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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Decoy state protocols for quantum cryptography with parametric down conversion sources

Xiongfeng Ma, Chi‐Hang Fred Fung, Hoi‐Kwong Lo

Published Jun 1, 2007• 0 citations
Publisher

Abstract

In this paper, we show that decoy states are very simple to implement for quantum key distribution (QKD) with parametric down conversion (PDC) sources. Indeed, with no modification in the hardware at all, by using the AYKI scheme, one can achieve a key generation rate that is close to the theoretical limit of infinite decoy states. Therefore, we expect decoy state QKD to become a standard technique not only in the coherent state QKD, but also in QKD with PDC sources.

DOI
10.1109/cleoe-iqec.2007.4387001

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In this paper, we show that decoy states are very simple to implement for quantum key distribution (QKD) with parametric down conversion (PDC) sources. Indeed, with no modification in the hardware at all, by using the AYKI scheme, one can achieve a key generation rate that is close to the theoretical limit of infinite decoy states. Therefore, we expect decoy state QKD to become a standard technique not only in the coherent state QKD, but also in QKD with PDC sources.
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.
We propose an efficient quantum key distribution protocol based on the photon-pair generation from parametric down-conversion (PDC). It uses the same experimental setup as the conventional protocol, but a refined data analysis enables detection of photon-number splitting attacks by utilizing information from a built-in decoy state. Assuming the use of practical detectors, we analyze the unconditional security of the new scheme and show that it improves the secure key generation rate by several orders of magnitude at long distances, using a high intensity PDC source.
Quantum cryptography or quantum key distribution (QKD) applies fundamental laws of quantum physics to guarantee secure communication. The security of quantum cryptography was proven in the last decade. Many security analyses are based on the assumption that QKD system components are idealized. In practice, inevitable device imperfections may compromise security unless these imperfections are well investigated.\n\nA highly attenuated laser pulse which gives a weak coherent state is widely used in QKD experiments. A weak coherent state has multi-photon components, which opens up a security loophole to the sophisticated eavesdropper. With a small adjustment of the hardware, we will prove that the decoy state method can close this loophole and substantially improve the QKD performance. We also propose a few practical decoy state protocols, study statistical fluctuations and perform experimental demonstrations. Moreover, we will apply the methods from entanglement distillation protocols based on two-way classical communication to improve the decoy state QKD performance. Furthermore, we study the decoy state methods for other single photon sources, such as triggering parametric down-conversion (PDC) source. Note that our work, decoy state protocol, has attracted a lot of scientific and media interest. The decoy state QKD becomes a standard technique for prepare-and-measure QKD schemes.\n\nAside from single-photon-based QKD schemes, there is another type of scheme based on entangled photon sources. A PDC source is commonly used as an entangled photon source. We propose a model and post-processing scheme for the entanglement-based QKD with a PDC source. Although the model is proposed to study the entanglement-based QKD, we emphasize that our generic model may also be useful for other non-QKD experiments involving a PDC source. By simulating a real PDC experiment, we show that the entanglement-based QKD can achieve longer maximal secure distance than the single-photon-based QKD schemes.\n\nWe propose a time-shift attack that exploits the efficiency mismatch of two single photon detectors in a QKD system. This eavesdropping strategy can be realized by current technology. We will also discuss counter measures against the attack and study the security of a QKD system with efficiency mismatch detectors.
For quantum key distribution (QKD) using spontaneous parametric-down-conversion sources (SPDCSs), the passive decoy-state protocol has been proved to be efficiently close to the theoretical limit of an infinite decoy-state protocol. In this paper, we apply a tight finite-key analysis for the passive decoy-state QKD using SPDCSs. Combining the security bound based on the uncertainty principle with the passive decoy-state protocol, a concise and stringent formula for calculating the key generation rate for QKD using SPDCSs is presented. The simulation shows that the secure distance under our formula can reach up to 182 km when the number of sifted data is 1010{10}^{10}1010. Our results also indicate that, under the same deviation of statistical fluctuation due to finite-size effects, the passive decoy-state QKD with SPDCSs can perform as well as the active decoy-state QKD with a weak coherent source.
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.
To increase dramatically the distance and the secure key generation rate of quantum key distribution (QKD), the idea of quantum decoys--signals of different intensities--has recently been proposed. Here, we present the first experimental implementation of decoy state QKD. By making simple modifications to a commercial quantum key distribution system, we show that a secure key generation rate of 165 bit/s, which is 1/4 of the theoretical limit, can be obtained over 15 km of a telecommunication fiber. We also show that with the same experimental parameters, not even a single bit of secure key can be extracted with a non-decoy-state protocol. Compared to building single photon sources, decoy state QKD is a much simpler method for increasing the distance and key generation rate of unconditionally secure QKD.