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IEEE Photonics Technology Letters

Single-Photon Interference Experiment Over 80 km With a Pulse-Driven Heralded Single-Photon Source

Tsuyoshi Nishioka, Alexandre Soujaeff, Toshio Hasegawa, Toyohiro Tsurumaru, Jun'ichi Abe, Shigeki Takeuchi

Published Jan 1, 2008• 2 citations

Abstract

We have conducted a single-photon interference experiment over an 80-km optical fiber using a pulse-driven heralded single-photon source (HSPS). To the best of our knowledge, this is, thus far, the longest distance over which a single-photon interference experiment has been conducted using HSPSs (continuous-wave-pumped or pulse-driven). The effect of the 80-km transmission on the dispersion and fluctuation of polarization are more severe than those in our previous 40-km quantum key distribution (QKD) experiment. We have overcome the difficulties by some fine tunings and low-jitter controlling. By conducting ten consecutive transmission experiments over a total time of 30 min, an average quantum bit-error rate (QBER) of 7.9 plusmn 1.2% has been obtained. This QBER is lower than the threshold QBER of 10.55% which is considered as a limit for unconditional security for the QKD under negligible multiphoton emission.

DOI: 10.1109/lpt.2007.915656

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Despite technically successful experiments, most of current quantum key distribution (QKD) systems based on the original BB84 protocols are insecure. The reason is the usage of weak coherent pulses (WCP) as pseudo single photon source (SPS), which allows an eavesdropper to get full information on the key in presence of high optical losses on the transmission channel. To overcome this problem, we have developed a pulse heralded single photon source (HSPS), which we have used for QKD over 40 km of fiber.

Quantum key distribution (QKD) protocols utilize single photons for key exchanging. Heralded single-photon sources (HSPSs) with spontaneous parametric down-conversion (SPDC) is a practical and effective way of producing single photons of high fidelity, bringing extra performance especially at long distances. A pulsed laser is usually used as the pump for HSPS QKD, but it has disadvantages in operational stability and practicality compared to a cw one. On the other hand, a precise model describing the performance of HSPS with a cw pump is still missing. It is inaccurate and unsafe to apply the key rate model of the pulse-pumped HSPS QKD directly to the cw one, and it also overlooks the timing jitter and dead time of the single-photon detectors. In this paper, we study the cw case in detail, including practical imperfections of the experimental instruments. Our model shows that a cw-pumped setup can reach a much longer transmission distance (31 % longer) than the fastest pulsed HSPS QKD system at present, together with other advantages such as higher operational stability and better spectral characteristics.

Mode-pairing quantum key distribution (MP-QKD) is a promising protocol immune to detector side-channel attacks and capable of breaking the repeaterless PLOB bound without global phase locking. Existing MP-QKD protocols employ weak coherent sources (WCSs) with a low probability of single-photon emission, hindering MP-QKD's performance improvement. We propose a new, to the best of our knowledge, MP-QKD protocol using the heralded single-photon source (HSPS) to significantly reduce the vacuum emission probability and increase the single-photon emission rate. Compared with the original WCS-MP-QKD protocol, this protocol has three advantages. First, its maximum transmission distance is extended from 488 km to 560 km. Second, the key rates at 100, 200, 300, and 400 km are 4.4 to 7.4 times those of the original protocol. Third, it does not change the original architecture of MP-QKD and is easy to implement experimentally. This HSPS-MP-QKD protocol has application potential in future metropolitan area QKD networks.

Decoy state quantum key distribution (QKD), being capable of beating PNS attack and being unconditionally secure, has become attractive recently. However, in many QKD systems, disturbances of transmission channel make the quantum bit error rate (QBER) increase, which limits both security distance and key bit rate of real-world decoy state QKD systems. We demonstrate the two-intensity decoy QKD with a one-way Faraday-Michelson phase modulation system, which is free of channel disturbance and keeps an interference fringe visibility (99%) long period, over a 120km single mode optical fibre in telecom (1550 nm) wavelength. This is the longest distance fibre decoy state QKD system based on the two-intensity protocol.

Secure long distance communication over optical fibres requires robust data encryption. While the encryption itself can be conducted using classical algorithms, there is no unconditionally secure method of classical key distribution. Quantum key distribution (QKD), on the other hand, can provide users of the optical networks with unconditionally secure keys. Since QKD is based on single-photon transmission, one of the challenging tasks is to overcome the distance limitation imposed by the losses in optical fibres. In this work we show that single-photon based QKD outperforms the industry-standard weak coherent pulse approach. We also present our recent experimental results on building a heralded single-photon source based on spontaneous parametric down-conversion of CW light and discuss problems and challenges of heralded single-photon generation in the CW regime.