Quantum cryptography and quantum-key distribution with single photons

A brief overview of the current status of quantum cryptography is given. The results are presented of our preliminary experiments with free-space quantum-key distribution by means of single linearly polarized photons with coded polarization states according to the BB84 protocol, for which purpose a quantum-cryptography communication system is designed. Single photons are detected with a 50% probability using specially designed high-speed photodetectors based on silicon avalanche photodiodes, operated in Geiger mode with active avalanche quenching. A key generation rate of 3.8 kbit/s is obtained, the mean photon number per pulse being 0.2.