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Acta Physica Sinica

A quantum key distribution system based on two polarization beam splitters

Ma Hai-Qiang, Yaling Li, Huan Zhao, Ling-An Wu, (1)中国科学院物理研究所光物理开放实验室, 北京 100080; (2)中国科学院物理研究所光物理开放实验室, 北京 100080;北京交通大学, 北京 100044; (3)中国科学院物理研究所光物理开放实验室, 北京 100080;东南大学, 南京 210096

Published Jan 1, 2005• 10 citations
Publisher

Abstract

We present a quantum key distribution system based on two polarization beam splitters, by means of which the phase modulator's polarization dependence is cancelled out. A high key generation rate has been obtained for the first time at 1310nm transmitted over a 25 km long fiber, with a fringe visibility of 99.4%. A sifted key rate of about 0.6 kbits/s and quantum bit error rate of about 0.5% are obtained.

DOI
10.7498/aps.54.5014

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We present a quantum key distribution system based on two polarization beam splitters, by means of which the phase modulator's polarization dependence is cancelled out. A high key generation rate has been obtained for the first time at 1310nm transmitted over a 25 km long fiber, with a fringe visibility of 99.4%. A sifted key rate of about 0.6 kbits/s and quantum bit error rate of about 0.5% are obtained.
We have implemented a quantum key distribution (QKD) system with polarization encoding at 850 nm over 1 km of optical fiber. The high-speed management of the bit-stream, generation of random numbers and processing of the sifting algorithm are all handled by a pair of custom data handling circuit boards. As a complete system using a clock rate of 1.25 Gbit/s, it produces sifted keys at a rate of 1.1 Mb/s with an error rate lower than 1.3% while operating at a transmission rate of 312.5 Mbit/s and a mean photon number μ = 0.1. With a number of proposed improvements this system has a potential for a higher key rate without an elevated error rate.
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A method that can generate all kinds of nonorthogonal polarization states is induced for long distance and high bit rate generation quantum key distribution. T he polarization states are synthesized by two linearly polarization states contr olled by phase modulator. The modulator is allowed to produced non-quarter_wave phase delay so that can compensate the influence of birefrmgence during transmis sion throuogh the fiber. The structures of the encoder and the decoder based on the method are also described that can effectively reduce the bit error rate.
Polarization-multiplexed phase modulation was demonstrated in a Sagnac interferometer for polarization-auto-compensating quantum key distribution. The Sagnac interferometer was composed of a polarization beam splitter to split the signal pulses into clockwise and counter-clockwise beams of different polarizations, a Faraday rotator to rotate the polarizations of the split clockwise and counter-clockwise beams, and two phase-modulators asymmetrically set in the Sagnac ring. With this structure, the delays of the electronic triggering signals for the two phase-modulators could be set separately, reducing the difficulty in the setup and achieving more precise phase modulation on different polarized beams. Such polarization-multiplexed phase modulation was appropriately gated in sequential time gates to ensure the long-term stability of automatic polarization compensation in the long-distance quantum key distribution system. Polarization-multiplexed phase modulation was implemented in a protocol quantum key distribution system of 25-km communicating fiber length, generating 20-kHz sifted key rate with a qubit error rate lower than 5%.