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arXiv:quant-ph/0408031 · Applied Physics Letters

Stability of phase-modulated quantum key distribution systems

Zheng‐Fu Han, Xiaofan Mo, You-Zhen Gui, Guang‐Can Guo

Published May 23, 2005• 46 citations
arXivHTMLPDF

Abstract

The stability of double Mach–Zehnder implementation has been investigated in experiment, which is important for practical quantum key distribution systems. The results of experiment show that the random fluctuation of interference visibility comes from polarization disturbance, especially from that of transmission fiber. A theoretical model is built and a necessary condition is given for intrinsic-stabilization unidirectional quantum key distribution system.

DOI
10.1063/1.1931824

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The stability of double Mach–Zehnder implementation has been investigated in experiment, which is important for practical quantum key distribution systems. The results of experiment show that the random fluctuation of interference visibility comes from polarization disturbance, especially from that of transmission fiber. A theoretical model is built and a necessary condition is given for intrinsic-stabilization unidirectional quantum key distribution system.
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.
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.
In this paper, we present a fully fiber-based one-way quantum-key-distribution (QKD) system implementing the Gaussian-modulated coherent-state (GMCS) protocol. The system employs a double Mach-Zehnder interferometer (MZI) configuration in which the weak quantum signal and the strong local oscillator (LO) go through the same fiber between Alice and Bob, and are separated into two paths inside Bob's terminal. To suppress the LO leakage into the signal path, which is an important contribution to the excess noise, we implemented a scheme combining polarization and frequency multiplexing, achieving an extinction ratio of 70 dB. To further minimize the system excess noise due to phase drift of the double MZI, we propose that, instead of employing phase feedback control, one simply let Alice remap her data by performing a rotation operation. We further present noise analysis both theoretically and experimentally. Our calculation shows that the combined polarization and frequency multiplexing scheme can achieve better stability in practice than the time-multiplexing scheme, because it allows one to use matched fiber lengths for the signal and the LO paths on both sides of the double MZI, greatly reducing the phase instability caused by unmatched fiber lengths. Our experimental noise analysis quantifies the three main contributions to the excess noise, which will be instructive to future studies of the GMCS QKD systems. Finally, we demonstrate, under the ``realistic model'' in which Eve cannot control the system within Bob's terminal, a secure key rate of 0.3bit∕pulse0.3\text{bit}∕\text{pulse}0.3bit∕pulse over a 5km fiber link. This key rate is about two orders of magnitude higher than that of a practical Bennett-Brassard 1984 protocol QKD system.
An intrinsically stable quantum key distribution system (QKD) with six polarization states encoded by phase modulation is introduced. The encoder and decoder are in the same structures that consist of two polarizing Sagnac interferometers connected in tandem. The six polarization states are determined and distinguished by different sets of phase shifts induced by two respective electrically-driven integrated phase modulators. A mean visibility of interference fringes is kept stable at 97.58% for an hour's performance. Theoretical and experimental analyses show that the proposed QKD system features intrinsically stability immune from environment fluctuation.
Quantum bit error rate (QBER) of the quantum key distribution system based on the unbalanced Mach-Zehnder interferometer is theoretically analyzed. The influence of the unsymmetry of the two Mach-Zehnder interferometers, the birefringence in the fiber and the fluctuation of the temperature, on the QBER have been discussed in detail. The analytical expression of the QBER is deduced. It shows that the QBER caused by the unsymmetry of the two Mach-Zehnder interferometers defers to the Gauss distribution, QBER caused by the birefringence in the fiber which results in the disaccord of the polarization of the two pulses defers to the cosine distribution, and the fluctuation of the temperature exert great influence on the QBER.