Emitter, communication system and method

1. An emitter configured to output a sequence of periodic light pulses with different polarisations, the emitter comprising:

a beam splitter configured to divide the pulses of a first sequence of pulses, such that each pulse is split between a first path and a second path, the first sequence of pulses having a varying phase and a first polarisation;

a polarisation rotator configured to rotate the polarisation state of pulses in one of the first path or the second path with respect to the polarisation state of pulses in the other path;

a time delay component configured to provide a time delay such that the first sequence of pulses in the first path are delayed by one period with respect to the first sequence of pulses in the second path;

an optical combination component configured to combine the delayed first sequence of pulses from the first path with the first sequence of pulses from the second path to produce an output sequence of pulses where each pulse in the output sequence is a combination of a pulse from the second path and a delayed pulse from the first path and has a polarisation determined from the phase difference between combined pulses and the polarisation of the first path and the second path.

2. The emitter of claim 1 , the emitter further comprising a first source configured to generate the first sequence of pulses of light having a varying phase and a first polarisation.

3. The emitter of claim 2 , further comprising a controller for said first source wherein said controller is configured to cause said first source to output pulses where pairs of pulses have a phase difference that allows one of a set number of phase differences to be randomly selected.

4. The emitter of claim 3 , wherein said controller is configured to cause said first source to output pairs of pulses having a set phase difference randomly selected from one of 0, π/2, −π/2 and π.

5. The emitter of claim 3 , wherein the controller is configured to cause said first source to output pairs of pulses with said set phase difference and for a random phase difference to be provided between the last pulse of one pair and the first pulse of the following pair.

6. The emitter of claim 3 , wherein the first source is a gain switched laser and said controller is configured to modify the gain applied to the first source.

7. The emitter of claim 6 , further comprising at least one further gain switched laser which is configured as a secondary laser and receives a seeding pulse from said first source.

8. The emitter of claim 7 , wherein the secondary laser is provided prior to said beam splitter such that the sequence of pulses output by the secondary laser is divided by said beam splitter.

9. The emitter of claim 7 , wherein the secondary laser is provided by a first gain switched secondary laser and a second gain switched secondary laser, wherein the first secondary laser is provided in the first path and the second secondary laser is provided in the second path.

10. The emitter of claim 9 , wherein a secondary laser controller is provided to control the first secondary laser not to emit a pulse when it receives the second pulse of a pair of pulses with the set phase difference and to control the second secondary laser not to emit a pulse when it receives the first pulse of the pair of pulses with the set phase difference.

11. The emitter of claim 1 , wherein the first path and second path comprise polarisation maintaining optical fibre and the polarisation rotator is provided by rotating the fibre birefringent axis in one path.

12. The emitter of claim 1 , wherein the optical combination component is a polarising beam splitter.

13. The emitter of claim 1 , further comprising a waveplate provided to rotate the polarisation of the output sequence of pulses.

14. The emitter of claim 1 , wherein the polarisation rotator is configured to rotate the polarisation state of pulses in one of the first path or the second path with respect to the polarisation state of pulses such that pulses in one path have an orthogonal polarisation to the pulses in the other path.

15. The emitter of claim 1 , wherein the emitter is configured to output pulses randomly selected from two polarisation basis.

16. The emitter of claim 15 , wherein the emitter is configured to output one of four states, said four states selected from two orthogonal states in each polarisation basis.

17. The emitter of claim 16 , wherein the emitter comprises at least one gain switched laser and said four states are selected by randomly switching the gain of the laser between values that allow the output of said four states.

18. A communication system comprising an emitter according to claim 15 , and a receiver, said receiver being configured to receive pulses from said emitter and measure said pulses in a first or a second polarisation basis.

19. A method of emitting a sequence of periodic light pulses with different polarisations, the method comprising:

receiving a first sequence of pulses of light having a varying phase and a first polarisation,

dividing the pulses of the first sequence, such that the pulse from each beam is split between a first path and a second path,

rotating the polarisation state of pulses in one of the first path or the second path with respect to the polarisation state of pulses in the other path;

delaying in time the first sequence of pulses in the first path by one period with respect to the first sequence of pulses in the second path;

combining the delayed first sequence of pulses from the first path with the first sequence of pulses from the second path to produce an output sequence of pulses where each pulse in the output sequence is a combination of a pulse from the second path and a delayed pulse from the first path and has a polarisation determined from the phase difference between combined pulses and the polarisation of the first path and the second path.

20. A quantum communication method, the method comprising:

emitting a sequence of periodic light pulses with different polarisations according to the method of claim 19 , wherein pulses in the sequence of periodic light pulses with different polarisations, comprise polarisations randomly selected from two polarisation basis; and

receiving at a receiver the sequence of periodic light pulses with different polarisations and varying the polarisation measurement basis in the receiver.