Patent ID: 6522749
Filing Date: 2003-02-18
Classification: H04L

Abstract:
A quantum cryptographic communication channel based on quantum coherence, the quantum cryptographic communication channel comprising:(a) a sender system having: (i) a light source; (ii) a first directing means; (iii) first and second sources each capable of generating a pair of photons emitted in the form of signal and idler light beams when energized by the light source, the first and second sources being arranged relative to each other such that the idler beam from the first source is incident upon the second source and aligned into the idler beam of the second source and the signal beams are directed by the first directing means to converge upon a first common point; (iv) a phase modulator for changing the phase of one of the idler beam from the first source, signal beam from the first source, or signal beam from the second source between first and second phase settings, said first and second phase settings being separated by 180Â°; and (v) a fist controller for controlling the timing of the phase change from the first phase setting to the second phase setting; (b) a receiver system having: (i) a second directing means for directing the signal beams from the sender system to converge upon a second common point; (ii) first and second detectors for detecting the incidence of the signal beams from the first and second sources; and (iii) a beam splitter disposed at the second common point for directing the signal beams to the first detector when the phase is changed to the first phase setting and to the second detector when the phase is changed to the second phase setting; and (c) an optical link in optical communication with the second and third light modulators thereby optically connecting the sender system and the receiver system for transmission of the signal beams therebetween wherein the first directing means comprises: a half waveplate disposed in the optical path of the signal beam from the second source for shifting the phase of the signal beam from the second source by 90 degrees; first and second mirrors disposed in the optical paths of the signal beams from the fist and second sources, respectively for directing the signal beams from the first and second sources to the first common point; a first polarization beam splitter disposed at the first common point for transmitting the signal beam from the first source and reflecting the signal beam from the second source shifted by 90 degrees; a second light modulator disposed in the optical path between the first polarization beam splitter and the optical link, the second light modulator being controlled such that the phase of the signal beam from the first source is unaltered and the phase of the signal beam from second source is again shifted by 90 degrees to thereby restore its phase as it was before being shifted by the first half waveplate; and wherein the second directing means comprises: a third light modulator for accepting the signal beams from the fiber link, the third light modulator being controlled such that the phase of the signal beam from the first source is unaltered and the phase of th signal beam from second source is shifted by 90 degrees; a second polarization beam splitter for transmitting the signal beam from the tint source and reflecting the signal beam from the second source as shifted by 90 degrees; a second half waveplate disposed in the optical path of the signal beam from the second source before converging onto the second common point for shifting the phase of the signal beam from the second source by 90 degrees, to thereby restore its phase as it was before being shifted by the third light modulator; and third and fourth mirrors disposed in the optical paths of the signal beams from the first and second sources, respectively for directing the signal beams from the first and second sources to the second common point.