Dual rail compensation in phase encoded communication

1. An apparatus for optical communication and quantum key distribution comprising:

a phase modulator configured to modify a phase of light incoming into the phase modulator to generate phase-encoded light;

a dual rail encoder configured to modify the phase-encoded light from the phase modulator in dependence on a control signal based on feedback data relating to polarization rotation from a receiver, thereby imparting a polarization rotation compensation into the phase-encoded light; and

a polarization rotator combiner coupled to the dual rail encoder, the polarization rotator combiner being configured to convert dual rail light from the dual rail encoder into polarization encoded light,

wherein the apparatus is arranged to transmit the polarization encoded light to the receiver.

2. The apparatus according to claim 1 , wherein the phase modulator is further configured to modify an amplitude of light incoming into the phase modulator.

3. The apparatus according to claim 1 , further comprising:

an attenuator configured to attenuate at least one of the phase-encoded light, the dual rail light and the polarization encoded light.

4. The apparatus according to claim 1 , further comprising:

a controller configured to receive the feedback data from the receiver, and to generate the control signal at least in part in dependence on the feedback data.

5. The apparatus according to claim 1 , further comprising:

a pulse modulator configured to pulse modulate light and to provide, directly or indirectly, pulse modulated light into the phase modulator,

wherein the dual rail encoder is configured to receive the phase encoded light onto a first rail of the dual rail encoder and a reference signal onto a second rail of the dual rail encoder.

6. The apparatus according to claim 5 , further comprising:

a switch configured to provide time multiplexing by alternatingly providing light from the pulse modulator to the second rail of the dual rail encoder and to the phase modulator.

7. The apparatus according to claim 5 further comprising:

a delay line arranged to provide time multiplexing by delaying progression of light from the pulse modulator toward the second rail of the dual rail encoder.

8. The apparatus according to claim 1 , further comprising:

a switch configured to provide polarization multiplexing by alternatingly providing pulse modulated light from a pulse modulator to the phase modulator and to a second phase modulator, the phase modulator corresponding to state |1> and being coupled to provide its output to a first rail of the dual rail encoder and the second phase modulator corresponding to state |0> and being coupled to provide its output to a second rail of the dual rail encoder.

9. The apparatus according to claim 1 , further comprising:

a qubit |0> and |1> relative amplitude encoder and a delay line configured to provide polarization multiplexing by delaying progression of pulse modulated light from a pulse modulator toward a second rail of the dual rail encoder, corresponding to state |0>,

wherein the phase modulator corresponds to state |1> and is coupled to provide its output to a first rail of the dual rail encoder.

10. An apparatus for optical communication and quantum key distribution comprising:

a polarization splitter rotator configured to convert polarization encoded light incoming into the apparatus into dual rail light, and

an optical receiver configured to receive the dual rail light, at least in part, and to determine a bit sequence phase encoded into the dual rail light,

wherein the optical receiver is configured to generate and to send feedback data relating to polarization rotation based on the dual rail light to a dual rail encoder disposed in one of an emitter or the apparatus and arranged to impart a polarization rotation compensation.

11. The apparatus according to claim 10 further comprising:

a dual rail state analyser configured to generate feedback data relating to polarization rotation based on the dual rail light to a dual rail encoder,

wherein the dual rail state analyser is coupled to receive a first part of the dual rail light and the optical receiver is arranged to receive a second part of the dual rail light.

12. The apparatus according to claim 10 , further comprising:

a dual rail encoder between the polarization splitter rotator and the optical receiver and arranged to impart a polarization rotation compensation onto the dual rail light generated by the dual rail encoder in dependence of the feedback data originating in the optical receiver.

13. The apparatus according to claim 12 , further comprising:

at least one optical waveguide arranged to convey dual rail light from the polarization splitter rotator, bypassing the dual rail encoder, to at least one auxiliary detector thereby to enable modification of the feedback data.

14. A method for optical communication and quantum key distribution comprising:

modifying a phase of light in a phase modulator to generate phase-encoded light;

modifying, in dual rail form, the phase-encoded light from the phase modulator in dependence on a control signal based on feedback data relating to polarization rotation from a receiver, thereby imparting a polarization rotation compensation into the phase-encoded light;

converting the dual rail light into polarization encoded light; and

transmitting the polarization encoded light to the receiver.

15. The method according to claim 14 , further comprising:

modifying an amplitude of light incoming into the phase modulator.

16. The method according to claim 14 , further comprising:

attenuating at least one of the phase-encoded light, the dual rail light and the polarization encoded light.

17. The method according to claim 14 , further comprising:

receiving feedback data from a receiver, and generating the control signal at least in part in dependence of the feedback data.

18. The method according to claim 14 , further comprising:

pulse modulating light and providing, directly or indirectly, pulse modulated light into the phase modulator; and

receiving the phase encoded light onto a first rail of the dual rail encoder and receiving a reference signal onto a second rail of the dual rail encoder.

19. A method for optical communication and quantum key distribution comprising:

converting polarization encoded light incoming into an apparatus into dual rail light;

receiving the dual rail light, at least in part, and determining a bit sequence phase encoded into the dual rail light; and

generating and sending feedback data relating to polarization rotation based on the dual rail light to a dual rail encoder disposed in one of an emitter or the apparatus and arranged to impart a polarization rotation compensation.

20. A non-transitory computer readable medium having stored thereon a set of computer readable instructions for optical communication and quantum key distribution that, when executed by at least one processor, cause an apparatus to at least:

modify a phase of light in a phase modulator to generate phase-encoded light;

modify, in dual rail form, the phase-encoded light from the phase modulator in dependence on a control signal based on feedback data relating to polarization rotation from a receiver, thereby imparting a polarization rotation compensation into the phase-encoded light and to obtain dual rail light from the phase-encoded light;

convert the dual rail light into polarization encoded light; and

transmit the polarization encoded light to the receiver.