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Applied Optics

Experimental demonstration of a local active phase compensation method for a phase encoding quantum key distribution system

Yue Zhang, Junyue Yin, Huiqing Zhao, Jindong Wang, Ruili Ma, Zihao Liu, Jiahao Wei, Ya‐Fei Yu, Zhengjun Wei, Zhi‐Ming Zhang

Published Aug 19, 2022• 2 citations
Publisher

Abstract

An efficient phase stabilization method is required in quantum key distribution (QKD) systems for stability in practical applications. The existing active phase compensation method has limitations in multi-node network applications, especially in network-scale applications based on measurement-device-independent QKD systems. In this study, we propose a local active phase compensation scheme that can realize phase compensation independently for each interferometer node. We performed experimental demonstrations in the BB84 phase encoding system based on a Faraday-Michelson interferometer. The average QBER rates of the system under two different forms of the reference light were found to be 1.9% and 1.6%. This scheme can also be applied to other QKD systems and has potential for application in future quantum communication networks.

DOI
10.1364/ao.457735

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An efficient phase stabilization method is required in quantum key distribution (QKD) systems for stability in practical applications. The existing active phase compensation method has limitations in multi-node network applications, especially in network-scale applications based on measurement-device-independent QKD systems. In this study, we propose a local active phase compensation scheme that can realize phase compensation independently for each interferometer node. We performed experimental demonstrations in the BB84 phase encoding system based on a Faraday-Michelson interferometer. The average QBER rates of the system under two different forms of the reference light were found to be 1.9% and 1.6%. This scheme can also be applied to other QKD systems and has potential for application in future quantum communication networks.
Quantum key distribution (QKD) system must be robust enough in practical communication. The system performance is notably affected by phase drift. For the fiber-based phase-encoded quantum key distribution system, the Faraday-Michelson QKD scheme can auto-compensate the birefringence of fiber, but phase drift caused by environment variations is still a serious problem in practical operation. In this paper, the major schemes to compensate for the phase drift are analyzed in detail and an “five-phase”effective active phase compensation method is proposed. The comparison with other two major active phase compensation schemes also given. The result shows that this method uses less time to acquire the parameter of phase drift with the same precision and it can be used in phase-encoded QKD systems without introducing additional devices.
Measurement-device-independent quantum key distribution (MDI QKD) is a substantial step toward practical information-theoretic security for key sharing between remote legitimate users (Alice and Bob). As with other standard device-dependent quantum key distribution protocols, such as BB84, MDI QKD assumes that the reference frames have been shared between Alice and Bob. In practice, a nontrivial alignment procedure is often necessary, which requires system resources and may significantly reduce the secure key generation rate. Here, we propose a phase-coding reference-frame-independent MDI QKD scheme that requires no phase alignment between the interferometers of two distant legitimate parties. As a demonstration, a proof-of-principle experiment using Faraday-Michelson interferometers is presented. The experimental system worked at 1 MHz, and an average secure key rate of 8.309 bps was obtained at a fiber length of 20 km between Alice and Bob. The system can maintain a positive key generation rate without phase compensation under normal conditions. The results exhibit the feasibility of our system for use in mature MDI QKD devices and its value for network scenarios.
Phase drift is an inevitable problem in the practical implementation of phase-coding quantum key distribution (QKD) systems. Conventional active phase tracking and compensation solutions cannot be implemented during the key transmission process, resulting in reduced efficiency of the system. In this paper, we propose a single-photon level real-time phase tracking scheme using only the estimated quantum bit error rate (QBER) and mismatched-basis data to acquire the phase drift parameter instead of a phase scanning process or reference light pulses, without extra hardware or interrupting the transmission of quantum signals. This scheme is applied into a phase-coding QKD system with active phase disturbance and obtains an average QBER of 1.01%1.01\%1.01% with a standard derivation of 0.37%0.37\%0.37% over 50 h of continuous operation. Experimental results show that this scheme enables QKD systems to operate continuously with long-term stability and keep a low level of QBER even with rapid phase drift, suggesting its suitability for practical applications.
Quantum key distribution (QKD) enables information-theoretically secure communication against eavesdropping. However, phase instability remains a challenge across many QKD applications, particularly in schemes such as twin-field QKD and measurement-device-independent QKD. The most dominant source of phase fluctuation arises from the frequency offset between independent lasers. Here we propose a method to address this issue by employing a classical photodiode to compensate for the laser frequency difference. As an application of this method, we implement this technique in a mode-pairing QKD system, achieving an error rate approaching the theoretical limit and surpassing the linear key-rate bound over a fiber distance of 296.8 km. This approach provides a practical solution for frequency matching between independent lasers and can be extended to other fields requiring precise phase stabilization.
Phase drift is an inherent problem in phase-encoded quantum key distribution (QKD) systems. The current active phase tracking and compensation solutions cannot satisfy the requirements of a system with nonlinearity in phase modulation. This paper presents a four-phase scanning method, which is based on the quantitative analysis of the quantum bit error rate (QBER) from phase drift and the performance requirements of phase compensation. By obtaining the four interference fringes and adjusting the coding matrix of the system, this method automatically calculates the accurate driving voltages for the phase modulator. The implementation and experimental tests show that the proposed method can compensate phase drift caused by environmental changes and the system’s nonlinearity, and is applicable to large-scale QKD networks.
Phase coding is frequently used in current quantum key distribution(QKD) systems.It has posed many technical challenges on the design of the control electronics.After the description to the overall design of our phase coding QKD system,the paper focuses on the special requirements and implementations on the synchronizing mechanism and automatic phase drift scanning and compensation.The electronics designed in this paper combining with the optical devices can be an independent node device of a QKD network.We have successfully used these node devices to build a practical encrypted communication system through the commercial communication optical fiber system of Wuhu Telecom in Anhui Province.Encrypted communication between multiple nodes by quantum cryptography is implemented in this system.The QKD system in the network actualizes BB84 phase coding protocol at 20MHz repetition rate of light pulses.It achieves quantum bit error rate(QBER)2.95% and secure key generation rate 4.91kbps with a 20km transmission distance.