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Register a new userHomodyne Detection Quadrature Phase Shift Keying Continuous-Variable Quantum Key Distribution with High Excess Noise Tolerance
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Discrete-modulated continuous-variable quantum key distribution with homodyne detection is widely known for the simplicity on implementation, the efficiency in error correction and the compatibility with modern optical communication devices. However, recent work indicates that using homodyne detection will lead to poor tolerance of excess noise and insufficient transmission distance, hence seriously restricting the large-scale deployment of quantum secure communication networks. Here, we propose a homodyne detection protocol using the technique of quadrature phase shift keying. By limiting information leakage, our protocol enhances excess noise tolerance to a high level. Furthermore, we demonstrate that using homodyne detection performs better than heterodyne detection in quaternary-modulated continuous-variable quantum key distribution under the untrusted detector noise scenario. The security is analyzed by tight numerical method against collective attacks in the asymptotic regime. Results imply that our protocol possesses the ability to distribute keys in nearly intercity area. This progress will make our protocol the main force in constructing low-cost quantum secure communication networks.
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Most quantum key distribution (QKD) protocols could be classified as either a discrete-variable (DV) protocol or continuous-variable (CV) protocol, based on how classical information is being encoded. We propose a protocol that combines the best of both worlds: the simplicity of quantum state preparation in DV protocols as well as the cost-effective and high bandwidth of homodyne detectors that are normally used in CV protocols. In addition, our protocol does not require the honest parties to share the same reference phase, in contrast to typical CV-QKD protocols. We then prove the security of the proposed protocol in the asymptotic limit under the assumption of collective attacks. Our simulation suggests that the protocol is suitable for secure and high-speed practical key distribution over short distances.
The value of residual phase noise, after phase compensation, is one of the key limitations of performance improvement for continuous-variable quantum key distribution using a local local oscillator (LLO CV-QKD) system, since it is the major excess noise. However, due to the non-ideality of the realistic devices implemented in practice, for example, imperfect lasers, detectors and unbalanced interferometers, the value of residual phase noise in current system is still relatively large. Here, we develop a phase noise model to improve the phase noise tolerance of the LLO CV-QKD schemes. In our model, part of the phase-reference measurement noise associated with detection efficiency and electronic noise of Bobs detector as well as a real-time monitored phasereference intensity at Bobs side is considered trusted because it can be locally calibrated by Bob. We show that using our phase noise model can significantly improve the secure key rate and transmission distance of the LLO CV-QKD system. We further conduct an experiment to substantiate the superiority of the phase noise model. Based on experimental data of a LLO CV-QKD system in the 25 km optical fiber channel, we demonstrate that the secure key rate under our phase noise model is approximately 40% higher than that under the conventional phase noise model.
Information reconciliation is crucial for continuous-variable quantum key distribution (CV-QKD) because its performance affects the secret key rate and maximal secure transmission distance. Fixed-rate error correction codes limit the potential applications of the CV-QKD because of the difficulty of optimizing such codes for different low SNRs. In this paper, we propose a rateless reconciliation protocol combined multidimensional scheme with Raptor codes that not only maintains the rateless property but also achieves high efficiency in different SNRs using just one degree distribution. It significantly decreases the complexity of optimization and increases the robustness of the system. Using this protocol, the CV-QKD system can operate with the optimal modulation variance which maximizes the secret key rate. Simulation results show that the proposed protocol can achieve reconciliation efficiency of more than 95% within the range of SNR from -20 dB to 0 dB. It also shows that we can obtain a high secret key rate at arbitrary distances in a certain range and achieve a secret key rate of about 5*10^(-4) bits/pulse at a maximum distance of 132 km (corresponding SNR is -20dB) that is higher than previous works. The proposed protocol can maintain high efficient key extraction under the wide range of SNRs and paves the way toward the practical application of CV-QKD systems in flexible scenarios.
We analyze the effect of phase fluctuations in an optical communication scheme based on collective detection of sequences of binary coherent state symbols using linear optics and photon counting. When the phase noise is absent, the scheme offers qualitatively improved nonlinear scaling of the spectral efficiency with the mean photon number in the low-power regime compared to individual detection. We show that this feature, providing a demonstration of superaddivitity of accessible information in classical communication over quantum channels, is preserved if random phases imprinted on transmitted symbols fluctuate around a reference fixed over the sequence length.
The shot-noise unit in continuous-variable quantum key distribution plays an important and fundamental role in experimental implementation as it is used as a normalization parameter that contribute to perform security analysis and distill the key information. However, the traditional calibration procedure and detector model can not cover all system noise in practical application, which will result in some loopholes and influence the practical security. Whats more, the traditional procedure is also rather complicated and has difficulty in compatible with automatic operating system. In this paper we propose a calibration model based on the proposed trusted detector model, which could naturally close the loopholes in practical application. It can help identify the shot-noise unit in only one step, which can not only effectively simplify the evaluation process but also reduce the statistical fluctuation, while two steps are needed in traditional method. We prove its feasibility and derive the complete version of the corresponding entanglement-based model. Detailed security analysis against arbitrary collective attacks and numerous simulation results in both the asymptotic limit regime and the finite-size regime are provided. A proof-of-principle experiment has been implemented and the results indicate that the one-time-calibration model can be employed as a powerful substitution to calibrate the shot-noise unit. Our method paves the way for the deployment of continuous-variable quantum key distribution with real time calibration and automatic operation.
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