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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Optimization of polarization control schemes for QKD systems

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

Published May 21, 2011• 3 citations
Publisher

Abstract

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.

DOI
10.1117/12.892221

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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.
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, a fully functional polarization-encoding quantum key distribution system (QKD) is demonstrated. At the transmitter side, the polarization states are prepared using an algorithm to determine the voltages to be applied on the electronic polarization controller (EPC), to generate up to four polarization states. At the receiver, a second EPC is used to perform the basis alignment. The symbol synchronization is achieved using a reference optical signal that is wavelength division multiplexed with the quantum signal which carries the polarization-encoded qubit. Frame synchronization is achieved at a protocol level. The presented QKD system works at a repetition rate of 500 Hz, with an average quantum bit error rate of 1.35%.
In a commercial fiber-based quantum key distribution system, the local and signal optical fields are transmitted through long distance fibers by using time division multiplexing and polarization multiplexing. The state of polarization of the optical field is inevitably disturbed by random birefringence of the standard single-mode fiber caused by external complex environments. This drift of the state of polarization significantly affects the balanced homodyne detection results and the secret key rate. Therefore, the key technology of the dynamic polarization control unit is crucial for the system in a large-scale commercial application. We theoretically analyze and prove that the polarization control unit only needs the combination of two degrees of freedom when considering the result of an arbitrary polarization extinction ratio at the receiver of the system. To overcome the influence of polarization variations, we propose a chaotic monkey algorithm based on Bayesian parameter estimation method and implement intelligence algorithm on field programmable gate array (FPGA) hardware under pulsed light with an integral-type detector for the dynamic polarization control unit. The simulation results show that the optimal combination is four degrees of freedom and the optimal prior distribution is an exponential distribution among various distributions in the dynamic polarization control unit. According to the simulation results, the experimental results show that the achieved polarization extinction ratio is over 30 dB and the average time of polarization control is 400 μs for a single random polarization scrambling. By combining the dynamic polarization control unit with the system, we demonstrate the continuous variable quantum key distribution (CV-QKD) under a continuous polarization scrambling scope of 0－2 krad/s and verify its effectiveness. In addition, the methods presented will improve the performance of the system and expand the range of applications even under strong external disturbance.
Detection-time-bin-shift (DTBS) is a scheme that projects the measurement bases or measured photon values into detection time-bins and then time division multiplexes a single photon detector in a quantum key distribution (QKD) system. This scheme can simplify the structure of a QKD system, reduce its cost and overcome the security problems caused by the dead-time introduced self-correlation and the unbalanced characteristics of detectors. In this paper, we present several DTBS schemes for QKD systems based on attenuated laser pulses and entangled photon sources. We study the security issues of these DTBS schemes, especially the time-bin-shift intercept-resend attack and its countermeasures. A fiber-based DTBS QKD system has been developed and its results are presented in this paper.