QKD stations with fast optical switches and QKD systems using same

A quantum key distribution (QKD) system for forming a quantum key, comprising:

• operably coupled first and second QKD stations configured to form the quantum key from quantum signals exchanged therebetween;
• in the first QKD station, a fast optical switch optically coupled to first and second round-trip optical paths, the fast optical switch including:
• an output port and first and second side ports;
• first and second Faraday mirrors optically coupled to the fast optical switch at the respective first and second side ports, thereby defining respective first and second round-trip optical paths having different optical path lengths;
• a phase modulator arranged in one of the optical paths and adapted to impart a selective random phase modulation to quantum signals passing therethrough;
• a light source optically coupled to the input port and adapted to generate quantum signals;
• and
• wherein the fast optical switch is switchable between a first operating state that divides an incident quantum signal into first and second quantum signals that travel over the respective first and second round-trip optical paths so as to return a modulated first quantum signal and an unmodulated second quantum signal to the optical switch, and second and third operating states that respectively direct the modulated first and unmodulated second quantum signals out of the output port.

A quantum key distribution (QKD) system for forming a quantum key, comprising:

• a first QKD station having a light source adapted to generate optical pulses, the light source being optically coupled to a first fast optical switch that is optically coupled to first and second round-trip optical paths having an optical path difference OPDA and that generates first and second quantum signals from each optical pulse;
• a second QKD station optically coupled to the first QKD station and having a second fast optical switch optically coupled to third and fourth round-trip optical paths that have an optical path difference OPDB=OPDA, the second QKD station including a single-photon detector unit optically coupled to the second fast optical switch;
• and
• wherein a plurality of first and second quantum signals from the first QKD station are received and interfered by the second fast-optical switch and detected by the single-photon detector unit based on phases selectively randomly imparted to the first and second quantum signals at the first and second QKD stations so as to be processed to form the quantum key.

A quantum key distribution (QKD) system for forming a quantum key, comprising:

• at a first QKD station:
• a first fast-switch interferometer arm adapted to generate first and second quantum signals from an optical pulse, selectively randomly encode one of the first and second quantum signals, and sequentially output the first and second quantum signals;
• at a second QKD station:
• a second fast-switch interferometer arm optically coupled to the first fast-switch interferometer arm and adapted to receive the first and second quantum signals from the first fast-switch interferometer arm, selectively randomly encode one of the first and second quantum signals, and interfere the first and second quantum signals to form an interfered quantum signal;
• a detector unit optically coupled to the second fast-switch interferometer arm to detect the interfered quantum signal;
• and
• a controller adapted to form the quantum key from a plurality of interfered quantum signals.

A method of performing quantum key distribution (QKD) to form a quantum key, comprising:

• sending optical pulses to a fast optical switch in a first QKD station;
• setting the fast optical switch to a first operating state that divides each optical pulse into first and second quantum signals;
• sending the first and second quantum signals over two different optical paths so that the first quantum signal returns to the fast optical switch before the second quantum signal, with at least one of the first and second quantum signals being selectively randomly modulated while traveling in at least one of the optical paths;
• switching the fast optical switch to the second operating state so that the first quantum signal is directed out of an output port of the fast optical switch;
• switching the fast optical switch to a third operating state so that the second quantum signal is directed out of the output port after the first quantum signal;
• and
• sending the first and second quantum signals to a second QKD station for processing to form the quantum key.

A method of interfering first and second quantum signals to form a quantum key in a quantum key distribution (QKD) system, comprising:

• sequentially receiving the first and second quantum signals sent from a first QKD station at a fast optical switch in a second QKD station;
• switching the fast optical switch between first and second operating states so as to send the first and second quantum signals over respective first and second optical paths that return the first and second quantum signals to the fast optical switch at the same time;
• switching the fast optical switch to a third operating state that forms an interfered quantum signal from the first and second quantum signals;
• and
• processing a plurality of interfered quantum signals to form the quantum key.