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International Journal of Quantum Information

Urban QKD test for phase and polarization encoding devices

A. A. Kanapin, Alexander Duplinskiy, Alexander Sokolov, S. Vorobey, Alexander V. Miller, V. L. Kurochkin, Yury Kurochkin

Published Dec 1, 2017• 0 citations
Publisher

Abstract

In this work, the results of quantum key distribution through an urban fiber communication line with a length of 30.6[Formula: see text]km and losses of 11.7[Formula: see text]dB, obtained by both phase and polarization encoding-based devices, are presented. For phase encoding, a two-pass auto-compensating optical scheme, commonly called “plug&play”, was used. For polarization encoding, a self-developed unconventional optical scheme was made. A continuous key distribution with a sifted key generation rate of 1.0[Formula: see text]kbit/s and a quantum bit error rate of 5.7% was implemented when using “plug&play” device, whereas 0.1[Formula: see text]kbit/s and 5.5% was observed when using one with polarization encoding. The features and conveniences of both implementations are discussed.

DOI
10.1142/s0219749917400184

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In this work, the results of quantum key distribution through an urban fiber communication line with a length of 30.6[Formula: see text]km and losses of 11.7[Formula: see text]dB, obtained by both phase and polarization encoding-based devices, are presented. For phase encoding, a two-pass auto-compensating optical scheme, commonly called “plug&play”, was used. For polarization encoding, a self-developed unconventional optical scheme was made. A continuous key distribution with a sifted key generation rate of 1.0[Formula: see text]kbit/s and a quantum bit error rate of 5.7% was implemented when using “plug&play” device, whereas 0.1[Formula: see text]kbit/s and 5.5% was observed when using one with polarization encoding. The features and conveniences of both implementations are discussed.
Quantum key distribution (QKD) as a technology is being actively implemented into existing urban telecommunication networks. QKD devices are commercially available products. While sending single photons through optical fiber, adjacent fibers, which are used to transfer classical information, might influence the amount of registrations of single photon detectors. This influence is registered, since it directly introduces a higher quantum bit error rate (QBER) into the final key [1-3]. Our report presents the results of the first tests of the QKD device, developed in the Russian Quantum Center. These tests were conducted in Moscow, and are the first of such a device in Russia in urban optical fiber telecommunication networks. The device in question is based on a two-pass auto-compensating optical scheme, which provides stable single photon transfer through urban optical fiber telecommunication networks [4,5]. The single photon detectors ID230 by ID Quantique were used. They operate in free-running mode, and with a quantum effectiveness of 10 % have a dark count ~10 Hz. The background signal level in the dedicated fiber was no less than 5.6∙10-14 W, which corresponds to 4.4∙104 detector clicks per second. The single mode fiber length in Moscow was 30.6 km, the total attenuation equal to 11.7 dB. The sifted quantum key bit rate reached values of 1.9 kbit/s with the QBER level equal to 5.1 %. Methods of lowering the influence of crosstalk on the QBER are considered.
Phase encoding in quantum key distribution (QKD) enables long-distance information-theoretic secure communication in optical fibers. We present a novel theoretical model characterizing errors from various sources in practical phase encoding-based QKD systems, namely the laser linewidth, detector dark counts, and channel dispersion. This model provides optimized optical pulse parameters and less distortion in pulses, which eliminates system imperfections and leads to a reduced quantum bit error rate (QBER) for practical QKD scenario. This analysis is applicable to various fiber-based phase and time encoding protocols. In particular, we implement this to a differential phase shift (DPS) QKD scheme operating at a 2.5 GHz clock, which produces a secure key rate of 193 bits/s at a fiber length of 265 km and an unprecedented QBER < 1[Formula: see text] up to 225 km length with standard telecom components. We show that by adjusting the quantum efficiency and dark count rates of detectors, proposed system can establish secure keys up to 380 km distance using standard telecom grade fiber with a QBER of 1.48%. Moreover, the system is compatible with existing optical fiber networks and capable of establishing a secure key exchange between two cities 432 km apart using ultra-low-loss (ULL) specialty fiber.
