System and method for key distribution using quantum cryptography

A communications system for use in a method of quantum cryptography comprising:

• a transmitter,
• one or more receivers,
• a network linking the transmitter to each receiver,
• a source for generating a single-photon signal,
• each receiver including a modulator arranged (a) to modulate a single-photon signal received from the source using a chosen modulation state and (b) to return the modulated single-photon signal to the transmitter, and
• the transmitter including a single-photon detector arranged to detect the returned single-photon signal.

A communications system for use in a method of quantum cryptography comprising:

• a transmitter,
• one or more receivers,
• a network linking the transmitter to the or each receiver,
• the transmitter including means for generating a single-photon signal and modulating the single-photon signal using a chosen encryption alphabet,
• each receiver including a single-photon detector for detecting a single-photon signal from the receiver,
• the network including a looped-back path for returning at least some of the single-photon signals output by the transmitter to the transmitter, and
• the transmitter including a single-photon detector arranged to detect the returned single-photon signal, in use the transmitter comparing the states of the single-photon signals as transmitted and returned.

A method of communication using quantum cryptography including:

• selecting at a transmitter one of a plurality of encryption alphabets corresponding to different, non-commuting quantum mechanical operators, and encoding a signal for transmission to the receiver using the selected operator,
• a receiver further modulating the single-photon signal received from the transmitter and returning it to the transmitter,
• the transmitter using the quantum mechanical operator selected for the outgoing signal in detecting the returned signal modulated by the receiver, and
• subsequently comparing the states of the signals as transmitted and received at the transmitter, thereby detecting the presence of any eavesdropper intercepting the single-photon signal.