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Quantum Information Processing

Four-state quantum key distribution exploiting maximum mutual information measurement strategy

Dong‐Xu Chen, Pei Zhang, Hongrong Li, Hong Gao, Fuli Li

Published Nov 20, 2015• 5 citations
Publisher
DOI
10.1007/s11128-015-1173-z

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Quantum key distribution (QKD) enables secure key sharing between distant parties, with several protocols proven resilient against conventional eavesdropping strategies. Here, we introduce a new attack scenario where an eavesdropper, Eve, exploits a quantum switch using the indefinite causal order to intercept and manipulate quantum communication channel. Using multiple metrics such as the information gain, mutual information, and Bell violation, we demonstrate that the presence of a quantum switch significantly compromises QKD security. Our results highlight a previously overlooked vulnerability, emphasizing the need for countermeasures against quantum-controlled adversarial strategies.
This paper investigates the security vulnerabilities within quantum communication protocols focused on key agreement. Specifically, we analyze the Bell state measurement (BSM) protocol and its variations, examining the potential for eavesdropping attacks and compromised key generation. Our analysis utilizes concepts from quantum information theory, including quantum entropy, quantum mutual information, and the density operators describing the quantum states involved. We identify critical weaknesses related to the measurement process and propose modifications to enhance security. The primary focus is on quantifying the information leakage during the BSM process and demonstrating how a sophisticated adversary can exploit this leakage to gain knowledge about the shared key. The proposed improvements involve introducing redundancy in the measurement process and employing post-processing techniques to mitigate the impact of eavesdropping. The goal is to provide a foundational understanding of the security challenges inherent in quantum key distribution (QKD) protocols and to offer practical strategies for building more robust systems.
Quantum-key-distribution protocols are known to be vulnerable against a side channel attack that exploits the time difference in detectors' responses used to obtain key bits. The recommended solution against this timing side channel attack is to use a large time bin width instead of high-resolution timing information. A common notion is that using a large bin width reduces the resolution of detectors' responses, hence supposedly minimizes the information leakage to an eavesdropper. We challenge this conventional wisdom and demonstrate that increasing the bin width does not monotonically reduce the mutual information between the key bits and the eavesdropper's observation of detectors' responses. Instead of randomly increasing the bin width, it should be carefully chosen because the mutual information fluctuates with respect to the bin width. We also examine the effect of full width half maximums (FWHMs) of the detectors' responses on the mutual information and show that decreasing the FWHM increases the mutual information. Lastly, the start time of binning is also shown to be important in the binning process and the mutual information fluctuates periodically with respect to it.