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Physical Review A

Security of reference-frame-independent quantum key distribution with source flaws

Shi‐Hai Sun

Published Aug 20, 2021• 11 citations
Publisher

Abstract

Recently, reference-frame-independent quantum key distribution (RFI-QKD) has attracted more attention, since no active calibration of the reference frame is required. Thus, it has potential advantages in practical applications, especially for QKD with a mobile-platform-based quantum channel. However, due to the imperfections of devices, practical security is still one of the main bottlenecks for the application of RFI-QKD. Although, based on ``qubit'' assumption, RFI-QKD with the inaccuracy of encoded state was analyzed and demonstrated, this assumption is hard to be guaranteed in practical situations, and some source flaws could not be considered. In this paper we evaluate the security of RFI-QKD with general source flaws, in which the qubit assumption is removed. Then, almost all the source flaws (not only the inaccuracy of encoded state, but also the side channel, the Trojan horse, and so on) can be considered together.

DOI
10.1103/physreva.104.022423

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Recently, reference-frame-independent quantum key distribution (RFI-QKD) has attracted more attention, since no active calibration of the reference frame is required. Thus, it has potential advantages in practical applications, especially for QKD with a mobile-platform-based quantum channel. However, due to the imperfections of devices, practical security is still one of the main bottlenecks for the application of RFI-QKD. Although, based on ``qubit'' assumption, RFI-QKD with the inaccuracy of encoded state was analyzed and demonstrated, this assumption is hard to be guaranteed in practical situations, and some source flaws could not be considered. In this paper we evaluate the security of RFI-QKD with general source flaws, in which the qubit assumption is removed. Then, almost all the source flaws (not only the inaccuracy of encoded state, but also the side channel, the Trojan horse, and so on) can be considered together.
The discrepancy between theory and experiment severely limits the development of quantum key distribution (QKD). Reference-frame-independent (RFI) protocols have been proposed to avoid alignment of the reference frame with the assumption of qubit systems. However, Trojan horse attacks and high-dimensional information leakage caused by equipment loopholes inevitably result in an imperfect non-qubit emitted signal. In this paper, we analyze the security of the RFI-QKD protocol with non-qubit source flaws based on a generalizing loss-tolerant method. The simulation results show that it is feasible to employ non-qubit flaw sources without compromising the security of the QKD system. Moreover, it only requires the preparation of four quantum states, which reduces the complexity of the experiment.
Compared with the traditional protocols of quantum key distribution (QKD), the reference-frame-independent (RFI)--QKD protocol has been generally proved to be very useful and practical, since its experimental implementation can be simplified without the alignment of a reference frame. In most RFI-QKD systems, the encoding states are always taken to be perfect, which, however, is not practical in realizations. In this paper, we consider the security of RFI QKD with source flaws based on the loss-tolerant method proposed by Tamaki et al. [Phys. Rev. A 90, 052314 (2014)]. As the six-state protocol can be realized with four states, we show that the RFI-QKD protocol can also be performed with only four encoding states instead of six encoding states in its standard version. Furthermore, the numerical simulation results show that the source flaws in the key-generation basis (Z(Z(Z basis) will reduce the key rate but are loss tolerant, while the ones in XXX and YYY bases almost have no effect and the key rate remains almost the same even when they are very large. Hence, our method and results will have important significance in practical experiments, especially in earth-to-satellite or chip-to-chip quantum communications.
Abstract Reference-frame-independent measurement-device-independent quantum key distribution (RFI-MDI-QKD) can remove all potential detector side-channel attacks and the requirement of real-time alignment of reference frames. However, all previous RFI-MDI-QKD implementations assume the perfect state preparation in sources, which is impractical and may lead to security loopholes. Here, we propose an RFI-MDI-QKD protocol which is robust against state preparation flaws. Comparing to the conventional six-state RFI-MDI-QKD, our scheme can be realized with only four flawed states, which improves the practical security of RFI-MDI-QKD and simplifies the experimental implementation. In addition, simulation results demonstrate that source flaws in Z basis have adverse effect on the performance of RFI-MDI-QKD while the source flaws in X and Y bases have almost no effect. We hope that this work could provide a valuable reference for practical implementations of RFI-MDI-QKD.
Reference-frame-independent quantum key distribution (RFI-QKD) has been proposed as a method to avoid the requirement of real-time alignment of reference frames. However, the source flaw has not been considered in most of the previously reported works. In this paper, based on the analysis of Wang et al. [Phys. Rev. A 92, 042319 (2015)], we perform a proof-of-concept experiment to demonstrate the loss-tolerant RFI-QKD by constructing a time-bin phase-encoded system and quantifying its source flaws. Moreover, the finite-key analysis is taken into account while estimating the secure key rate. Our experimental demonstration makes a positive contribution to the realization of quantum cryptography in a complex and imperfect real-world environment.
Reference-frame-independent quantum key distribution (RFI QKD) significantly alleviates alignment requirements for reference frame in practical quantum communication systems. While the original protocol requires Alice to prepare six quantum states in ZZZ, XXX, and YYY bases, its reliance on active modulation introduces inherent side-channel vulnerabilities from device imperfections. We address this security limitation by integrating a fully passive source into the RFI framework. In this paper, we propose a fully passive RFI QKD protocol. Our protocol avoids active modulation entirely, suppressing side-channel risks through passive quantum state generation. Moreover, by making full utilization of the quantum states generated by the fully passive source, we enhance the secure key rate of fully passive protocol. We establish a system model to analyze the performance of the protocol. Through the optimization of post-selection intervals and intensities, we obtain the maximum secure key transmission rate of the protocol. Under ideal circumstances, the secure key transmission rate of our protocol can reach more than 50% of that of the ideal QKD. Under practical conditions, we have considered the finite-length effect. When pulse number generated by the source reaches 101210^{12}1012, the maximum communication distances of the protocol can reach 167 km and 136 km with a reference frame misalignment of $0 $ and $45^{\circ} $ respectively. We believe that our protocol can contribute to the development of practical QKD systems.
Reference-frame-independent measurement-device-independent quantum key distribution (RFI MDI QKD) can resist side-channel attacks against detectors and slow variations of reference frames, thus presenting an advantageous approach for long-distance secure quantum communication. Nevertheless, if an eavesdropper implements a Trojan-horse attack (THA) by injecting bright light into sources, the back-reflected light may carry encoding information into the channel, thereby causing serious information leakage. In this paper, we theoretically evaluate the finite-key security of decoy-state RFI MDI QKD against a THA by employing the reference-state technique and a particular concentration inequality. Additionally, dual-parameter optimization and multibasis-estimation strategies are introduced to reduce statistical fluctuations effectively. Our work demonstrates that RFI MDI QKD can maintain a better performance even in the presence of large reference-frame drifts and a THA, representing a key advancement in strengthening the security of quantum communication in real-world implementations.