Richard Penty

Continuous-variable quantum key distribution (CV QKD) using optical coherent detectors is practically favorable due to its low implementation cost, flexibility of wavelength division multiplexing, and compatibility with standard coherent communication technologies. However, the security analysis and parameter estimation of CV QKD are complicated due to the infinite-dimensional latent Hilbert space. Also, the transmission of strong reference pulses undermines the security and complicates the experiments. In this work, we tackle these two problems by presenting a time-bin-encoding CV protocol with a simple phase-error-based security analysis valid under general coherent attacks. With the key encoded into the relative intensity between two optical modes, the need for global references is removed. Furthermore, phase randomization can be introduced to decouple the security analysis of different photon-number components. We can hence tag the photon number for each round, effectively estimate the associated privacy using a carefully designed coherent-detection method, and independently extract encryption keys from each component. Simulations manifest that the protocol using multi-photon components increases the key rate by two orders of magnitude compared to the one using only the single-photon component. Meanwhile, the protocol with four-intensity decoy analysis is sufficient to yield tight parameter estimation with a short-distance key-rate performance comparable to the best Bennett-Brassard-1984 (BB84) implementation.

Running single photon detectors (such as single photon avalanche photodiodes or superconducting nanowire detectors) at high speed is challenging and requires innovative driving and signal processing electronics. Self-differencing has been shown to be a very successful method to achieve detection rates in excess of 1 GHz using APDs while keeping low error rates from dark counts and afterpulses. Here, we present first results of high bit-rate QKD using a novel compact self-differencing setup which is integrated completely on a single printed circuit board (PCB). We achieve bit rates higher than 1 Mbit/s for a fibre distance of 50 km.