Abstract

Atoms exposed to a modified spectrum of the vacuum fluctuations show a different energy from those exposed to the full spectrum. In order to investigate this very small energy shift the atoms have to be brought in a confined space. The cavity geometry adopted in our work is made up of two plane parallel plates representing a prototype geometry in Cavity Quantum Electrodynamics. Rydberg atoms are suitable for this purpose since the transitions to neighbouring levels are in the microwave region which can thus be influenced by plate distances in the mm region. Limiting the analysis to principal quantum numbers ranging between 20 and 30, these changes are expected to be of a few hundred Hz when thermal radiation at room temperature is additionally present within the cavity. With the vacuum field alone the change of the energy expected is on the order of roughly 100 Hz [1]. The measurement of this shift with particular emphasis on vacuum effects constitutes the main aim of this work. The experimental technique involves a combination of a Doppler-free two-photon absorption with the optical Ramsey method of two successive oscillatory fields. The small expected effect requires an extremely stable light source: a dye laser with frequency fluctuations at the Hz level [2]. Between the two interaction regions of the Ramsey set-up Rubidium atoms pass through a pair of conducting plates with a relative distance d which can be changed by using piezoelectric step-motors. The experimental evidence obtained so far indicates an on-resonance variation of about 200 Hz for the 24S level at room temperature. Energy shifts have also been observed when the set-up is cooled to liquid Helium temperature. These variations amount to about 90 Hz and 120 Hz for the 26S and 22S level respectively, the first being coupled to the parallel plate structure through the 26S_1/2 → 25P_3/2 transition (see figure) and the second through the 22S_1/2 → 21P_3/2 transition. The figure shows the energy shift when the cut-off frequency is tuned with the distance of the plates.

© 1998 IEEE