Patent ID: 7853020

Claim:
A method of forming a quantum key by exchanging quantum signals between first and second QKD stations of an actively stabilized QKD system that has a system visibility, comprising: measuring the system visibility between the first and second QKD stations during the exchanging of quantum signals, using an unmodulated, non overlapping classical signal multiplexed with the quantum signals; generating an error signal S E representative of a transient degradation in the system visibility; setting an error-signal threshold S TH that defines an upper bound on the transient degradation of the system visibility; detecting the quantum signals to form qubits based on the quantum signals; and forming a raw quantum key by storing only below-threshold qubits for which S E ≦S TH , wherein the QKD system is a phase-encoding system and the quantum signal error is a phase error, generating a final quantum key from the raw quantum key by sifting, error-correcting and privacy amplifying the raw key.