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Abstract

A laser pulse representing a bit of a quantum key is split into two pulses. In addition to known round trip phase encoding schema, a secret phase key is modulated into one of the two pulses: P1 and P2. The secret phase key is used to identify whether the returning pulses originated from the sender, i.e., whether the key distribution has been attacked by an eavesdropper. A secret key phase modulator randomly modulates pulse P1. An attenuator then reduces the average photon number of the modulated pulse P1 to a selected level greater than one to increase the likelihood of efficient, successful transmission while reducing the possibility of eavesdropping, e.g., .mu.=10. Both pulses P1 and P2 are sent to the intended recipient and reflected to the sender. Pulse P2 is modulated upon return to the sender using the same secret phase key previously modulated into pulse P1. Therefore, when both pulses meet together at a coupler/beamsplitter of the sender, both pulses should contain the same secret key in their phase and therefore exhibit no resulting phase difference if the photon pulse is the same pulse originated by the sender. If the returning pulse is not the pulse originated by the sender then phase differences indicative of a so-called intercept-resend attack applied by an eavesdropper EVE are indicated by a large quantum bit error rate ('QBER') will be detectable. If EVE applies photon-split attack, the secret phase key modulated by the sender prevents Eve from knowing the encoded key information in the photon(s). Therefore, double phase encoding QKD enables use of multi-photon pulses without unacceptable loss of security, thereby enhancing QKD bit rate.

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