Patent ID: 7502476

Claim:
A method of forming a key in a quantum key distribution (QKD) system having optically coupled first and second QKD stations Alice and Bob, comprising: generating at Alice, via a single light source that operates via spontaneous parametric down conversion using a non-linear medium, correlated pairs of photon pulses, with one of the pulses in each pair serving as an outgoing pulse having either a single-photon or multiple photons; at Alice, identifying time slots and a number of photons in each outgoing pulse by detecting the other of the outgoing pulse for each pulse pair using a photon-number-resolving (PNR) detector; communicating the time slot identities from Alice to Bob; altering the state of each outgoing pulse; transmitting a block of altered-state outgoing pulses from Alice to Bob; performing at Bob a second state alteration of each outgoing pulse received at Bob and then detecting each twice-altered outgoing pulse; determining at Bob a first number of detected single-photon pulses and a second number of detected multi-photon pulses using said time slot identities; and comparing the first and second numbers to respective probabilities of detecting said first and second numbers.