Transmitter for a quantum communication system, a quantum communication system and a method of generating intensity modulated photon pulses

1. A transmitter for a quantum communication system, comprising:

a photon source unit comprising a photon source; and

a first intensity modulator, configured to receive an input light pulse from the photon source unit, the first intensity modulator comprising:

a first element, configured to split the input light pulse into two components,

a phase modulator, configured to apply a phase shift between the two components, and

a second element, configured to interfere the two components,

wherein at least one of the first element and the second element has a splitting ratio less than 1, where the splitting ratio is a ratio of a signal intensity through a first port to a signal intensity through a second port, for a signal applied at a third port,

wherein the transmitter is configured to implement decoy state quantum communication,

wherein the photon source unit is configured to emit light pulses having a first intensity and a second intensity, the second intensity being lower than the first intensity, wherein the phase modulator comprises a first setting, corresponding to one of destructive interference or constructive interference and a second setting, corresponding to the other of destructive or constructive interference, and wherein the first intensity modulator outputs light pulses having three or more different intensities, dependent on selection between the first and second setting of the phase modulator and the first and second intensity of the photon source unit,

wherein a first of the three or more intensities is generated when the photon source unit is configured to emit light pulses having the first intensity and the phase modulator is configured to the first setting,

wherein a second of the three or more intensities is generated when the photon source unit is configured to emit light pulses having the first intensity and the phase modulator is configured to the second setting, and

wherein a third of the three or more intensities is generated when the photon source unit is configured to emit light pulses having the second intensity.

2. The transmitter according to claim 1 , wherein the second intensity corresponds to a vacuum state.

3. The transmitter according to claim 1 , wherein the first intensity modulator has an extinction ratio of less than or equal to 0.99.

4. The transmitter according to claim 1 , wherein the first intensity modulator has a variable extinction ratio.

5. The transmitter according to claim 1 , wherein the first element or the second element comprises an asymmetric beam splitter.

6. The transmitter according to claim 1 , wherein the first element or the second element comprises a tunable ratio splitter.

7. The transmitter according to claim 6 , wherein the tunable ratio splitter comprises a first symmetric beam splitter and a second symmetric beam splitter, wherein one output of the first symmetric beam splitter is coupled to one input of the second symmetric beam splitter forming a first arm, and the other output of the first symmetric beam splitter is coupled to the other input of the second symmetric beam splitter forming a second arm, wherein at least one of the first arm and the second arm comprises a phase modulator, configured to tune the splitting ratio of the tunable ratio splitter.

8. The transmitter according to claim 1 , wherein the first element comprises a polarisation controller and a polarizing beam splitter, configured to split the input light pulse into the two components, the two components having different polarisations.

9. The transmitter according to claim 8 , wherein the second element comprises the polarising beam splitter.

10. The transmitter according to claim 1 , wherein the first element comprises a first polarisation controller and an interface between a polarization maintaining fibre and a single mode fibre, configured to split the input light pulse into the two components, the two components having different polarisations.

11. The transmitter according to claim 10 , wherein the second element comprises the fibre interface, a second polarisation controller and a polarising beam splitter.

12. The transmitter according to claim 1 , wherein the photon source unit comprises a pulsed laser, wherein the first intensity corresponds to an “on” state of a gain switched laser and the second intensity corresponds to an “off” state of the laser.

13. The transmitter according to claim 1 , wherein the photon source unit comprises a laser and a second intensity modulator, wherein the second intensity modulator is symmetric and wherein the second intensity corresponds to a minimum transmission state of the second intensity modulator.

14. The transmitter according to claim 1 , wherein at least one part of the first element also forms at least one part of the second element.

15. The transmitter according to claim 1 , wherein the components are integrated onto a photonic chip.

16. A quantum communication system comprising:

the transmitter of claim 1 ; and

a receiver.

17. The quantum communication system according to claim 16 , further comprising a second transmitter.

18. A method of generating intensity modulated photon pulses performed by a transmitter, the method comprising:

generating light pulses at a photon source unit comprising a photon source;

receiving an input light pulse from the photon source unit at a first intensity modulator;

splitting the input light pulses into two components at a first element in the first intensity modulator;

applying a phase shift between the two components at a phase modulator in the first intensity modulator; and

interfering the two components at a second element in the first intensity modulator, wherein at least one of the first element and the second element has a splitting ratio less than 1, where the splitting ratio is the ratio of a signal intensity through a first port to a signal intensity through a second port, for a signal applied at a third port,

wherein the transmitter is configured to implement decoy state quantum communication;

wherein the photon source unit is configured to emit light pulses having a first intensity and a second intensity, the second intensity being lower than the first intensity, wherein the phase modulator comprises a first setting, corresponding to one of destructive interference or constructive interference and a second setting, corresponding to the other of destructive or constructive interference, and wherein the first intensity modulator outputs light pulses having three or more different intensities, dependent on selection between the first and second setting of the phase modulator and the first and second intensity of the photon source unit,

wherein a first of the three or more intensities is generated when the photon source unit is configured to emit light pulses having the first intensity and the phase modulator is configured to the first setting,

wherein a second of the three or more intensities is generated when the photon source unit is configured to emit light pulses having the first intensity and the phase modulator is configured to the second setting, and

wherein a third of the three or more intensities is generated when the photon source unit is configured to emit light pulses having the second intensity.