ECAMP 15

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).