Quantum secure direct communication (QSDC) directly transmits confidential information over a quantum channel. In this context, it is important to take into account the multi-photon effect when using weak coherent laser pulses as the quantum source. Here we propose a multi-intensity QSDC (MI-QSDC) protocol where laser pulses having different intensities are used for secure communication in a fashion reminiscent of decoy states, where only signal intensity pulses are used for key exchange. Pulses of all intensities are used both for information transmission and eavesdropping detection in the MI-QSDC. The statistical fluctuations associated with practical finite block-length in QSDC are also considered and a tight secrecy capacity bound is given. The simulation results show that the proposed protocol is capable of communicating over a distance on the order of 100 km by relying on a finite block-length of sending 1014pulses.
Quantum secure direct communication (QSDC) directly transmits confidential information over a quantum channel. In this context, it is important to take into account the multi-photon effect when using weak coherent laser pulses as the quantum source. Here we propose a multi-intensity QSDC (MI-QSDC) protocol where laser pulses having different intensities are used for secure communication in a fashion reminiscent of decoy states, where only signal intensity pulses are used for key exchange. Pulses of all intensities are used both for information transmission and eavesdropping detection in the MI-QSDC. The statistical fluctuations associated with practical finite block-length in QSDC are also considered and a tight secrecy capacity bound is given. The simulation results show that the proposed protocol is capable of communicating over a distance on the order of 100 km by relying on a finite block-length of sending 1014pulses.
As a novel paradigm in quantum communication, quantum secure direct communication (QSDC) enables secure, reliable, and deterministic information transmission, leveraging the principles of quantum mechanics. One-photon-interference QSDC is particularly attractive because it mitigates the vulnerabilities in measurement devices while extending transmission distances. In this paper, we propose a high-dimensional one-photon-interference QSDC protocol that exploits the advantages of high-dimensional encoding in the phase of weak coherent pluses to further enhance transmission distances and improve secrecy channel capacity. The security of this protocol is analyzed using quantum wiretap channel theory, and its resistance to common quantum threats is discussed. Numerical simulations demonstrate that our protocol outperforms its predecessor in terms of its secrecy capacity and extends the maximum communication distance achievable up to 494 km, which is over 13% longer than the two-dimensional case, effectively doubling the transmission length of traditional protocols. These improvements highlight the protocol's potential for use in quantum communication applications in this era of frequent data breaches and information leaks.
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.
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.
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.
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 β in the other two directions. To improve the protocol's robustness against reference frame fluctuations, we introduce a β-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 8.765×10−6 bit/pulse and 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.