero-phonon line and the resonant cavity mode (see Christian Elsässer’s poster at EQEP). This presentation is focused on the detailed study of these SPS.
HBT experiments under cw non-resonant pumping unveils a very clean antibunching behavior (g(2)(0)<0.05) over the full range of pump powers, up to saturation of W’s PL. This result highlights the absence of parasitic emitters in the cavity.
We have performed HBT experiments under pulsed excitation, playing with different pump powers and repetition rates. In the weak pumping regime (1-10% of the saturation power), the area A0 of the peak at zero delay is typically 20 smaller than reference peaks at long delays, showing emission of at most one photon as a response to excitation pulses.
This good single photon purity is however lost at large pump powers close to the saturation power, and A0 reaches about 0.5 in normalized units. A careful analysis of the central peak using a fine time binning to build the photon correlation histogram reveals a narrow and well-defined antibunching dip at zero delay at the center of that peak. We attribute these behaviors to a repumping of the W center after emission of a first photon, due to the presence of residual electron-hole pairs inside Si. Quasi-resonant pumping and/or the application of a weak static electric field should mitigate this issue.
We also show that the Purcell effect is at work in our system, as witnessed by a 500-fold increase of the photoluminescence signal at saturation, and by a 3-fold reduction of the emitter’s lifetime. These results demonstrate that the W center ZPL emission experiences a Purcell enhancement in the 6-10 range, to be compared with 13, the highest possible value for a W center that is ideally coupled to the cavity mode. This confirms the accurate positioning of the W center close to the mode antinode, with +/-50 nm precision.
On the basis of these results, we will present the attractive expected performances of a waveguide-coupled SPS made of a single W center in a high-Fp “nanobeam” photonic crystal cavity.
[1] Redjem, W. et al. Nat Electron 3, 738–743 (2020)
[2] Durand, A. et al, PRL 126, 083602 (2021)
[3] Y. Baron et al., ACS Photonics 9 (7), 2337-2345 (2022).
[4] B. Lefaucher et al, APL 122, 061109 (2023)
[5] B. Lefaucher et al., ACS Photonics 11 (1), 24-32 (2024)