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Journal of Lightwave Technology

Impact of Third-Order Intermodulation on the Performance of Subcarrier Multiplexed Quantum Key Distribution

J. Capmany, Carlos R. Fernández‐Pousa

Published Aug 25, 2011• 13 citations
Publisher

Abstract

This paper extends the analysis presented in a previous contribution to account for the effect of third-order intermodulation distortion in the system quantum bit error rate attainable in subcarrier multiplexed quantum key distribution (SCM-QKD) systems, showing that under practical conditions this effect can be safely neglected. When the unconditionally secure key distribution rate is computed for BB84 SCM-QKD systems based on weak coherent pulses and strong reference, it is found a per-channel additional decrease due to nonlinear distortions as low as 1% when compared to the dedicated nonmultiplexed BB84 key distribution channels operated under the same conditions.

DOI
10.1109/jlt.2011.2165526

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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.
We provide, to our knowledge, the first experimental demonstration of the feasibility of sending several parallel keys by exploiting the technique of subcarrier multiplexing (SCM) widely employed in microwave photonics. This approach brings several advantages such as high spectral efficiency compatible with the actual secure key rates, the sharing of the optical fainted pulse by all the quantum multiplexed channels reducing the system complexity, and the possibility of upgrading with wavelength division multiplexing in a two-tier scheme, to increase the number of parallel keys. Two independent quantum SCM channels featuring a sifted key rate of 10 Kb/s/channel over a link with quantum bit error rate <2% is reported.
In this paper we investigate the chromatic dispersion impact on the quantum key distribution system based on multi-mode weak coherent phase-coded states. We provide an asymptotic secure key rate estimation, taking into account error detection probability due to chromatic dispersion. We demonstrate numerically and experimentally that the effect of chromatic dispersion in an optical fiber without any compensation hinders the secret key distribution at a distance more than 53 km. Finally, we propose a modification to the considered quantum communication system in order to mitigate the influence of chromatic dispersion on its performance.
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