| 摘要 |
A method and apparatus for secure distribution of cryptographic key information is described based on quantum cryptography. The apparatus incorporates a transmitter 1 comprising a source of pairs of dim, depolarised light pulses together with a phase modulator 5 and random number generator 6 that are used to encode the pulse pairs with the binary key information by changing the relative phases of the pulses of some pairs. The apparatus incorporates a receiver 13 comprising a polarisation beam splitter 14, and a pair of interferometers 16,17 and optical detectors 19,20. Detector 19 determines when the phase difference between the pulses of a received pulse pair is indicative at a 'O' output from the random number generator, and detector 20 determines when the phase difference is indicative of a '1' output. The invention overcomes problems associated with polarisation evolution in a non-polarisation-preserving optical channel 12 (e.g. standard optical fibre). In addition the invention removes the need for an active random number generator and phase modulator at the receiver, because the polarisation beam splitter acts as if it were a random router.
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