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IET conference proceedings.

Pilot-free digital clock synchronization for continuous-variable quantum key distribution

Patrick Matalla, M. S. Mahmud, Christian G. Koos, Sebastian Randel

Published Nov 13, 2023• 3 citations
Publisher

Abstract

A feedforward blind digital clock synchronization method is investigated for continuousvariable quantum-key distribution. We demonstrate timing synchronization for an 1-GBd QPSK signal over a 20 km quantum channel at signal-to-noise ratios as low as -20 dB and various clock frequency offsets.
DOI
10.1049/icp.2023.2552

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Frame synchronization is a crucial step in quantum key distribution (QKD) and a prerequisite for secure key distillation. Due to the extremely low signal-to-noise ratio (SNR) and fast phase drift in the continuous-variable QKD system, efficient frame synchronization is highly demanded. In this paper, a frame-synchronization scheme based on weak coherent states is proposed. This scheme uses a train of coherent-state pulses encoded by a Zadoff-Chu sequence to achieve frame synchronization. The synchronization performance is analyzed under different transmission distances, transmit powers, frequency offsets, and phase drifts through Monte Carlo simulation. Its performance is also compared with other synchronization sequences, confirming its noise-tolerance capability. An optical experiment is conducted to verify the feasibility of the synchronization scheme under low SNR (as low as $\ensuremath{-}20$ dB), and the applicable SNR range can be further reduced by increasing the sequence length. Due to its efficiency and noise tolerance, this scheme can provide technical support for weak-coherent-state-based QKD and future quantum network construction.
Clock synchronization is crucial for a practical continuous-variable quantum key distribution system to precisely get the measurement result. Three different synchronization schemes for continuous-variable quantum key distribution system are presented to demonstrate the optimal scheme. The performance of synchronization scheme is evaluated by measuring the excess noise which is the critical parameter for the continuous-variable quantum key distribution system. The experiment results show that distilling the synchronization signal from the local oscillator has the simplest physical implemention and superior effect of synchronization, but a stronger local oscillator is required. Transmitting synchronization signal and quantum signal in the same fiber by wave-length division multiplex is also a fine way to provide stable clock when we take no account of the phsical device and wave-length source.
In practicable CV-QKD systems one major challenge is to synchronise the time scales of the free running electronics at Alice (A) and Bob's (B) site and the free running true local oscillator (TLO) in the coherent receiver. For the correct phase estimation of the discrete symbols and the mitigation of the phase noise of the lasers some phase reference is required. Reference signals can be multiplexed with the quantum signal in time, frequency or polarisation domain [1]. In some CV-QKD experiments this is achieved by using an extra wavelength which requires additional hardware [2]. In [3] we proposed the use of auxiliary signals in an 8-PSK 40 MBd system. The advantage of this pilot-assisted clock recovery is that it works independently of the quantum signal and without training symbols. On the other hand, the necessary second pilot occupies DAC and ADC bandwidth. In the receiver DSP constructive interference of both pilots must be realized. This requires knowledge of the clock ratio before phase noise compensation. To simplify the signal processing, we propose to use two very stable Rubidium clocks at A and B which can be disciplined with the time reference broadcast by GPS [4].