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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Synchronization schemes for continuous-variable quantum key distribution

Changjiang Liu, Yijia Zhao, Yichen Zhang, Ziyong Zheng, Song Yu

Published Jan 5, 2017• 2 citations
Publisher

Abstract

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.

DOI
10.1117/12.2264528

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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 a practical continuous variable quantum key distribution (CVQKD) system, frame synchronization is crucial for its operation, especially in conditions of low signal-to-noise ratio (SNR) and phase drift. This paper introduces a robust frame synchronization scheme for CVQKD systems that only utilizes quantum signals. The proposed scheme effectively employs randomly selected segments of quantum signals to achieve frame synchronization, eliminating the need for additional modulation. The performance of this scheme applied in a local local oscillator scenario is thoroughly analyzed through numerical simulations. The results demonstrate that the proposed scheme is capable of withstanding low SNR and arbitrary slow phase drift, as well as fast phase drift originates from two independent lasers, while also slightly improving the secret key rate compared to the scheme using inserted synchronization frames. These findings validate the feasibility of implementing the proposed scheme for long-distance CVQKD in practical scenarios.
It is well known that accurate clock synchronization is the backbone of quantum information processing tasks. Continuous-variable quantum key distribution (CV QKD), when proposed, benefited from its natural synchronization signals, i.e., local oscillator (LO), which provides a precise and automatic clock synchronization without a special clock board. The application of LO, however, incurs security loopholes, exposing the system to the risk of being attacked. We resolve this unsafe synchronization by proposing a virtual clock synchronization method for the CV QKD protocol without sending a real LO. The proposed method, which is based on the interpolation algorithm, extracts the clock information from the measured quadrature of the quantum signal rather than the classical LO. By formulating the measured quadrature as the synchronization signal, Bob recovers the clock information without both security loopholes and a specific clock board. Numerical results test the robustness of our method and show its potential in a real CV QKD implementation.
Three clock synchronization schemes for a quantum key distribution system are compared experimentally through the outdoor fibre and the interaction physical model of the the clock signal and the the quantum signal in the quantum key distribution system is analysed to propose a new synchronization scheme based on time division multiplexing and wavelength division multiplexing technology to reduce quantum bits error rates under some transmission rate conditions. The proposed synchronization scheme can not only completely eliminate noise photons from the bright background light of the the clock signal, but also suppress the fibre nonlinear crosstalk.
We propose a carrier synchronization scheme for continuous-variable measurement-device-independent quantum key distribution (CV-MDI-QKD) with a real local oscillator (LO). The middle node Charlie evaluates carrier frequency offsets and phase drift values based on the pilot signal sent by the legal parties Alice and Bob and a part of their public data. Alice and Bob then calibrate their modulation data according to the evaluation parameters to achieve carrier synchronization. We simulate the entire CV-MDI-QKD protocol and verify the feasibility of this scheme. The influences of pilot segment size and public data size on the excess noise are also analyzed. Besides, we experiment to measure the actual frequency offsets and phase drifts in the channel, which gives guidance on the actual implementation. This solution can ensure the implementation of CV-MDI-QKD with a real LO, paving the way for the practical application of this high-rate QKD protocol.
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.
Considering a practical continuous variable quantum key distribution(CVQKD) system, synchronization is of significant importance as it is hardly possible to extract secret keys from unsynchronized strings. In this paper, we proposed a high performance frame synchronization method for CVQKD systems which is capable to operate under low signal-to-noise(SNR) ratios and is compatible with random phase shift induced by quantum channel. A practical implementation of this method with low complexity is presented and its performance is analysed. By adjusting the length of synchronization frame, this method can work well with large range of SNR values which paves the way for longer distance CVQKD.