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arXiv:quant-ph/0102062 · Journal of Modern Optics

Faint laser quantum key distribution: Eavesdropping exploiting multiphoton pulses

StÉPhane Félix, Nicolas Gisin, AndrÉ Stefanov, Hugo Zbinden

Published Nov 1, 2001• 47 citations
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Abstract

The technological possibilities of a realistic eavesdropper are discussed. Two eavesdropping strategies taking advantage of multiphoton pulses in faint laser QKD are presented. We conclude that, as long as storage of qubits is technically impossible, faint laser QKD is not limited by this security issue, but mostly by the detector noise.
DOI
10.1080/09500340108240903

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We present a two-way protocol for the quantum key distribution with practical faint laser pulses. It is secure when the faint laser pulses contain no more than two photons. The key distribution task is completed in two transmissions. Bob first sends the laser pulses to Alice, and Alice encodes the key message through certain unitary operations and returns the laser pulses to Bob. Security is achieved by placing eavesdropping check procedures in both transmissions. This protocol is secure and is close to practical conditions. In addition, it does not require the exchange of measuring basis information between Alice and Bob, hence saving a lot of storage space.
Any imperfections in a practical quantum key distribution (QKD) system may be exploited by an eavesdropper to collect information about the key without being discovered. We propose a modified photon-number-splitting attack scheme against QKD systems based on weak laser pulses taking advantage of possible multiphoton pulses. Proof-of-principle experiments are demonstrated. The results show that the eavesdropper can get information about the key generated between the legitimate parties without being detected. Since the equivalent attenuation introduced by the eavesdropper for pulses of different average photon numbers are different, the decoy-state method is effective in fighting against this kind of attack. This has also been proven in our experiments.
A highly attenuated laser pulse which gives a weak coherent state is widely used in quantum key distribution (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 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 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. 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. We present 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 also discuss counter measures against the attack and study the security of a QKD system with efficiency mismatch detectors.