Abstract

The strong atom-photon coupling generated inside high-finesse optical micro cavity lets us detect a single atom trajectory in real time. Such a strong coupling is a candidate for trapping a single atom with a single photon, which is very suitable to realize quantum gate, single photon generator [1], and so on. Several preliminary experimental results have already been reported [2]. However, they are limited to the use of longitudinal cavity mode. Here we propose a novel type of single-atom trapping using a high-order transverse (TEM₀₁) mode of the cavity. In this novel trap, both laser frequency and cavity resonance are locked to the atomic resonance. When a single atom enters the cavity, the cavity transmission decreases due to vacuum Rabi splitting (Fig. 1a). By using this signal as a trigger, the laser frequency is suddenly shifted to blue side of the atomic resonance by Rabi frequency (Fig. 1b) When the atom is on axis of the cavity, i.e., the atom is in trapping region, the atom dose not interact with an optical field and thus laser beam can not enter the cavity. However, if the atom tries to escape from this trapping region, atomic position starts to overlap with spatial profile of the transverse mode and thus atom-photon coupling is recovered (Fig. 1c). Blue-shifted laser beam enters the cavity and the atom feels repulsive restoring force. This novel atom trap has a lot of advantages. When the atom is in trapping region, there is no laser beam in the cavity and thus heating due to spontaneous emission can be dramatically suppressed. Induced heating caused by mechanical fluctuation of the cavity length which is mainly limiting present single atom trapping lifetime [2] can be also suppressed. Dynamics of trapped atoms can be observed in situ and in real time by detecting intensity change on the transmission of blue-shifted laser beam. We cooled down and trapped rubidium atoms by using conventional cell magneto optical trap (MOT) and launched them into the micro optical cavity which locates 35mm above the MOT. The finesse and length of the cavity are 1.5×10⁵ and 100µm, respectively. In figure 2, we show an experimental result of real-time change in transmission of the atom-cavity system produced by a single atom transit through the transverse mode . By using this signal as trigger and changing the laser frequency, it is, in principle, possible to trap the atom in the cavity. Figure 1. Schematic diagram of single-atom Figure 2. Observed reduction in the cavity trapping with a high-order transverse mode. transmission by a single atom transit.

© 2000 IEEE