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IEEE EUROCON - International Conference on Computer as a Tool (EUROCON)

Critical issues in polarization encoded quantum key distribution systems

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

Published Apr 1, 2011• 2 citations
Publisher

Abstract

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.

DOI
10.1109/eurocon.2011.5929234

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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.
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.
We analyze the impact of fiber random birefringence in realistic quantum communication systems in which information is encoded in single-photon polarization qubits. We discuss an algorithm to dynamically compensate the polarization random rotations. We assess the algorithm performance considering both the polarization drift dynamics and the QBER threshold. We apply the polarization drift control algorithm to a realistic quantum key distribution system based on the BB84 and we assess its performance in terms of key generation rate for different values of polarization linewidth.
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.
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.
The polarization mode dispersion (PMD) in optical fibers poses a major challenge for maintaining the fidelity of quantum states for quantum communications. In this work, a comprehensive model linking the probability of quantum measurement errors (infidelity) to PMD is developed and validated by experimental measurements of differential group delay and quantum bit error rate (QBER). Our research proposes effective methods to mitigate PMD effects for broadband entangled photons and evaluates the impact of higher-order PMD effects. The model provides an experimentally verified framework for the optimization of commercial quantum key distribution systems in deployed fiber-optic lines.
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.