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

1 . An optical transmitter for quantum key distribution according to a quantum cryptography technique, the 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.

2 . The optical transmitter of claim 1 , wherein the first and the second lasers, the optical combiner and the asymmetric interferometer are formed on an integrated chip.

3 . The optical transmitter of claim 1 , wherein the adjustable optical combiner further comprises an intensity control element configured to adjustably modulate an intensity of the pulses emitted by the first laser, the combining ratio of the combiner being adjustable by adjusting the modulation of the intensity control element.

4 . The optical transmitter of claim 3 , wherein the intensity control element is a first intensity control element, and the adjustable optical combiner further comprises a second intensity control element configured to adjustably modulate an intensity of the pulses emitted by the second laser, the combining ratio of the combiner being adjustable by adjusting the modulation of the first and second intensity control elements.

5 . The optical transmitter of claim 4 , wherein the first intensity control element and/or the second intensity control element comprises at least one of an electro-absorption modulator, a Mach-Zehnder modulator, a tuneable interferometer, and a semiconductor optical amplifier.

6 . The optical transmitter of claim 1 , wherein the combining ratio of the adjustable optical combiner is adjusted such that, at the output coupler, an intensity of the pulses emitted by the first laser is equal to a first predetermined intensity, and an intensity of the pulses emitted by the second laser is equal to a second predetermined intensity, the second predetermined intensity being different from the first predetermined intensity.

7 . The optical transmitter of claim 1 , wherein the quantum cryptography technique is a decoy state quantum cryptography technique, and the optical transmitter is configured to encode either signal states or decoy states in pulses emitted by the first laser encodes, and the other one of signal states or decoy states in pulses emitted by the second laser.

8 . The optical transmitter of claim 1 , further comprising a further phase controlling element configured receive the output of the asymmetric interferometer, control a phase of the output of the asymmetric interferometer, and provide the phase-controlled output of the asymmetric interferometer to the output coupler.

9 . The optical transmitter of claim 1 , wherein the optical transmitter is configured such that during each cycle of a clock rate at most one of the first and second lasers emits a pulse.

10 . The optical transmitter of claim 9 , wherein the optical transmitter is configured to perform, for each cycle of the clock rate and based on a random number, one of:

emitting a pulse from the first laser;

emitting a pulse from the second laser, and

emitting a vacuum pulse from each of the first and second lasers of the transmitter.

11 . The optical transmitter of claim 10 , further comprising a random number generator configured to generate the random number.

12 . The optical transmitter of claim 1 , wherein a time delay between the short arm and the long arm is 500 ps or less.

13 . The optical transmitter of claim 9 , wherein the clock rate is 1 GHz or greater.

14 . The optical transmitter of claim 1 , wherein the first and second lasers are gain-switched lasers.

15 . 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.

16 . The quantum communication system of claim 15 , wherein the optical receiver further comprises:

an asymmetric receiver interferometer with a short arm and a long arm, the receiver interferometer having an input port configured to receive the optical pulses from the quantum channel and an output port configured to provide an output of the asymmetric receiver interferometer, and

a single photon detector connected to the output port of the asymmetric receiver interferometer,

wherein a time delay between the short arm and the long arm of the asymmetric receiver interferometer is equal to a time delay between the short arm and the long arm of the interferometer of the optical transmitter.

17 . The quantum communication system of claim 16 , wherein at least one of the short and long arm of the asymmetric receiver interferometer comprises a receiver phase controlling element configured to control a relative phase between the short and the long arm of the asymmetric receiver interferometer.

18 . 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.

19 . 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.