Since it is hard to realize all quantum information processes in a single quantum system, interfaces between various quantum systems like photons, atoms, ions, quantum dots, and Josephson junctions are becoming more important. We are focusing on a photon-atom quantum interface based on an atom-cavity system. Above all, we are working on enhancing the efficiency of deterministic single photon generation. A single photon is an ideal carrier of quantum information, playing an important role in quantum optics research and quantum information science like quantum communication and quantum computation. The atom-cavity system has an advantage in high directionality of radiated photons. We realized a single atom-cavity system by trapping a single rubidium atom in a Fabry-Perot cavity and excited the atom with 10 MHz nanosecond π-ulse. As a result, we achieved the repetition rate of 1.7×10⁶ per second which is the highest record among the atom-cavity systems based single photon sources. Efficiency of the single photon generation is defined as the probability of a single photon radiation per a single pumping pulse, and 17% was achieved in this experiment. We are working on making the efficiency of single photon generation over 50%.

A single-atom-cavity system can be used to realize a single-qubit quantum memory. Instead of trapping an atom in the cavity, if we trap some atoms in an one-dimensional optical lattice by one atom per one lattice site and drag the optical lattice, we can selectively couple the cavity with each atom. Using this method, the multi-qubit quantum memory can be realized. In this case, the atoms should be moved as fast as possible, while keeping the atoms in their motional ground state of the lattice potential. In this regard, we study the change of atomic dynamics in moving optical lattice. The resonance fluorescence spectrum of cold atoms provides the information on temperature and quantum mechanical motional states of the atoms. The resonance fluorescence spectrum of a few atoms is measured by photon-counting second-order-correlation spectroscopy(PCSOCS) method, developed by our laboratory[H. Hong et al., Opt. Lett. 31, 3182 (2006)]. By using PCSOCS, we can measure the spectrum of extremely small signals, about femto-Watt level. In the previous study with PCSOCS method, we measured the resonance fluorescence of single Rubidium atoms in optical lattice micro-potential formed at the center of magneto-optical trap. When the atoms are localized in the optical lattice minima, the spectrum exhibits the three narrow peaks, which are affected by Lamb-Dicke effect. As increasing the speed of the moving optical lattice, broadened Doppler peaks start to grow while the narrow peaks get smaller. These Doppler peaks reflect non-localized atoms. Comparison of the ratio between two atom groups(localized, non-localized) is currently under investigation.