We theoretically and experimentally study quantum mechanical properties of photons, electrons and atoms. In particular, we want to apply the quantum states of single photons to future communication, represented as the quantum key distribution, and we try to realize and to control quantum entanglements between photons and materials.
Differential-phase-shift quantum key distribution (QKD) protocol, which had been proposed by NTT and Stanford Univ. in 2002, has a potential to realize an ultimate secure communication via optical fiber in a stable and fast manner. Recently, we succeeded for the first time in generating secure keys against generalized individual attack through an optical fiber of 200 km using superconducting single photon detectors in collaboration with NIST. We have also demonstrated stable field experiments with 17.6 km optical fiber and frequency up-conversion single photon detectors using PPLN wave-guide and Si-APD. Proposal of new QKD protocols and their security proofs are other issues of the group. For future quantum communications, we are exploring noise-free and stable source of telecom-band entangle photon pairs. In particular, we have succeeded in generating entangle photon pairs using silicon wire waveguide. We also theoretically study the physics of cold atoms trapped in an optical lattice made by laser lights and compare with experimental results.