Patent ID: 7555127

Claim:
A quantum cryptography key distribution system for use in sharing a secret key between a transmitter site and a receiver site on the basis of a quantum cryptography key distribution protocol of four non-orthogonal states that are represented by superposition of quantum wave packet states each consisting of a single photon with different time and space characteristics, the transmitter site comprising: an unbalanced Mach-Zehnder interferometer system composed of two different optical paths of unequal lengths, a light splitting portion, and a light coupling portion; wherein either one of the light splitting portion and the light coupling portion is a variable branching-ratio optical coupler composed of a balanced Mach-Zehnder interferometer system having two optical paths of an equal length and a phase modulator which is disposed in one of the two optical paths and which is controllable by an applied voltage; wherein the unbalanced Mach-Zehnder interferometer system and the balanced Mach-Zehnder interferometer system constitute a serially coupled interferometer system; wherein the transmitter site transmits, through a quantum channel to the receiver site, the photon which is specified by single photon packet states selected from (|s> A , |l> A ) and (|l> A +|s> A , |l> A −|s> A ) that are different in time and space characteristics from each other in response to a pair of phase modulation values that are randomly selected from (0,π) and (π/2, 3π/2) to phase modulate a light pulse: wherein the transmitter site produces, as the photon, single photons that correspond to the phase modulation values of each pair, the receiver site comprising: photon detectors; and a recording device for recording photon detection times at which the photon is received by each of the photon detectors.