Optical transmitter, a quantum communication system and a method of operating an optical transmitter

A quantum communication system for quantum key distribution according to a quantum cryptography technique, the system comprising:

• an optical transmitter;
• an optical receiver;
• and
• an optical channel configured to receive optical pulses from an output coupler of the optical transmitter and to propagate the optical pulses to the optical receiver,
• wherein the optical transmitter comprises
• a first laser and a second laser, each laser configured to emit a respective plurality of optical pulses such that a phase of each pulse of the respective plurality of optical pulses is randomized;
• an adjustable optical combiner configured to combine, based on an adjustable combining ratio, the pulses emitted by the first laser and second laser into a combined stream of pulses;
• an asymmetric interferometer with a short arm and a long arm, the interferometer having an input port configured to receive the combined stream of pulses and an output port configured to provide an output of the interferometer;
• and
• the output coupler configured to provide the output of the asymmetric interferometer to a quantum channel,
• wherein at least one of the short and long arm comprises a phase controlling element configured to control a relative phase between the short and the long arm, and
• the adjustable optical combiner comprises an interferometer having two arms of equal length and an adjustable phase controlling element in at least one arm, the combining ratio of the combiner being adjustable by adjusting the adjustable phase controlling element.

A method of operating an optical transmitter for quantum key distribution according to a quantum cryptography technique, the optical transmitter comprising

• a first laser and a second laser, each laser configured to emit a respective plurality of optical pulses such that a phase of each pulse of the respective plurality of optical pulses is randomized;
• an adjustable optical combiner configured to combine, based on an adjustable combining ratio, the pulses emitted by the first laser and second laser into a combined stream of pulses;
• an asymmetric interferometer with a short arm and a long arm, the interferometer having an input port configured to receive the combined stream of pulses and an output port configured to provide an output of the interferometer;
• and
• an output coupler configured to provide the output of the asymmetric interferometer to a quantum channel,
• wherein at least one of the short and long arm comprises a phase controlling element configured to control a relative phase between the short and the long arm, and
• the adjustable optical combiner comprises an interferometer having two arms of equal length and an adjustable phase controlling element in at least one arm, the combining ratio of the combiner being adjustable by adjusting the adjustable phase controlling element,
• the method comprising:
• selecting one of:
• (i) emitting a pulse from the first laser of the optical transmitter, and
• (ii) emitting a pulse from the second laser of the optical transmitter, and emitting a pulse based on the selection.

A method of operating an optical transmitter for quantum key distribution according to a quantum cryptography technique, the optical transmitter comprising

• a first laser and a second laser, each laser configured to emit a respective plurality of optical pulses such that a phase of each pulse of the respective plurality of optical pulses is randomized;
• an adjustable optical combiner configured to combine, based on an adjustable combining ratio, the pulses emitted by the first laser and second laser into a combined stream of pulses;
• an asymmetric interferometer with a short arm and a long arm, the interferometer having an input port configured to receive the combined stream of pulses and an output port configured to provide an output of the interferometer;
• and
• an output coupler configured to provide the output of the asymmetric interferometer to a quantum channel,
• wherein at least one of the short and long arm comprises a phase controlling element configured to control a relative phase between the short and the long arm, and
• the adjustable optical combiner comprises an interferometer having two arms of equal length and an adjustable phase controlling element in at least one arm, the combining ratio of the combiner being adjustable by adjusting the adjustable phase controlling element,
• the method comprising:
• selecting one of:
• (i) emitting a pulse from the first laser of the optical transmitter;
• (ii) emitting a pulse from the second laser of the optical transmitter, and
• (iii) emitting a vacuum state pulse from each of the first and second lasers of the optical transmitter, and
• emitting a pulse based on the selection.