We propose a simple method for passive preparation of decoy states in quantum key distribution with coherent light. It involves linear optics together with a photo-detector. The performance is comparable to the active decoy schemes.
Signal state preparation in quantum key distribution schemes can be realized using either an active or a passive source. Passive sources might be valuable in some scenarios; for instance, in those experimental setups operating at high transmission rates, since no externally driven element is required. Typical passive transmitters involve parametric down-conversion. More recently, it has been shown that phase-randomized coherent pulses also allow passive generation of decoy states and Bennett–Brassard 1984 (BB84) polarization signals, though the combination of both setups in a single passive source is cumbersome. In this paper, we present a complete passive transmitter that prepares decoy-state BB84 signals using coherent light. Our method employs sum-frequency generation together with linear optical components and classical photodetectors. In the asymptotic limit of an infinite long experiment, the resulting secret key rate (per pulse) is comparable to the one delivered by an active decoy-state BB84 setup with an infinite number of decoy settings.
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.
In this paper, we present a scheme on realizing the passive decoy-state quantum key distribution. In this scheme, two weak coherent pulses interfere at a beam-splitter, and pulses in one output port are split by one beam-splitter into two parts which are sent into two local detectors individually. Then, all the clicking and nonclicking events from the local detectors are used to herald the arriving and nonarriving of the signal pulse in the other output port. Accordingly, we can obtain more decoy states in the photon-number space and can thus, achieve more precise estimations for single-photon contributions. In this scheme, we use only one intensity at each input of the beam-splitter to avoid intensity modulation fluctuations existing in other active decoy-state methods. We compare the present scheme with Curty et al.'s passive scheme and the conventional three-intensity decoy-state scheme with the weak coherent sources. Through numerical simulations, we demonstrate that this scheme is better than the other two methods.
The decoy-state method could effectively enhance the performance of quantum key distribution (QKD) with a practical phase randomized weak coherent source. Although active modulation of the source intensity is effective and has been implemented in many experiments, passive preparation of decoy states is also an important addition to the family of decoy-state QKD protocols. In this paper, following the theory of Curty et al. [Phys. Rev. A 81, 022310 (2010)], we experimentally demonstrate the phase-encoding passive-decoy-state QKD with only linear optical setups and threshold single-photon detectors. In our experiment, two homemade independent pulsed lasers, with visibility of Hong-Ou-Mandel interference $0.53(\ifmmode\pm\else\textpm\fi{}0.003)$, have been implemented and used to passively generate the different decoy states. Finally, a secret key rate of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$/pulse is obtained with 10-km commercial fiber between Alice and Bob.
The decoy-state method is widely used in practical quantum key distribution systems to replace ideal single photon sources with realistic light sources by varying intensities. Instead of active modulation, the passive decoy-state method employs built-in decoy states in a parametric down-conversion photon source, which can decrease the side channel information leakage in decoy state preparation and hence increase the security. By employing low dark count up-conversion single photon detectors, we have experimentally demonstrated the passive decoy-state method over a 50-km-long optical fiber and have obtained a key rate of about 100 bit/s. Our result suggests that the passive decoy-state source is a practical candidate for future quantum communication implementation.
Decoy states have recently been proposed as a useful protocol for substantially improving the performance of quantum key distribution (QKD). The passive decoy-state protocol has the advantage of easier implementation, higher transmission rate and better security compared with the active decoy-state protocol. This paper analyzes the passive decoy-state protocol with different light sources, including the lastest passive state protocol with weak coherent state (WCS) and the simple passive state protocol (AYKI) with heralded single photon source (HSPS). Based on the theoretic discussion and numerical simulation, we obtain the conclusion that the passive decoy-state protocol with HSPS is easier to implement than the passive decoy-state protocol with WCS because the latter need a hardware change but the former dose not involve any hardware change on the statnard QKD scheme. The key generation rate of the passive decoy-state protocol with WCS is better than that of the passive decoy-state protocol with HSPS, but the transmission distance is less than the latter. Anyhow, the performance of the passive scheme with a threshold detector is close to the active one.