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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Multi-photon free-space quantum key distribution system with synchronously delayed classical signals

Yang Li, Lei Hu, Songhou Liu

Published Jan 26, 2006• 0 citations
Publisher

Abstract

We present a multi-photon scheme to improve the efficiency of free-space quantum key distribution system. Two essential features of free-space quantum key distribution system are its classical timing pulses and frequency tracking devices, we make use of these two features to design a new type of quantum key distribution system with multi-photon quantum signals and synchronously delayed classical signals. This scheme is much more efficient then the usual one, and in the same time, more secure since its synchronously randomizing of frequency and arriving time of quantum signals makes Eves have almost no opportunity to catch the quantum signals in time.

DOI
10.1117/12.668187

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We present a multi-photon scheme to improve the efficiency of free-space quantum key distribution system. Two essential features of free-space quantum key distribution system are its classical timing pulses and frequency tracking devices, we make use of these two features to design a new type of quantum key distribution system with multi-photon quantum signals and synchronously delayed classical signals. This scheme is much more efficient then the usual one, and in the same time, more secure since its synchronously randomizing of frequency and arriving time of quantum signals makes Eves have almost no opportunity to catch the quantum signals in time.
We present a multi-photon scheme to improve the efficiency of free-space quantum key distribution system. Two essential features of free-space quantum key distribution system are its classical timing pulses and frequency tracking devices, we make use of these two features to design a new type of quantum key distribution system with multi-photon quantum signals and synchronously delayed classical signals. This scheme is much more efficient then the usual one, and in the same time, more secure since its synchronously randomizing of frequency and arriving time of quantum signals makes Eves have almost no opportunity to catch the quantum signals in time.
Two essential features of free-space quantum key distribution system are its timing pulses and frequency tracking devices. We make use of these two features to design a new type of quantum key distribution system with synchronously delayed classical signals. This scheme is much more secure since its synchronously randomizing of frequency and arriving time of quantum signals makes Eves have almost no opportunity to catch the quantum signals in time. Besides, this scheme can improve the intrinsic photon utilization efficient from 50% to 100%. Because the scheme breaks the symmetry between Bob and Eve before open discussion, it extends the concept of quantum key distribution and increases our choice of protocol bases.
To establish a time reference frame between two users in quantum key distribution, a synchronization calibration process is usually applied for the case of using gated mode single-photon detectors (SPDs). Traditionally, the synchronization calibration is independently implemented by the line length measurement for each SPD. However, this will leave a loophole which has been experimentally demonstrated by a special attack. Here, we propose an alternative synchronization scheme by fixing the relative delay of the signal time window among all SPDs and jointly performing the line length measurement with multiple SPDs under combining low-precision with high-precision synchronization. The new scheme is not only immune to the vulnerability but also improves the synchronization time from usually a few seconds to tens of milliseconds.
A new scheme of quantum key distribution (QKD) using frequency and time coding is proposed, in which the security is based on the frequency-time uncertainty relation. In this scheme, the binary information sequence is encoded randomly on either the central frequency or the time delay of the optical pulse at the sender. The central frequency of the single photon pulse is set as ω 1 for bit 0 and set as ω 2 for bit 1 when frequency coding is selected. However, the single photon pulse is not delayed for bit 0 and is delayed in τ for 1 when time coding is selected. At the receiver, either the frequency or the time delay of the pulse is measured randomly, and the final key is obtained after basis comparison, data reconciliation and privacy amplification. With the proposed method, the effect of the noise in the fiber channel and environment on the QKD system can be reduced effectively.
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.
We propose a novel synchronization scheme using a periodic light pulse for free-space quantum key distribution (QKD) system. For our 10MHz system, the repetition rate of timing pulse in the novel synchronization scheme can be reduced to 100 KHz or even lower. It allows employing a KHz repetition rate ultrashort pulse laser as timing synchronization source to implement high precision time synchronization. In this paper, short term time drift induced by atmospheric turbulence is analyzed, and how the time deviation impacts on the final secure key generation rate in our QKD system is calculated. In addition, we carried out the QKD experiments over the distance of 1.38 km. The results of the experiments indicate that our scheme is feasible.