We have implemented a practical plug-and-play measurement-device-independent quantum key distribution (MDI QKD) system with polarization division multiplexing (PDM). MDI QKD is known as a secure protocol that can inherently prevent detection loopholes. Significant efforts have been made toward implementing MDI QKD systems. Recently, a plug-and-play architecture has been proposed for implementing the MDI QKD system, and its feasibility has been experimentally verified in both free space and fiber channel. However, in order to apply it to the real world, it is necessary to develop a more practical architecture including multiplexing methods and self-compensation techniques. In this paper, we have proposed and implemented a practical plug-and-play MDI QKD architecture that can be implemented regardless of whether the quantum channel lengths of Alice and Bob are symmetric or asymmetric using PDM and can be operated even under ambient-temperature-changing environments through an optical path-length self-compensation technique. Experimentally, we have achieved6.25×10−66.25\times 10^{-6}6.25×10−6bits per pulse as the key rate and the quantum bit error rates under 3% on 25-km quantum channels.
Polarization encoding is a promising approach for practical quantum key distribution (QKD) systems due to its simple encoding and decoding methodology. In this study, we propose a self-compensating polarization encoder (SCPE) based on a phase modulator, which can be composed of commercial off-the-shelf (COT) devices. We conducted a proof-of-concept experiment to test the SCPE, which demonstrated an in-system quantum bit error rate (QBER) of 0.53% and long-term running stability without any active adjustments. Additionally, we conducted experiments with transmission over commercial fiber spools of lengths up to 100 km and obtained a secure finite key rate of 3 kbps. Our polarization encoder is a promising solution for various polarization encoding protocols, including BB84, MDI, and RFI.
Quantum key distribution (QKD) is a cryptographic technique that uses quantum mechanical principles to enable secure key distribution, offering information-theoretic security guaranteed by physical laws. Practical deployment of QKD requires robust, cost-effective systems that can operate in challenging field environments. A major challenge is achieving reliable clock synchronization without adding hardware complexity. Conventional approaches often use separate classical light signals, which increase costs and introduce noise that degrades quantum channel performance. To address this limitation, we demonstrate a QKD system incorporating a recently proposed qubit-based distributed frame synchronization method, deployed over a metropolitan fiber network in Nanning, China. Using the polarization-encoded one-decoy-state BB84 protocol and the recently proposed qubit-based distributed frame synchronization method, our system achieves synchronization directly from the quantum signal, eliminating the need for dedicated synchronization hardware. Furthermore, to counteract dynamic polarization disturbances in urban fibers, the system integrates qubit-based polarization feedback control, enabling real-time polarization compensation through an automated polarization controller using data recovered from the qubit-based synchronization signals. During 12 hours of continuous operation, the system maintained a low average quantum bit error rate of 1.12 ± 0.48%, achieving a secure key rate of 26.6 kbit/s under 18 dB channel loss. Even under a high channel loss of 40 dB, a finite-key secure rate of 115 bit/s was achieved. This study represents a successful long-term validation of a frame-synchronization-based QKD scheme in a real urban environment, demonstrating exceptional stability and high-loss tolerance, and offering an alternative for building practical, scalable, and cost-efficient quantum-secure communication networks.
We have implemented a quantum key distribution (QKD) system with polarization encoding at 850 nm over 1 km of optical fiber. The high-speed management of the bit-stream, generation of random numbers and processing of the sifting algorithm are all handled by a pair of custom data handling circuit boards. As a complete system using a clock rate of 1.25 Gbit/s, it produces sifted keys at a rate of 1.1 Mb/s with an error rate lower than 1.3% while operating at a transmission rate of 312.5 Mbit/s and a mean photon number μ = 0.1. With a number of proposed improvements this system has a potential for a higher key rate without an elevated error rate.