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Journal of Lightwave Technology

QBER Estimation in QKD Systems With Polarization Encoding

Nelson Jesus Muga, Mário F. S. Ferreira, Armando N. Pinto

Published Dec 22, 2010• 43 citations
Publisher

Abstract

A model for quantum BER estimation in polarization encoded quantum key distribution systems is presented. Both TDM and WDM based polarization control schemes are analyzed. It is shown that TDM presents some important advantages when compared with the WDM control scheme. In WDM, the polarization decorrelation between the reference and data signals is an intrinsic and very limitative impairment. This effect has a contribution to quantum BER that increases with the propagation distance, and is highly dependent on the fiber polarization mode dispersion. In the TDM control scheme, the polarization decorrelation is less critical and other issues, like the single photon detector and feedback polarization control system performance tend to dominate. We show that for long distances the fiber losses represent the main contribution to the total quantum BER. Nevertheless, for distances shorter than 70 km and frequencies higher than 5 MHz the after pulse detections provide an important contribution to the total quantum BER.

DOI
10.1109/jlt.2010.2099643

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A model for quantum BER estimation in polarization encoded quantum key distribution systems is presented. Both TDM and WDM based polarization control schemes are analyzed. It is shown that TDM presents some important advantages when compared with the WDM control scheme. In WDM, the polarization decorrelation between the reference and data signals is an intrinsic and very limitative impairment. This effect has a contribution to quantum BER that increases with the propagation distance, and is highly dependent on the fiber polarization mode dispersion. In the TDM control scheme, the polarization decorrelation is less critical and other issues, like the single photon detector and feedback polarization control system performance tend to dominate. We show that for long distances the fiber losses represent the main contribution to the total quantum BER. Nevertheless, for distances shorter than 70 km and frequencies higher than 5 MHz the after pulse detections provide an important contribution to the total quantum BER.
An analysis of the most critical issues in quantum key distribution systems with polarization encoded photons is presented. Effects like the fiber birefringence, fiber losses and some technical aspects can difficult the state of polarization control in such kind of systems, leading to an undesired quantum bit-error-rate increment. It is shown that a system with a long fiber, presenting a typical PMD value, only assures a strong correlation between two Stokes vectors for narrow wavelength separations. In terms of time correlation, it is shown that the typical drift times are much longer than delays between reference and data signals. The problematic of the reference pulse leakage to the data detector is also analyzed; a good agreement between experimental and theoretical data is observed.
In this work we develop an analysis of polarization control schemes suitable for quantum key distribution systems. Both time division multiplexing and wavelength division multiplexing based schemes are considered. A model for the optimization of the temporal separation between reference pulses and polarization encoded photons is presented. The model accounts for the reference pulse shape, the single photon detector gate width, and the respective temporal separation between them. The theoretical results are validated through experimental measurements. These results can be used to optimize the performance of polarization control schemes and therefore to optimize the polarization encoded quantum key distribution systems.
Polarization-encoding provides a promising approach for the practical quantum key distribution (QKD) system due to the simple encoding and decoding method. Here we present a long-term stable polarization-encoding QKD with a real-time polarization control. The polarization of the signal photons in the optical fiber was maintained stable by using time-division-multiplexed reference pulses for feedback control. We implemented this technique in the QKD experiment in 50 km fiber to show that it could facilitate polarization-encoded quantum communication. The system operated stably for ∼460 min, and the quantum bit error rate was ∼5.27%. The raw key generating rate in this system was kept stable at ∼500 bits s -1 . The advantages and disadvantages of the previous polarization-control schemes were also rigorously analyzed.
Background: Quantum Key Distribution (QKD( is a promising technology for secure communication, leveraging the principles of quantum mechanics to provide theoretically unbreakable encryption. With the exponential growth in data traffic and the increasing need for secure communication in backbone fiber networks, integrating high-bit-rate multiplexing techniques into QKD systems can enhance their efficiency and scalability. Problem: Traditional QKD systems face limitations in terms of data rate and network scalability, particularly in high-capacity optical communication networks. As data demands increase, there is a critical need for methods that can support high-bit-rate multiplexing while maintaining the security and performance of QKD. Method: This study proposes a novel QKD approach using high-bit-rate multiplexing in backbone fiber networks. The method involves encoding quantum keys using multiple optical channels simultaneously to increase the data throughput of the QKD system. We employ a combination of time-division multiplexing (TDM( and wavelength-division multiplexing (WDM( to optimize the use of fiber resources and enhance key distribution rates. Results: Simulation results demonstrate that the proposed method achieves a key distribution rate of 10 Mbps over a 200 km fiber link with a quantum bit error rate (QBER( of 1.5%. This represents a 50% improvement in key rate compared to conventional QKD systems without multiplexing. Additionally, the method shows enhanced scalability and network utilization, supporting up to 16 multiplexed channels with minimal impact on security.
Polarizations of single-photon pulses have been controlled with long-term stability of more than 10 hours by using an active feedback technique for auto-compensation of unpredictable polarization scrambling in long-distance fiber. Experimental tests of long-term operations in 50, 75 and 100 km fibers demonstrated that such a single-photon polarization control supported stable polarization encoding in long-distance fibers to facilitate stable one-way fiber system for polarization-encoded quantum key distribution, providing quantum bit error rates below the absolute security threshold.