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Journal of Optics B Quantum and Semiclassical Optics

Secure communication with a heralded single-photon source

Alexei S. Trifonov, Anton Zavriyev

Published Nov 23, 2005• 31 citations
Publisher

Abstract

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.

DOI
10.1088/1464-4266/7/12/050

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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.
The security of quantum key distribution (QKD) is based on the basic principles of quantum mechanics, and therefore has unconditional security in theory. In existing quantum key distribution systems, weakly coherent sources (WCSs) are often used as light sources due to a high probability of vacuum pulses in these sources, resulting in limited transmission distances. Besides, there inevitably exist equipment defects in actual QKD systems, such as certain defects in phase modulators and intensity modulators, which lead to distinguishability of quantum states in higher dimensions and result in side-channel vulnerabilities. An eavesdropper can carry out corresponding attacks, thereby threatening the actual security of QKD systems. To overcome the above limitations, we propose an improved protocol on quantum key distribution based on monitoring heralded single-photon sources. Due to the simultaneity of parametric down-conversion photon pairs, we can accurately predict the arrival of one photon by measuring the arrival time of another photon. Through this way, we can greatly reduce the probability of vacuum states in the signal light, and increase the longest transmission distance of the QKD system. Moreover, a light source monitoring module is inserted into the sender’s side. By randomly selecting a certain period of time through the source monitoring module to measure the Hong-Ou-Mandel interference between the signal light and the idle light , we can estimate the side-channel information leakage of the source and then obtain the key generation rate. Compared with the QKD protocol based on monitoring weak coherent sources, our present protocol can give a better performance in either the transmission distance or the key generation rate, especially when the interference error is large. In addition, in principle, our present protocol can also be extended to other quantum key distribution protocols, such as the measurement-device-independent protocols, to further improve the security and practicability of QKD systems. Therefore, our present work can provide valuable references for realizing the large-scale application of quantum communication networks in the near future.
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.
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.
Recently a long distance and high key rate quantum key distribution (QKD) has become possible by the idea of the decoy state method. We show that a longer distance QKD is possible by utilizing a heralded single photon source (utilizing spontaneous parametric down-conversion) as a source instead of a weak coherent pulse (WCP) as proposed in the original decoy state method. Moreover, the key rate is improved by utilizing a presently available photon number resolving detector as a trigger detector of the heralded single photon source and it is shown to approach the key rate of the WCP.
We report our recent results in development of the secure fiber-optics communication system based upon quantum key distribution (QKD). Emphasize is made on the limitation imposed by the state-of-the-art components crucial for the system performance. We discuss the problem of the interferometer design and highlight the possible security loopholes known. Together with single photon counting performance it places the main restriction on the distance range and the secure key rate of the QKD system based upon the weak coherent pulses. Finally we describe the result of the first test of the system using single photons produced by non-degenerate parametric down-conversion as a source.
Recently, a long-distance and high-key-rate quantum key distribution (QKD) has become possible by the idea of the decoy state method. We show that a longer distance QKD is possible by utilizing a heralded single-photon source (HSPS) utilizing spontaneous parametric down-conversion as a source instead of a weak coherent pulse (WCP) as proposed in the original decoy state method. Moreover, the key rate is improved by utilizing a presently available photon number resolving detector as a trigger detector of the HSPS, and it is shown to approach the key rate of the WCP.