In a quantum cryptography key distribution system for sharing a secret key between a transmitter and a receiver site, an unbalanced interferometer system in the transmitter site has a Mach-Zehnder interferometer switch with a phase modulator while the receiver site records photon arrival time slots. The system utilizes a whole of arrival photons in the receiver site and dispenses with any phase modulator in the receiver site. This system serves to improve a photon utilization efficiency.