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.
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.
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.
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.
Abstract Frame synchronization is essential in continuous‐variable quantum key distribution (CVQKD), typically relying on active modulation to establish a specific frame structure. In passive CVQKD systems, however, the elimination of modulation requirements poses a significant challenge for frame synchronization, especially in local‐local‐oscillator (LLO) configurations. This paper introduces a robust, modulation‐free frame synchronization method tailored for passive LLO CVQKD systems. This method leverages the complex cross‐correlation of quantum signals exchanged between Alice and Bob for synchronization purposes. Validated through an analytical model and numerical simulations, this approach demonstrates high synchronization success rates, robustness to noise, and resilience to both slow phase drifts between local oscillators and quantum signals and fast phase drifts stemming from two independent lasers. These results underscore the viability of the proposed method for implementing long‐distance CVQKD in real‐world scenarios.
In continuous-variable quantum key distribution (CVQKD) systems, high-quality data synchronization between two legitimate parties, Alice and Bob, is the premise of the generation of shared secret keys. Synchronization with specially designed frames is an efficient way, but it requires special modulating devices to generate these special frames. Moreover, the extra requirement of special modulating devices makes it technically impossible for some passive preparation schemes. We propose a novel approach to realize synchronization in this paper, which is different from those special-frame-based methods. In our proposed scheme, Alice publishes parts of the original signals as the synchronization frames and Bob takes these frames to perform the synchronization algorithm. Besides, a synchronization feature is applied to deal with phase shifts. The simulation results based on practical data demonstrate that the proposed synchronization scheme not only maintains a high success rate but simplifies the data processing flow at the same time, which dramatically reduces the computational complexity.
We proposed a passive continuous-variable quantum key distribution (CVQKD) using a locally generated local oscillator (LLO) at reception. Different from the LLO-based Gaussian-modulated coherent-state quantum key distribution, our protocol is implemented by taking advantage of a multimode thermal source, beam splitters, optical attenuators, and homodyne detectors rather than the amplitude and phase modulators, which could be a cost-effective solution for LLO-based CVQKD. We adopt a general LLO noise model to analyze the performance of our protocol. That is to say, our consideration for noise in the proposed scheme contains the excess noise caused by passive state preparation, the phase noise, and the photon-leakage noise from the reference path. Because our protocol waives the necessity of the use of amplitude and phase modulators, there is no need to bear the effect of modulation noise caused by finite dynamics. Simulation results show that the improvement of the mode-overlap coefficient can suppress the excess noise caused by passive state preparation and thus enhance the performance of our protocol. Because lower relative phase drift and the photon-leakage noise can be obtained by using the multiplexing technique, we can still achieve reasonable performance of our protocol with the mode-overlap coefficient values which are very close to the experimental reference. Furthermore, we perform the finite-size analysis for the proposed protocol, which is more practical than that achieved in the asymptotic limit. This work confirms the feasibility of passive CVQKD implemented in the LLO configuration within the metropolitan area.
We propose a pilot-multiplexed continuous-variable quantum key distribution (CVQKD) scheme based on a local local oscillator (LLO). Our scheme utilizes time-multiplexing and polarization-multiplexing techniques to dramatically isolate the quantum signal from the pilot, employs two heterodyne detectors to separately detect the signal and the pilot, and adopts a phase compensation method to almost eliminate the multifrequency phase jitter. In order to analyze the performance of our scheme, a general LLO noise model is constructed. Besides the phase noise and the modulation noise, the photon-leakage noise from the reference path and the quantization noise due to the analog-to-digital converter (ADC) are also considered, which are first analyzed in the LLO regime. Under such general noise model, our scheme has a higher key rate and longer secure distance compared with the preexisting LLO schemes. Moreover, we also conduct an experiment to verify our pilot-multiplexed scheme. Results show that it maintains a low level of the phase noise and is expected to obtain a 554-Kbps secure key rate within a 15-km distance under the finite-size effect.