Optical emitter, communication system and method

1. An optical emitter comprising:

a primary laser and a plurality of secondary lasers wherein each secondary laser is optically injection locked to said primary laser;

at least one polarization controller configured to control the polarization of output of at least one of the secondary lasers;

a combination unit that is configured to combine the outputs of the secondary lasers into an output signal; and

an optical power splitter configured to connect output of the primary laser to each of the secondary lasers.

2. The optical emitter according to claim 1 , wherein the primary laser and the secondary lasers are pulsed lasers configured to output optical pulses.

3. The optical emitter according to claim 2 , further comprising a selection unit configured to select one of said plurality of secondary lasers to contribute to the output signal at a time such that the output signal comprises a sequence of pulses wherein the sequence randomly switches between the outputs of the secondary lasers such that the polarization of the pulses change for each pulse.

4. The optical emitter according to claim 3 , wherein the primary laser and the plurality of secondary lasers are gain-switched lasers.

5. The optical emitter according to claim 4 , wherein the primary laser is controlled to emit pulses with a random phase between the pulses.

6. The optical emitter according to claim 4 , wherein the selection unit comprises a gain control unit that is configured to control the gain of the selected secondary laser to be above its lasing threshold.

7. The optical emitter according to claim 6 , wherein the gain control unit is configured to hold the non-selected secondary lasers below their lasing threshold.

8. The optical emitter according to claim 6 , wherein the gain control unit is configured to control the primary laser and the plurality of secondary lasers.

9. The optical emitter according to claim 8 , wherein the gain control unit is configured to increase the gain of a selected second secondary laser above its lasing threshold as or just after it receives a pulse from the primary laser.

10. The optical emitter according to claim 2 , further comprising an attenuator configured to attenuate the pulses output by the optical emitter such that they, on average, contain less than one photon.

11. The optical emitter according to claim 1 , wherein the secondary lasers are fixed polarization lasers.

12. The optical emitter according to claim 1 , wherein the polarization controller is selected from a wave-plate, a polarization maintaining optical fibre or a mount configured to mount one of the secondary lasers such that its polarization axis is rotated with respect to at least one other secondary laser.

13. The optical emitter according to claim 1 , wherein the optical emitter is configured to output polarization pulses where the polarization is randomly selected from two polarization basis.

14. The optical emitter according to claim 13 , comprising four secondary lasers and wherein the optical emitter is configured such that the output from each secondary laser when received at the combination unit is in one of four polarization states, wherein the four polarization states are two orthogonal states that form a first polarization basis and two orthogonal states that form a second polarization basis which is different to the first polarization basis.

15. A quantum communication system comprising the optical emitter according to claim 13 , and a receiver, said receiver being configured to receive pulses from said emitter and measure said pulses in a first or a second polarization basis.

16. The quantum communication system according to claim 15 , wherein the receiver is configured to passively select the polarization basis.

17. The quantum communication system according to claim 15 , wherein the receiver comprises a polarization beam splitter and a detector configured to measure output of said polarization beam splitter for each polarization basis.

18. A method of emitting an optical signal with varying polarization states, the method comprising:

emitting an optical signal from a primary laser;

directing, by an optical power splitter, the optical signal from said primary laser to a plurality of secondary lasers such that the optical signal from said primary laser optically injection locks said plurality of secondary lasers; and

combining outputs of the secondary lasers into an output signal,

wherein the output of at least one of the secondary lasers is rotated in polarization with respect to the output of at least one other secondary lasers prior to combining.

19. A quantum communication method, the method comprising:

emitting an optical signal with varying polarization states according to the method of claim 18 , wherein the polarization states are randomly selected from two polarization basis; and

receiving at a receiver the optical signal with varying polarization states and varying the polarization measurement basis in the receiver.

20. An optical emitter comprising:

a primary laser and a plurality of secondary lasers wherein each secondary laser is optically injection locked to said primary laser;

at least one polarization controller configured to control the polarization of output of at least one of the secondary lasers;

a combination unit that is configured to combine the outputs of the secondary lasers modules into an output signal;

wherein the primary laser and the secondary lasers are pulsed lasers configured to output optical pulses;

the optical emitter further comprising a selection unit configured to select one of said plurality of secondary lasers to contribute to the output signal at a time such that the output signal comprises a sequence of pulses wherein the sequence randomly switches between the outputs of the secondary lasers such that the polarization of the pulses change for each pulse;

wherein the primary laser and the plurality of secondary lasers are gain-switched lasers; and

wherein the primary laser is controlled to emit pulses with a random phase between the pulses.

21. A method of emitting an optical signal with varying polarization states, the method comprising:

emitting an optical signal from a gain-switched pulsed primary laser, wherein the gain-switched pulsed primary laser outputs optical pulses with a random phase between the optical pulses;

receiving said optical signal from said primary laser at a plurality of gain-switched pulsed secondary lasers such that the optical signal from said primary laser optically injection locks said plurality of secondary lasers;

emitting optical pulses from the secondary lasers;

combining outputs of the secondary lasers into an output signal,

wherein the output of at least one of the secondary lasers is rotated in polarization with respect to the output of at least one other secondary lasers prior to combining; and

selecting one of the plurality of secondary lasers to contribute to the output signal at a time such that the output signal comprises a sequence of pulses wherein the sequence randomly switches between the outputs of the secondary lasers such that the polarization of the pulses change for each pulse.

22. An optical emitter comprising:

a primary laser and a plurality of secondary lasers wherein each secondary laser is optically injection locked to said primary laser;

at least one polarization controller configured to control the polarization of output of at least one of the secondary lasers;

a combination unit that is configured to combine the outputs of the secondary lasers into an output signal;

wherein the primary laser and the secondary lasers are pulsed lasers configured to output optical pulses;

the optical emitter further comprising a selection unit configured to select one of said plurality of secondary lasers to contribute to the output signal at a time such that the output signal comprises a sequence of pulses wherein the sequence randomly switches between the outputs of the secondary lasers such that the polarization of the pulses change for each pulse;

wherein the primary laser and the plurality of secondary lasers are gain-switched lasers; and

wherein the selection unit comprises a gain control unit that is configured to control the gain of the selected secondary laser to be above its lasing threshold.

23. A method of emitting an optical signal with varying polarization states, the method comprising:

emitting an optical signal from a gain-switched pulsed primary laser, wherein the gain-switched pulsed primary laser outputs optical pulses;

receiving said optical signal from said primary laser at a plurality of gain-switched pulsed secondary lasers such that the optical signal from said primary laser optically injection locks said plurality of secondary lasers;

emitting optical pulses from the secondary lasers;

combining outputs of the secondary lasers into an output signal,

wherein the output of at least one of the secondary lasers is rotated in polarization with respect to the output of at least one other secondary lasers prior to combining;

selecting one of the plurality of secondary lasers to contribute to the output signal at a time such that the output signal comprises a sequence of pulses wherein the sequence randomly switches between the outputs of the secondary lasers such that the polarization of the pulses change for each pulse; and

controlling the gain of the selected secondary laser to be above its lasing threshold.