PROBLEM TO BE SOLVED: To prolong a transmission distance by reducing losses of photons in a receiver section and using a single photon detector having a high efficiency low dark count probability, in a quantum cipher system.;SOLUTION: In a cryptograph key information receiving section, a laser beam pulse, having a wavelength of 1,550 nm, is split into a horizontally polarized reference optical pulse and a vertically polarized signal optical pulse. The signal optical pulse is delayed and transmitted. In a cryptograph key information transmitting section, the polarization surface of the reference optical pulse is rotated by 90°. A random-phase shift is provided to the signal optical pulse to rotate the polarization surface by 90°. These pulses are attenuated to be formed into single-photon pulses and are sent back. In the cryptograph key information receiving section, a random phase shift is provided to a feedback reference optical pulse and overlapped on a feedback signal optical pulse, is split, in response to a polarization state by a polarization optical splitter, and is detected by an APD (avalanche photodiode) which is suspended only for a release time of trap carriers, thereby obtaining a cryptograph key.;COPYRIGHT: (C)2004,JPO
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