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.