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Chinese Journal of Quantum Electronics

Design of high-precision synchronization in quantum key distribution system

Xu Wan, Kehang Cui, Yuan Gao, Hongfei Zhang, Luo Chun-li, Wang Jian

Published Jan 1, 2011• 1 citation
OpenAlex

Abstract

A light synchronization scheme to solve the key problem of signal synchronization in quantum key distribution(QKD)system is proposed.At side of the transmitter,the field-programmable gate array (FPGA)chip produces a synchronization pulse sequence of certain pattern.The pulse sequence drives laser to transmit sync light through one fiber along with signal light to the receiver.The sync light reverts to sync pulse sequence through fast discrimination at side of the receiver,and the pulses are provided to the single-photon detector as gate after high-precision delay adjustment.Simultaneously,the receiver uses a high-frequency clock to sample the sync pulse sequence in FPGA for encoding,synchronizing the transmitter and receiver.The scheme has a high-precision,low-cost,high fault tolerance feature,and was successfully applied to metro quantum communication network.

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A light synchronization scheme to solve the key problem of signal synchronization in quantum key distribution(QKD)system is proposed.At side of the transmitter,the field-programmable gate array (FPGA)chip produces a synchronization pulse sequence of certain pattern.The pulse sequence drives laser to transmit sync light through one fiber along with signal light to the receiver.The sync light reverts to sync pulse sequence through fast discrimination at side of the receiver,and the pulses are provided to the single-photon detector as gate after high-precision delay adjustment.Simultaneously,the receiver uses a high-frequency clock to sample the sync pulse sequence in FPGA for encoding,synchronizing the transmitter and receiver.The scheme has a high-precision,low-cost,high fault tolerance feature,and was successfully applied to metro quantum communication network.
Aiming at the difficulty in synchronization of signal in the system of Quantum Key Distribution (QKD) in free space, we proposed a method that deals with this problem with outer light signal, named light-synchronization method. In transmitter, continuous signal of an external laser is modulated into a light pulse sequence by using an acousto-optic modulator. And the light pulse sequence is sent to the receiver. The receiver incepts the light pulse signal by using a multiplier phototube, and generates synchronal signal as time base, then take count of single photon. The inner clock signal of high frequency is used for monitoring the generated synchronizing signal for enhancing precision. The scheme has the advantage of long-distance, wireless and low-complexity, and it has been applied in a Free-Space QKD system which is based on B92 protocol.
A real-time quantum key distribution (QKD) system is developed in this paper. In the system, based on the feature of a field-programmable gate array, secure key extraction control and algorithm have been optimally designed to perform sifting, error correction, and privacy amplification altogether in real time. In the QKD experiment, information synchronization mechanism and high-speed classic data channel are designed to ensure the steady operation of the system. Decoy state and synchronous laser light source are used in the system, while the length of optical fiber between Alice and Bob is 20 km. With photons repetition frequency of 20 MHz, the final key rate could reach 17 kb/s. Smooth and robust operation is verified with 6 h continuous test and associated with encrypted voice communication test.
Quantum key distribution (QKD) is the most matured application of quantum communication. The time synchronization and system timing jitter are essential to a practical QKD experiment, which can directly influence the quantum bit error rate (QBER) and the final key rate. High precision time-to-digital converters (TDCs) can effectively reduce the system timing jitter, resulting in lower QBER and higher key rate. Here, we introduce a TDC based on a field programmable gate array (FPGA). Its high resolution (LSB) of 50 ps with precision (RMS) less than 50 ps, low dead time and large dynamic range can well meet the requirement of the QKD experiment. A free space QKD system setup using the FPGA-based TDC has been assembled. The timing precision of the synchronous light and the system timing jitter were determined. The results demonstrate that the FPGA-based TDC could be successfully applied in the QKD experiment.
At the long-distance quantum key distribution(QKD) experiments,high-speed quantum transmission is needed.In this paper,quantum transmission rate at 500 Mbit/s is fulfilled based on SER/DES module inside FPGA,and a time synchronization design between transmitter and receiver using a low frequency sync-pulse(2 MHz) is proposed.For the experiment′s requirements,signal discriminator,pseudo-random number generator,time synchronization and data coding are developed.This design has been applied successfully to build decoy-state based long-distance QKD experiment.
Quantum Key Distribution (QKD) systems based on single photon transmission can generate unconditionally secure common key between remote users. Improvement of QKD performance, particularly on key generation rate, has been required to meet current network traffic. High key generation rate is realized by increasing photon detection rate and by reducing error rate. Therefore, a high-speed QKD system should be equipped with low loss receivers with high visibility, highly efficient photon detectors with small dark count probability, and a stable clock synchronization system with low stray light to the quantum signals. A solution for these issues are given by employing planar lightwave circuit interferometers, single photon detection circuits and modules, and clock synchronization based on Wavelength Division Multiplexing (WDM) technique.
Abstract Time synchronization is one of the most crucial issues that must be addressed in developing quantum key distribution (QKD) systems. It not only lets the transmitter and the receiver to assign a sequence number to each event and then do correct basis reconciliation, but also allows to increase signal-to-noise ratio. Time synchronization in satellite communications is especially complicated due to such factors as high loss, signal fading and Doppler effect. In this work, a simple, efficient and robust algorithm for time synchronization is proposed. It was tested during experiments on QKD between Micius, the world’s first quantum communications satellite, and an optical ground station located in Russia. The obtained synchronization precision lies in the range from 467 to 497 ps. The authors compare their algorithm for time synchronization with the previously used methods. The proposed approach can also be applied to terrestrial QKD systems.