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arXiv:2502.12652

Quantum secure direct communication based on fully passive source

Jia‐Wei Ying, Qi Zhang, Shi‐Pu Gu, Xing-Fu Wang, Zhou, Lan, Yu‐Bo Sheng

Published Feb 18, 2025• 0 citations
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Abstract

In practical quantum communication, imperfect devices may introduce side channels, creating opportunities for eavesdroppers. Especially on the source side, the side channels created by active modulation may compromise the security of the protocol. We proposes a passively-sourced quantum secure direct communication (QSDC) protocol based on fully passive source. By passively modulating both the quantum state and the intensity of the decoy state, we can avoid active modulation operations at the source, thereby enhancing the robustness of QSDC against side-channel attacks. We developed a system model and conducted parameter optimization to obtain the maximum secrecy message transmission rate achievable by the protocol for each channel attenuation. At a channel attenuation of 2, 4, 6 dB (corresponding to a communication distance of 5, 10, 15 km), the secrecy message transmission rates are 5.76 * 10^-5, 9.92 * 10^-6, and 4.99 * 10^-7 bit/sec. And its maximum communication distance is about 16.875 km, which is about 94.4% of that of actively modulated QSDC.

DOI
10.48550/arxiv.2502.12652

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In practical quantum communication, imperfect devices may introduce side channels, creating opportunities for eavesdroppers. Especially on the source side, the side channels created by active modulation may compromise the security of the protocol. We proposes a passively-sourced quantum secure direct communication (QSDC) protocol based on fully passive source. By passively modulating both the quantum state and the intensity of the decoy state, we can avoid active modulation operations at the source, thereby enhancing the robustness of QSDC against side-channel attacks. We developed a system model and conducted parameter optimization to obtain the maximum secrecy message transmission rate achievable by the protocol for each channel attenuation. At a channel attenuation of 2, 4, 6 dB (corresponding to a communication distance of 5, 10, 15 km), the secrecy message transmission rates are 5.76 * 10^-5, 9.92 * 10^-6, and 4.99 * 10^-7 bit/sec. And its maximum communication distance is about 16.875 km, which is about 94.4% of that of actively modulated QSDC.
The decoy state method is an effective approach for addressing the source imperfections in quantum communication. By employing pulses of varying intensities to estimate error rates and yields, this method requires active modulation of the source intensity. However, the active modulation operation may introduce side-channel vulnerabilities, potentially allowing an eavesdropper to execute Trojan horse attacks. The passive decoy state method circumvents this issue by eliminating the need for active intensity modulation. In this paper, we propose a passive decoy state quantum secure direct communication (QSDC) protocol that generates weak coherent states with random intensities using two phase-randomized coherent pulses. Leveraging linear optics, our protocol facilitates straightforward implementation and is robust against eavesdropping side-channel attacks targeting source intensity modulators.We establish a system model for the protocol and derive its secrecy message capacity. Additionally, we optimize the input intensities of the protocol and obtain the maximum secrecy message capacity for each channel attenuation value. Compared with the previous protocol, our protocol exhibits an approximately 25% enhancement in the secrecy message transmission rate within a channel attenuation range of 6 dB. We also compare the performance of the three-decoy-state protocol with that of the infinite-decoy-state protocol. Within a channel attenuation range of 6.8 dB, the secrecy message capacity of the three-decoy-state protocol achieves approximately 94%-96% of that of the infinite-decoy-state.
Quantum secure direct communications (QSDC) can directly transmit secret messages through a quantum channel without keys. The imperfect photon source is a major obstacle for QSDC's practical implementation. The unwanted vacuum state and multiphoton components emitted from imperfect photon source largely reduce QSDC's secrecy message capacity and even threaten its security. In the paper, we propose a high-efficient passive decoy-state QSDC protocol with the heralded single-photon source (HSPS). We adopt a spontaneous parametric down-conversion source to emit entangled photon pairs in two spatial modes. By detecting the photons in one of the two correlated spatial modes, we can infer the photon-number distribution of the other spatial mode. Meanwhile, our protocol allows a simple passive preparation of the signal states and decoy state. The HSPS can effectively reduce the probability of vacuum state and increase QSDC's secrecy message capacity. Meanwhile, the passive decoy-state method can simplify the experimental operations and enhance QSDC's robustness against the third-party side-channel attacks. Under the communication distance of 10 km, the secrecy message capacity of our QSDC protocol can achieve 81.85 times with average photon number of 0.1 and 12.79 times with average photon number of 0.01 of that in the original single-photon-based QSDC protocol without the HSPS. Our QSDC protocol has longer maximal communication distance about 17.975 km with average photon number of 0.01. Our work serves as a major step toward the further development of practical passive decoy-state QSDC systems.
