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arXiv:2002.04944 · Optics Letters

Experimental three-state measurement-device-independent quantum key distribution with uncharacterized sources

Xingyu Zhou, Hua-Jian Ding, Chunhui Zhang, Jian Li, Chun-Mei Zhang, Qin Wang

Published Jun 29, 2020• 34 citations
arXivHTMLPDF

Abstract

Measurement-device-independent quantum key distribution (MDI-QKD) removes all detector side-channel attacks and guarantees a promising way for remote secret keys sharing. Several proof-of-principal experiments have been demonstrated to show its security and practicality. However, these practical implementations demand mostly, for example, perfect state preparation or completely characterized sources to ensure security, which are difficult to realize with prior art. Here, we investigate a three-state MDI-QKD using uncharacterized sources, with the simple requirement that the encoding state is bidimensional, which eliminates security threats from both the source flaws and detection loopholes. As a demonstration, a proof-of-principal experiment over 170 km transmission distance based on Faraday-Michelson interferometers is achieved, representing, to the best of our knowledge, the longest transmission distance recorded under the same security level.

DOI
10.1364/ol.398993

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Measurement-device-independent quantum key distribution (MDI-QKD) removes all detector side-channel attacks and guarantees a promising way for remote secret keys sharing. Several proof-of-principal experiments have been demonstrated to show its security and practicality. However, these practical implementations demand mostly, for example, perfect state preparation or completely characterized sources to ensure security, which are difficult to realize with prior art. Here, we investigate a three-state MDI-QKD using uncharacterized sources, with the simple requirement that the encoding state is bidimensional, which eliminates security threats from both the source flaws and detection loopholes. As a demonstration, a proof-of-principal experiment over 170 km transmission distance based on Faraday-Michelson interferometers is achieved, representing, to the best of our knowledge, the longest transmission distance recorded under the same security level.
Measurement-device-independent quantum key distribution (MDI QKD) is an efficient way to share secrets using untrusted measurement devices. However, the assumption on the characterizations of encoding states is still necessary in this promising protocol, which may lead to unnecessary complexity and potential loopholes in realistic implementations. Here, by using the mismatched-basis statistics, we present the first proof-of-principle experiment of MDI QKD with uncharacterized encoding sources. In this demonstration, the encoded states are only required to be constrained in a two-dimensional Hilbert space, and two distant parties (Alice and Bob) are resistant to state preparation flaws even if they have no idea about the detailed information of their encoding states. The positive final secure key rates of our system exhibit the feasibility of this novel protocol, and demonstrate its value for the application of secure communication with uncharacterized devices.
Measurement-device-independent quantum key distribution (MDI-QKD), which is immune to all detector side-channel attacks, is the most promising solution to the security issues in practical quantum key distribution systems. Although several experimental demonstrations of MDI-QKD have been reported, they all make one crucial but not yet verified assumption, that is, there are no flaws in state preparation. Such an assumption is unrealistic and security loopholes remain in the source. Here we present a MDI-QKD experiment with the modulation error taken into consideration. By applying the loss-tolerant security proof by Tamaki et al. [Phys. Rev. A 90, 052314 (2014)], we distribute secure keys over fiber links up to 40 km with imperfect sources, which would not have been possible under previous security proofs. By simultaneously closing loopholes at the detectors and a critical loophole---modulation error in the source, our work shows the feasibility of secure QKD with practical imperfect devices.
Quantum key distribution (QKD) enables two legitimate parties to share a secret key even in the presence of an eavesdropper. The unconditional security of QKD is based on the fundamental laws of quantum physics. Original security proofs of QKD are based on a few assumptions, e.g., perfect single photon sources and perfect single-photon detectors. However, practical implementations of QKD systems do not fully comply with such assumptions due to technical limitations. The gap between theory and implementations leads to security loopholes in most QKD systems, and several attacks have been launched on sophisticated QKD systems. Particularly, the detectors have been found to be the most vulnerable part of QKD. Much effort has been put to build side-channel-free QKD systems. Solutions such as security patches and device-independent QKD have been proposed. However, the former are normally ad-hoc, and cannot close unidentified loopholes. The latter, while having the advantages of removing all assumptions on devices, is impractical to implement today. Measurement-device-independent QKD (MDI-QKD) turns out to be a promising solution to the security problem of QKD. In MDI-QKD, all security loopholes, including those yet-to-be discovered, have been removed from the detectors, the most critical part in QKD. In this thesis, we investigate issues related to the practical implementation and security of MDI-QKD. We first present a demonstration of polarization-encoding MDI-QKD. Taking finite key effect into account, we achieve a secret key rate of 0.005 bit per second (bps) over 10 km spooled telecom fiber, and a 1600-bit key is distributed. This work, together with other demonstrations, shows the practicality of MDI-QKD. Next we investigate a critical assumption of MDI-QKD: perfect state preparation. We apply the loss-tolerant QKD protocol and adapt it to MDI-QKD to quantify information leakage due to imperfect state preparation. We then present an experimental demonstration of MDI-QKD over 10 km and 40 km of spooled fiber, which for the first time considers the impact of inaccurate polarization state preparation on the secret key rate. This would not have been possible under previous security proofs, given the same amount of state preparation flaws.
Quantum key distribution (QKD) promises theoretically secure communication. However, it encounters challenges in implementation security and performance due to inevitable device imperfections. Since the proposal of measurement-device-independent (MDI) QKD, the critical step toward practical security is to secure QKD with imperfect sources. The source imperfections manifest as state-preparation uncertainty (SPU) in various aspects, e.g., encoding uncertainty, intensity fluctuation, and imperfect vacuum states. Here, we perform an MDI-QKD experiment and achieve both high practical security and superior performance. We address the general form of SPU and guarantee a tight estimation of the secret key rate based on the operator dominance method. We achieve secure key distribution over 303.37 km, which not only represents the farthest distance in experiments involving SPU but also considers the most SPU scenarios. Our experimental results represent a significant step toward promoting practical and secure quantum communication.
Measurement-device-independent quantum key distribution (MDI-QKD) is designed to be secure against any detector-side channel attacks. However, assumption of characterized sources in preparation is a demanding requirement and can not be fully satisfied in real-life implementations. Luckily, by utilizing the data of mismatched bases, the system can even generate secure keys with uncharacterized sources. Here, we give a framework on investigating the practical performance of an improved MDI-QKD with uncharacterized sources, where the three-intensity decoy-state method is employed and full parameter optimizations are applied. Simulation results show that the improved method can significantly enhance the performance compared with the original MDI-QKD with uncharacterized sources.