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Physical Review A

Performance enhancement of a decoy-state quantum key distribution using a conditionally prepared down-conversion source in the Poisson distribution

Qin Wang, Anders Karlsson

Published Jul 27, 2007• 32 citations
Publisher

Abstract

We study the behavior of decoy-state quantum key distribution systems using a conditionally prepared down-conversion source, analyzing such a source running in a mode of operation with either a thermal photon number distribution or a Poisson photon number distribution. By comparing both modes of operations to the three-intensity proposal of Wang et al., to the one-intensity proposal of Adachi et al., and to the currently demonstrated faint-laser pulse system, we demonstrate that a down-conversion source in a Poissonian mode of operation can largely enhance the performance of decoy-state quantum key distribution systems. This makes the down-conversion-based decoy-state system an interesting alternative to current technological implementations based on faint-laser pulses.

DOI
10.1103/physreva.76.014309

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We study the behavior of decoy-state quantum key distribution systems using a conditionally prepared down-conversion source, analyzing such a source running in a mode of operation with either a thermal photon number distribution or a Poisson photon number distribution. By comparing both modes of operations to the three-intensity proposal of Wang et al., to the one-intensity proposal of Adachi et al., and to the currently demonstrated faint-laser pulse system, we demonstrate that a down-conversion source in a Poissonian mode of operation can largely enhance the performance of decoy-state quantum key distribution systems. This makes the down-conversion-based decoy-state system an interesting alternative to current technological implementations based on faint-laser pulses.
The decoy-state method and the non-orthogonal encoding protocol, being capable of resisting the photon-number splitting attack, have become attractive recently. Here, we combine both the methods and propose a new protocol using a conditionally prepared down-conversion source, following Poisson photon number distribution. In the protocol, Alice randomly changes the intensities of the pump light so that the intensity of signal photon of photon pair is randomly changed. The signal state is used for generating keys, the decoy states for detecting the eavesdropping and estimating the fraction of single-photon and two-photon pulses. We have simulated the final key rate over transmission distance, showing our scheme can enhance the performance of quantum key distribution.
Decoy states have been proven to be a very useful method for significantly enhancing the performance of quantum key distribution systems with practical light sources. Although active modulation of the intensity of the laser pulses is an effective way of preparing decoy states in principle, in practice passive preparation might be desirable in some scenarios. Typical passive schemes involve parametric down-conversion. More recently, it has been shown that phase-randomized weak coherent pulses (WCP) can also be used for the same purpose [M. Curty et al., Opt. Lett. 34, 3238 (2009).] This proposal requires only linear optics together with a simple threshold photon detector, which shows the practical feasibility of the method. Most importantly, the resulting secret key rate is comparable to the one delivered by an active decoy-state setup with an infinite number of decoy settings. In this article we extend these results, now showing specifically the analysis for other practical scenarios with different light sources and photodetectors. In particular, we consider sources emitting thermal states, phase-randomized WCP, and strong coherent light in combination with several types of photodetectors, like, for instance, threshold photon detectors, photon number resolving detectors, and classical photodetectors. Our analysis includes as well the effect that detection inefficiencies and noise in the form of dark counts shown by current threshold detectors might have on the final secret key rate. Moreover, we provide estimations on the effects that statistical fluctuations due to a finite data size can have in practical implementations.
Decoy-state quantum key distribution (QKD) requires information about the photon number distribution of the light sources. The photon number distribution of weak coherent photon sources has been assumed to follow a Poisson distribution. However, it is not yet clear whether picosecond laser pulses, used in high-speed QKD systems, satisfy this assumption due to the significant and rapid changes in carrier density and photon density in the laser cavity. It is necessary to establish a method to experimentally evaluate the photon number distribution of laser pulses with a duration of less than 100 ps. We have implemented three methods to identify a suitable approach for evaluating light sources in quantum key distribution systems. Our findings indicate that all three methods conclude that the photon numbers of the gain-switching laser pulse are consistent with a Poisson distribution. We then compared these methods based on their accuracy of photon number probability and ease of implementation. These considerations will establish a method suitable for the security certification of high-speed quantum key distributed devices.
In practical quantum key distribution (QKD) systems, the heralded single photon source (HSPS) based on parametric down-conversion (PDC) has attracted significant attentions from many researchers because its various advantages compared to the weak coherent source. We propose a heralded single photon source-based decoy-state QKD with dual detectors and demonstrate in a theoretical simulation that this method improves the performance of QKD system. The secure key generation rate is increased significantly with our proposal both in thermal and Poisson distributions of photons. The secure key rate produced by our scheme is higher than the state of the art key rate of current high-speed QKD system. The decoy-state QKD system using the heralded single photon source with dual detectors can be an important alternative for the present practical QKD systems.
Parametric down-conversion (PDC) sources can be used as triggered single photon sources in quantum key distribution (QKD). Recently, there have been several practical proposals of the decoy state QKD with triggering PDC sources. In this paper, we generalize the passive decoy state idea, originally proposed by Mauerer and Silberhorn. The generalized passive decoy state idea can be applied to cases where either threshold detectors or photon-number-resolving detectors are used. The decoy state protocol proposed by Adachi, Yamamoto, Koashi and Imoto (AYKI) can be treated as a special case of the generalized passive decoy state method. By simulating a recent PDC experiment, we compare various practical decoy state protocols with the infinite decoy state protocol and also compare the cases using threshold detectors and photon-number-resolving detectors. Our simulation result shows that with the AYKI protocol, one can achieve a key generation rate that is close to the theoretical limit of an infinite decoy state protocol. Furthermore, our simulation result shows that a photon-number-resolving detector appears to be not very useful for improving QKD performance in this case. Although our analysis is focused on QKD with PDC sources, we emphasize that it can also be applied to QKD setups with other triggered single photon sources.