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Nanosystems Physics Chemistry Mathematics

Synchronization signal distortion in quantum communication systems

V.D. Dubrovskaia, Sergey A. Chivilikhin

Published Jun 15, 2017• 0 citations
Publisher

Abstract

An important problem in the practical implementation of fiber optical quantum communication systems is to synchronize the sender and receiver modules using a separate optical channel.The signal visibility in the quantum channel, which contributes to quantum bit error rate, is influenced by the synchronization signal delay.In this work, we investigate the dependence of the synchronization signal parameters on the dispersive effects in the fiber for a subcarrier wave quantum communication system (SCWQC), which is promising for quantum networking applications.The ITU-T G.652D standard single mode optical fiber was used for modeling.The maximum calculated phase mismatch of the synchronization signal for the system operating at 100 km fiber length corresponds to 1.7 ps signal time delay.The results show that dispersion causes significant signal distortion, therefore additional phase adjustment at least every 2.3 hours is required for stable system operation.

DOI
10.17586/2220-8054-2017-8-3-382-385

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An important problem in the practical implementation of fiber optical quantum communication systems is to synchronize the sender and receiver modules using a separate optical channel.The signal visibility in the quantum channel, which contributes to quantum bit error rate, is influenced by the synchronization signal delay.In this work, we investigate the dependence of the synchronization signal parameters on the dispersive effects in the fiber for a subcarrier wave quantum communication system (SCWQC), which is promising for quantum networking applications.The ITU-T G.652D standard single mode optical fiber was used for modeling.The maximum calculated phase mismatch of the synchronization signal for the system operating at 100 km fiber length corresponds to 1.7 ps signal time delay.The results show that dispersion causes significant signal distortion, therefore additional phase adjustment at least every 2.3 hours is required for stable system operation.
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Clock synchronization is critical for maintaining low error rates in quantum key distribution. Here, we describe how a frequency mismatch between the transmitter and receiver clocks affects the quantum bit error rate in quantum key distribution, and derive from this model a simple synchronization algorithm together with clock stability requirements for practical operation. Our algorithm continuously compensates for both frequency mismatch and time-offset fluctuations directly from detection timestamps. It does not require a dedicated synchronization channel or auxiliary qubit sequence, converges from a large frequency mismatch within approximately one second of photon acquisition, and remains effective in low-photon-count regimes (more than 30 dB of channel loss) using standard hardware. We validate our approach by demonstrating successful key exchange over 100 km of fiber and continuous operation over 24 hours in a 16 km metropolitan network using commercial systems, with performance equivalent to using a service channel for clock synchronization.
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This paper provides an in-depth theoretical analysis of subcarrier multiplexed quantum key distribution (SCM-QKD) systems, taking into account as many factors of impairment as possible and especially considering the influence of nonlinear signal mixing on the end-to-end quantum bit error rate (QBER) and the useful key rate. A detailed analysis of SCM-QKD is performed considering the different factors affecting the sideband visibility (drifts in the modulator bias points, modulation index mismatch between Alice and Bob subcarriers) and the impact of nonlinear signal mixing leaking into otherwise void subcarrier sidebands. In a similar way to classical photonic radio-over-fiber telecommunication and cable TV systems, the impact of this nonlinear signal mixing can be accounted in terms of a quantum carrier to noise ratio that depends on the specific frequency plan that is implemented. QBER and useful key rate results for three different frequency plans featuringNNN$=15(low−countchannelsystem),15 (low-count channel system),15(low−countchannelsystem),N$$=$30 (intermediate-count channel system), and$N$$=50(high−countchannelsystem)channelsareprovided,showingthatphotonnonlinearmixingcanbeofimportanceinmiddle−andhigh−countSCM−QKDsystems(50 (high-count channel system) channels are provided, showing that photon nonlinear mixing can be of importance in middle- and high-count SCM-QKD systems (50(high−countchannelsystem)channelsareprovided,showingthatphotonnonlinearmixingcanbeofimportanceinmiddle−andhigh−countSCM−QKDsystems(N≫30),withmoderateRFmodulationindexes(≫ 30), with moderate RF modulation indexes (≫30),withmoderateRFmodulationindexes(m≫5≫ 5%). In practical terms, nonlinear signal mixing can be neglected if low modulation indexes (≫5m$≪ 2%) are employed to encode the photons in the subcarrier sidebands.
The implementation of quantum communication protocols in long optical fibers is limited by several decoherence mechanisms. In this contribution we review this mechanisms and analysis their effect on the quantum states. Because of asymmetry in the real fibers, the two orthogonal polarization modes are propagated at different phase and group velocities. The difference in group velocities results in Polarization Mode Dispersion (PMD). It is created by random fluctuations of the residual birefringence in optical fibers, such that the State of Polarization (SOP) of an optical signal will turn randomly over time, in an unpredictable way. In optical quantum communication, quantum state measurement is necessary. The bottleneck for communication between far apart nodes is the increasing of the error probability with the length of the channel connecting the nodes. For an optical fiber, the value of absorption and depolarization of a photon (i.e., the qubit) increases exponentially with the length of the fiber.