Patent ID: 6188768
Filing Date: 2001-02-13
Classification: H04L

Abstract:
A communication channel for transmission of cryptographic key information between first and second channel ends comprising:a source of discrete light signals at the first channel end;a first polarizing beamsplitter at the first channel end for splitting each of the discrete light signals into a pair of first and second light pulses orthogonally polarized with respect to each other, the first pulse being directed from the first polarizing beamsplitter to the second channel end;a delay stage at the first channel end through which the second pulse from the first polarizing beamsplitter travels, the second pulse being directed to the second channel end with a time delay relative to the first pulse;a first phase modulator at the delay stage for modulating the phase of at least one of the pulses in the pair on its return back to the first channel end from the second channel end to thereby change the relative phase of the first and second return pulses;a second phase modulator at the second channel end for modulating the phase of at least one of the pulses in the pair received from the first channel end to thereby change the relative phase of the first and second pulses, said at least one pulse being split into its horizontal and vertical polarization components at the second channel end, the second phase modulator transmitting light of only one polarization but equally modulating both polarization components of said at least one pulse;a Faraday mirror at the second channel end for reflecting each of the pulses back to the first channel end as first and second return pulses, the first return pulse having its polarization state rotated ninety degrees, the second return pulse having its polarization state rotated ninety degrees, and the first and second return pulses being orthogonally polarized with respect to each other;an attenuator in the channel for assuring that the return pulses are substantially single-photon pulses; anda second polarizing beamsplitter at the first channel end for directing the first return pulse through the delay stage, the second return pulse being directed to the first polarizing beamsplitter; whereby the first return pulse travels through the delay stage and the first phase modulator and combines with the second return pulse at the first polarizing beamsplitter to form a recombined return light signal having a polarization state dependent on the relative phase change applied to the first and second return pulses by the first phase modulator and the relative phase change applied to the first and second pulses by the second phase modulator; anda detection stage at the first channel end having first and second detectors and means for directing the recombined return light signal to only one detector selected from the first and second detectors, the first detector receiving the recombined return light signal only if the recombined return light signal has a first polarization state and the second detector receiving the recombined return light signal only if the recombined return light signal has a second polarization state.