Phase reference sharing schemes for continuous-variable quantum cryptography

A system for coherent optical communication of quantum information between an emitter A and a distant receiver B connected by an optical channel, wherein emitter A comprises a laser LA and wherein receiver B comprises a laser LB used as a local local oscillator LLO to operate a coherent receiver, said system further comprising:

• at A, a laser LA configured to derive phase-coherently one or two optical pulses from a same optical wavefront at cycle i, said pulses being self-coherent with the laser LA;
• at A, a coherent multiplexed encoder to encode logical quantum information Qi and physical phase reference information Ri onto those one or two self-coherent optical pulses;
• an optical channel from A to B configured to send multiplexed Qi and Ri;
• at B, a multiplexed coherent measurement device configured to measure logical quantum information Qi and physical phase reference information Ri with said one or two self-coherent pulses derived from a single optical wavefront produced by laser LB and used as local oscillators to perform a coherent detection, thereby obtaining classical measurements Qmi of Qi and Rmi of Ri;
• at B, a device configured to determine the relative phase Phii at interference at cycle i between Qi optical pulse and corresponding LO pulse by estimating the relative phase between R(i) optical pulse and corresponding LO pulse;
• and
• at B, a device configured to determine logical quantum information Qi by correcting Qmi measurement using relative phase Phii estimation.

The system according to claim further comprising a heterodyne detector and a local-local oscillator LLO wherein:

• information sent from A to B is encoded on a single optical coherent state pulse, derived from the optical wavefront produced by LA at cycle i;
• quantum information Qi is encoded on both quadratures of the coherent state while phase reference information R(i) is encoded as a displacement of amplitude Delta of the quadrature in phase with LA at cycle i;
• a heterodyne detector measures both two quadratures of the received pulse at cycle i at wherein the laser LB is used as a local-local oscillator LLO to produce a coherent state pulse mode matched with the received pulse;
• and wherein quadrature measurement results are used to estimate the phase drift between LA and LB, and then to determine the quantum information Qi.

The system according to claim further comprising a splitter, an optical delay line and a homodyne or heterodyne detector wherein:

• quantum information Qi and phase reference information Ri are encoded onto two different optical coherent state pulses, this two pulses being generated by splitting onto a beamsplitter an optical pulse produced by LA (at cycle i) and by delaying the second pulse by a time DeltaT with respect to the first one, with an optical delay line;
• quantum information Qi is encoded on the quadratures of one of the pulses QPi;
• phase reference information Ri is encoded on the second pulse RPi whose quadrature value is in phase with LA and displaced by a value Delta, while its other quadrature value is not modulated;
• a measurement DQ of the quadratures of the received pulse QPi is performed at a homodyne or heterodyne detector at Bob at cycle i, with LB used as local-local oscillator to produce a coherent state pulse LLOQi mode-matched with QPi;
• a heterodyne measurement DR of the two quadratures of the received reference pulse RPi is performed at Bob at cycle i. The laser LB is used as local local oscillator LLO to produce a coherent state pulse LLORi that is obtained by delaying using a delayline a part of the wavefront used to produced LLOQi by at cycle i with a delay line;
• the quadrature measurement results obtained with DR are used to determine the phase drift Phii between LA and LB, said determined phase drift being then used to correct the quadrature measurement results of QP(i) and to determine the quantum information Qi.