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Chinese Physics Letters

A Fully Symmetrical Quantum Key Distribution System Capable of Preparing and Measuring Quantum States*

Tianqi Dou, Jipeng Wang, Zhenhua Li, Wenxiu Qu, Shunyu Yang, Zhongqi Sun, Fen Zhou, Yanxin Han, Yuqing Huang, Haiqiang Ma

Published Nov 1, 2020• 2 citations
Publisher

Abstract

We propose a fully symmetrical QKD system that enables quantum states to be prepared and measured simultaneously without compromising system performance. Over a 25.6 km fiber channel, we demonstrate point-to-point QKD operations with asymmetric Mach–Zehnder interferometer modules. Two interference visibilities of above 99% indicate that the proposed system has excellent stability. Consequently, the scheme not only improves the feasibility of distributing secret keys, but also enables QKD closer to more practical applications.

DOI
10.1088/0256-307x/37/11/110301

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We propose a fully symmetrical QKD system that enables quantum states to be prepared and measured simultaneously without compromising system performance. Over a 25.6 km fiber channel, we demonstrate point-to-point QKD operations with asymmetric Mach–Zehnder interferometer modules. Two interference visibilities of above 99% indicate that the proposed system has excellent stability. Consequently, the scheme not only improves the feasibility of distributing secret keys, but also enables QKD closer to more practical applications.
Quantum Key Distribution (QKD) system based on the two asymmetric Mach-Zehnder optical fiber interferometers is easily influenced by the external environmental interference, which causes phase excursion. And the high interference visibility of QKD system can’t keep for long and the instability of interference could cause the bit error of QKD. By tracking the stochastic phase excursion in statistical manner, the experiment of quantum key distribution over 75km was done successfully during the time of high interference visibility. The method of QKD in this paper is possessed of high security, less component, and easy operation.
Stability and robustness are important criteria to evaluate the performance of a quantum-key-distribution (QKD) system in real-life applications. However, the inherent birefringence effect of the fiber channel and disturbance caused by the variation of the deployment environment of the QKD system tremendously decrease its performance. To eliminate this adverse impact, we propose a polarization-insensitive phase-coding QKD system based on Sagnac--Mach-Zehnder interferometers. Verified by theoretical analysis and experimental tests, this QKD system is robust against channel polarization disturbance. The robustness and long-term stability of the QKD system is confirmed with a 10-day continuous operation over a 12.6-dB channel, which consists of a 50-km fiber spool and a polarization scrambler (2 rad/s). As a result, an average quantum bit error rate of 0.9580.958%0.958 and a sustained secure key rate of 3.68 kbps are obtained. Moreover, the secure key rate of the QKD system for a typical channel loss of 10 dB reaches 6.89 kbps, and the achievable maximum transmission distance exceeds 125 km.
We propose a unidirectional quantum key distribution (QKD) system based upon a time-division Mach–Zehnder interferometer (MZ) using phase stabilized optical fibers (PSOFs) for the sake of achieving the phase stability during the key distribution. A simple comparison between MZs constituent with PSOF and normal single mode fiber (SMF) is made. During the key distribution over 25 km of standard optical fiber, the phase drift of 4.6 × 10 -4 π/s is obtained. We discuss the demonstration of the quantum key distribution when the asymmetric MZ (AMZ) of PSOF is applied on the proposed QKD system.
Secure communication networks enabled by commercial quantum key distribution (QKD) are already available. However, their widespread deployment will require great efforts towards reducing the currently prohibitive cost of QKD systems. Here, we propose a compact and cost-effective alternative to the asymmetric Mach-Zehnder interferometer commonly used to implement phase encoding in the Bennett-Brassard 1984 (BB84) QKD protocol. Our solution consists of an all-fiber, in-line, highly birefringent interferometer (HBI). The HBI shows improved tolerance to length mismatches and a simpler assembly, making it particularly desirable for the fabrication of multi-user systems where several interferometers must have matched delays and where cost and space considerations can be most critical, such as quantum access networks. As a proof-of-principle, we demonstrate point-to-point QKD operation with HBIs over 15.5 km drop fiber and an 8-port passive optical network splitter. We achieve a secure key generation rate of 299.4 ± 16.4 kbit/s with a quantum bit error rate of 2.89 ± 0.31% for a continuous 25 h operation period.
We present a stable and deterministic quantum key distribution (QKD) system based on differential phase shift. With three cascaded Mach-Zehnder interferometers with different arm-length differences for creating key, its key creation efficiency can be improved to be 7/8, more than other systems. Any birefringence effects and polarization-dependent losses in the long-distance fiber are automatically compensated with a Faraday mirror. Added an eavesdropping check, this system is more secure than some other phase-coding-based QKD systems. Moreover, the classical information exchanged is reduced largely and the modulation of phase shifts is simplified. All these features make this QKD system more convenient than others in a practical application.