We present a pilot-assisted coherent intradyne reception methodology for CV-QKD with true local oscillator. An optically phase-locked reference tone, prepared using carrier-suppressed optical single-sideband modulation, is multiplexed in polarisation and frequency to the 250 Mbaud quantum signal in order to provide optical frequency- and phase matching between quantum signal and local oscillator. Our concept allows for high symbol rates and can be operated at an extremely low excess-noise level, as validated by experimental measurements.
QKD can be exercised with both, single photons or weak coherent states. The latter approach encodes the quantum information in the continuous phase and amplitude of the electromagnetic field of light and is therefore referred to as continuous-variable quantum key distribution (CV-QKD). As a main advantage of CV-QKD, this approach uses standard off-the-shelf components from the optical-telecommunication industry and can therefore be seamlessly integrated into existing telecom networks.
This article addresses an open problem in CV-QKD: to establish a stable phase- and frequency reference between transmitter and receiver (required for coherent detection) that does neither compromise the low noise level nor the symbol rate. As we believe, our method to co-transmit an optical reference signal, multiplexed to the quantum signal in modulation frequency and polarisation, constitutes a convincing answer to this problem. Harnessing the best practices from the highly advnced telecom industry, our transceiver operates at exceptionally high rate and low noise and thereby elevates QKD to a new level of technological maturity.
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