Current quantum secure direct communication (QSDC) protocols guarantee communication security by estimating the error rates of photons in the X and Z bases. This take the reference frame calibration between communicating parties as a necessary prerequisite. However, in mobile communications scenarios, achieving continuous and accurate reference frame calibration poses significant challenges. To address this issue, this paper proposes a reference-frame-independent (RFI) QSDC protocol. This protocol only requires ensuring the calibration accuracy of one direction of the reference frame, while allowing a misalignment angle βββ in the other two directions. To improve the protocol's robustness against reference frame fluctuations, we introduce a βββ-independent parameter C into the security analysis framework and rederive the protocol's security bounds. Additionally, we construct a system model and optimize the pulse intensity of the signal states, enabling the protocol to achieve optimal performance under each level of channel attenuation. At an attenuation of 10 dB (corresponding to a communication distance of 25 km), the secrecy message capacities for $β= 0^{ \circ} $ and $45^{ \circ} $ are 8.765×10−68.765 \times10^{-6}8.765×10−6 bit/pulse and 4.150×10−64.150 \times10^{-6}4.150×10−6 bit/pulse, respectively. Compared with the single-photon-based QSDC, the communication distance of the protocol proposed in this paper is significantly extended. When $β= 0^{ \circ} $ and $45^{ \circ} $, the maximum transmission distances of the RFI QSDC protocol are 27.875 km and 26.750 km, which is about 155.9 % and 149.7 % of that of the single-photon-based QSDC protocol.
Quantum secure direct communications (QSDC) can directly transmit secret messages through a quantum channel without keys. The imperfect photon source is a major obstacle for QSDC's practical implementation. The unwanted vacuum state and multiphoton components emitted from imperfect photon source largely reduce QSDC's secrecy message capacity and even threaten its security. In the paper, we propose a high-efficient passive decoy-state QSDC protocol with the heralded single-photon source (HSPS). We adopt a spontaneous parametric down-conversion source to emit entangled photon pairs in two spatial modes. By detecting the photons in one of the two correlated spatial modes, we can infer the photon-number distribution of the other spatial mode. Meanwhile, our protocol allows a simple passive preparation of the signal states and decoy state. The HSPS can effectively reduce the probability of vacuum state and increase QSDC's secrecy message capacity. Meanwhile, the passive decoy-state method can simplify the experimental operations and enhance QSDC's robustness against the third-party side-channel attacks. Under the communication distance of 10 km, the secrecy message capacity of our QSDC protocol can achieve 81.85 times with average photon number of 0.1 and 12.79 times with average photon number of 0.01 of that in the original single-photon-based QSDC protocol without the HSPS. Our QSDC protocol has longer maximal communication distance about 17.975 km with average photon number of 0.01. Our work serves as a major step toward the further development of practical passive decoy-state QSDC systems.
The fully passive source is capable of passively generating decoy states and performing passive encoding simultaneously, avoiding the side-channel risks caused by active modulation operations at the source end, thus effectively enhance the security in quantum key distribution (QKD). Existing fully passive QKD protocol and experiments exploit phase-randomized coherent pulses. In this paper, we propose a fully passive QKD protocol using parametric down-conversion source. The decoy state generation and encoding operation can be carried out passively by parameter down-conversion progress. This protocol has several advantages. First, it can also eliminate all side channels in active modulators. Second, compared with fully passive QKD protocol with phase-randomized coherent pulses, our protocol can significantly increase the key rate and extend the communication distance. Meanwhile, in terms of the transmission rate, our protocol is also closer to that of actively modulated QKD and can achieve fully passive modulation with fewer resources. Moreover, combined with measurement-device-independent (MDI) QKD, this protocol can even potentially achieve robustness against side channels in both detectors and modulators.
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.