SYSTEM AND METHOD FOR KEY DISTRIBUTION USING QUANTUM CRYPTOGRAPHY

A method of communication based on quantum cryptography is modified to include an initial step of outputting from, e.g., a transmitter, a single-photon signal, which may be unmodulated. A receiver then randomly selects one of a plurality of encryption alphabets corresponding to different, non-commuting quantum mechanical operators. The receiver modulates the single-photon signal with the selected operator and returns the signal to the transmitter. The transmitter in turn randomly selects a quantum mechanical operator and uses that operator in detecting the returned signal modulated by the receiver. Alternatively, the transmitter may randomly select one of a plurality of encryption alphabets and use that encryption alphabet in modulating the signal. The signal is then further modulated at the receiver using a predetermined encryption alphabet. When the signal is received back at the transmitter, it is detected using the same quantum mechanical operator as was initially used to modulate it. Comparison is made of the states of the single-photon signals as transmitted and received to detect the presence of any eavesdropper.