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Chinese Physics Letters

Polarization Encoded Quantum Key Distribution over Special Optical Fibres

Liu Wei-Tao, Wei Wu, Lin-Mei Liang, Cheng-Zu Li, Yuan Jian-Min

Published Jan 30, 2006• 11 citations
Publisher

Abstract

Employing a polarization compensator, an optical fibre quantum key distribution (QKD) system based on polarization coding has been developed. To obtain the compensator setting parameters, the measurement of the laser pulse polarization is performed with one single photon detector. We obtain a sifted key bit rate of about 2 kbits/s and a qubit error rate lower than 10% within 3.5 h. It is shown that polarization coding can be used for QKD over optical fibres as well. At the same time, the system is simple, easy to operate, practical and user-friendly. It gains more advantages than other systems over optical fibres when used in local area quantum communications and where the functional agility is important.

DOI
10.1088/0256-307x/23/2/004

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Employing a polarization compensator, an optical fibre quantum key distribution (QKD) system based on polarization coding has been developed. To obtain the compensator setting parameters, the measurement of the laser pulse polarization is performed with one single photon detector. We obtain a sifted key bit rate of about 2 kbits/s and a qubit error rate lower than 10% within 3.5 h. It is shown that polarization coding can be used for QKD over optical fibres as well. At the same time, the system is simple, easy to operate, practical and user-friendly. It gains more advantages than other systems over optical fibres when used in local area quantum communications and where the functional agility is important.
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.
Quantum key distribution (QKD) relies on quantum communication to allow distant parties to share a secure cryptographic key. Widespread adoption of QKD in current telecommunication networks will require the development of simple, low-cost, and stable systems. However, current QKD implementations usually include additional hardware that perform auxiliary tasks such as temporal synchronization and polarization basis tracking. Here we present a polarization-based QKD system operating at 1550 nm that performs synchronization and polarization compensation by exploiting only the hardware already needed for the quantum communication task. Polarization encoding is performed by a self-compensating Sagnac loop modulator that exhibits high temporal stability and the lowest intrinsic quantum bit error rate reported so far. The QKD system was tested over a fiber-optic link, demonstrating tolerance up to about 40 dB of channel losses. Due to its reduced hardware requirements and the quality of the source, this work represents an important step towards technologically mature QKD systems.
We propose and investigate a fibre-based quantum key distribution system, which employs polarization qubits encoded into faint laser pulses. As a novel feature, it allows sending of classical framing information via sequences of strong laser pulses that precede the quantum data. This allows synchronization, sender and receiver identification, and compensation of time-varying birefringence in the communication channel. Furthermore, this method also provides a platform to communicate implementation specific information such as encoding and protocol in view of future optical quantum networks. All optical components can be operated at Gbps rates, which is a first requirement for Mbps secret key rates.
We propose a fibre-based quantum key distribution system, which employs polarization qubits encoded into faint laser pulses. As a novel feature, it allows sending of classical framing information via sequences of strong laser pulses that precede the quantum data. This allows synchronization, sender and receiver identification and compensation of time-varying birefringence in the communication channel. In addition, this method also provides a platform to communicate implementation specific information such as encoding and protocol in view of future optical quantum networks. We demonstrate in a long-term (37 h) proof-of-principle study that polarization information encoded in the classical control frames can indeed be used to stabilize unwanted qubit transformation in the quantum channel. All optical elements in our setup can be operated at Gbps rates, which is a first requirement for a future system delivering secret keys at Mbps. In order to remove another bottleneck towards a high rate system, we investigate forward error correction based on low-density parity-check codes.
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.
A complete fiber-based polarization encoding quantum key distribution (QKD) system based on the BB84 protocol has been developed at National Institute of Standard and Technology (NIST). The system can be operated at a sifted key rate of more than 4 Mbit/s over optical fiber of length 1 km and mean photon number 0.1. The quantum channel uses 850 nm photons from attenuated high speed VCSELs and the classical channel uses 1550 nm light from normal commercial coarse wavelength division multiplexing devices. Sifted-key rates and quantum error rates at different transmission rates are measured as a function of distance (fiber length). A polarization auto-compensation module has been developed and utilized to recover the polarization state and to compensate for temporal drift. An automatic timing alignment device has also been developed to quickly handle the initial configuration of quantum channels so that detection events fall into the correct timing window. These automated functions make the system more practical for integration into existing optical local area networks.