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Project Area C:
Rydberg Interfaces

The remarkable properties of Rydberg atoms do not only lead to strong interactions between them, but also make them extremely susceptible to environmental influences. This includes both AC and DC electric and magnetic fields as well as Casimir-type dispersive interactions which result from "vacuum" fluctuations of the quantised electromagnetic field. For example, the electric polarisability of a Rydberg atom increases with the principal quantum number n as n7, thereby magnifying its response to external electric fields by several orders of magnitude. This sensitivity already allowed for the development of an accurate standard for microwave electric fields at 10 GHz with a minimum detectable field amplitude of ~8µV cm-1 and a sensitivity of ~30µV cm-1 Hz-1/2 based on Rydberg atoms in vapour cells (1), which already outperforms the currently used traceable standard at PTB with ~30µV cm-1 detectable field amplitude.

The development of vapour cells, both for thermal as well as for ultracold atoms, represented a major step forward from free-space atom manipulation in magneto-optical traps to miniaturized structures that can be used as portable platforms for, e.g. field measurements. The proximity of the atoms to the microcell walls leads to strong coupling to surface plasmon polaritons (SPP) and Casimir-Polder-type dispersion interactions. Rydberg atoms, utilizing their giant interactions and the Rydberg blockade mechanism, provide the means to generate large, tunable interactions that are necessary to implement quantum information processing protocols such as single-photon subtraction and generation schemes in compact systems. Environments with strong optical mode confinement include hollow-core optical fibres (see figure) in which the coupling to the material interface via SPPs or atom-surface interactions is strongly enhanced.

The huge response to external electromagnetic fields results in large shifts of the atoms’ energy levels. In case the shifts are due to fluctuating fields, the associated van-der-Waals interaction leads to the Rydberg blockade mechanism. Similarly, Casimir-Polder shifts between Rydberg atoms and material surfaces have been shown to be as large as several GHz, leading to large state mixing and appreciable forces that are strong enough to deflect single sheets of graphene, thus opening up the possibility to design truly hybrid atom-solid state quantum systems (2).

The design and development of hybrid quantum systems has already proved to be a successful route to merge the advantages of long quantum coherence times in atomic systems with the flexibility of solid-state technologies. One of the currently most promising developments is based on electro-mechanical resonators (3) whose interaction with the electromagnetic field is mediated by the radiation pressure force. Such systems possess resonance frequencies in the MHz-GHz range and have already been cooled to their quantum mechanical ground state. Together with superconducting strip line cavities, they represent a novel route to study the quantum behaviour of macroscopic systems and their interaction with the electromagnetic field. Their resonance frequencies are easily covered by transitions in Rydberg atoms. This facilitates a strong-coupling between microwave photons and Rydberg atoms, leading to the possibility of mutually coherent manipulation (4).

1. J. A. Sedlacek, A. Schwettmann, H. Kübler, R. Löw, T. Pfau, and J. P. Shaffer. Microwave electrometry with Rydberg atoms in a vapour cell using bright atomic resonances. Nature Phys. 8, 819 (2012).

2. S. Ribeiro, S.Scheel. Controlled ripple texturing of suspended graphene membranes due to coupling with ultracold atoms. Phys. Rev. A 88, 052521 (2013).

3. M. Aspelmeyer, T. J. Kippenberg, and F. Marquardt. Cavity Optomechanics. arXiv:1303.0733, accepted for publication in Rev. Mod. Phys. (2014).

4. S. D. Hogan, J. A. Agner, F. Merkt, T. Thiele, S. Filipp, and A. Wallraff. Driving Rydberg-Rydberg transitions from a coplanar microwave waveguide. Phys. Rev. Lett. 108, 063004 (2012).