ABSTRACT Quantum secure direct communication (QSDC) encodes and transmits information by directly using quantum states as carriers, without the need to establish a private key session in advance. This paper introduces a QSDC protocol that utilizes variable‐length encoding with hyperentangled states. It employs two‐photon six‐qubit hyperentangled states as carriers in information transmission and incorporates variable‐length encoding to enhance secrecy capacity. Specifically, each photon pair can carry at least 2 bits and, at most, 12 bits of information. The proposed protocol's security is examined from two aspects: eavesdropping detection probability and Wyner's wiretap channel theory. The analysis results indicate that the eavesdropping detection probability of this protocol has increased from 0.5 to 0.875 compared with the two‐step protocol when Eve attempts to steal the same information. Furthermore, in the same channel environment, the security threshold of this protocol is approximately 1.6 times that of the two‐step protocol, and its maximum secrecy capacity is 7.97, compared to existing Bell‐state protocols. Thus, the proposed protocol has higher tolerance to error rates under collective attacks and greater robustness to channel losses, enabling stable long‐distance transmission of secret messages.
ABSTRACT Quantum secure direct communication (QSDC) encodes and transmits information by directly using quantum states as carriers, without the need to establish a private key session in advance. This paper introduces a QSDC protocol that utilizes variable‐length encoding with hyperentangled states. It employs two‐photon six‐qubit hyperentangled states as carriers in information transmission and incorporates variable‐length encoding to enhance secrecy capacity. Specifically, each photon pair can carry at least 2 bits and, at most, 12 bits of information. The proposed protocol's security is examined from two aspects: eavesdropping detection probability and Wyner's wiretap channel theory. The analysis results indicate that the eavesdropping detection probability of this protocol has increased from 0.5 to 0.875 compared with the two‐step protocol when Eve attempts to steal the same information. Furthermore, in the same channel environment, the security threshold of this protocol is approximately 1.6 times that of the two‐step protocol, and its maximum secrecy capacity is 7.97, compared to existing Bell‐state protocols. Thus, the proposed protocol has higher tolerance to error rates under collective attacks and greater robustness to channel losses, enabling stable long‐distance transmission of secret messages.
Quantum Secure Direct Communication (QSDC) is a promising approach for secure information exchange. This paper proposes an efficient and secure four-party QSDC scheme utilizing hyperentangled Bell states in the polarization degree of freedom, the first longitudinal momentum degree of freedom and the second longitudinal momentum degree of freedom. The four participants can perform different unitary operations to independently encode their secret messages onto photons in three degrees of freedom, subsequently transmitting them directly through the quantum channel. In this proposed protocol, each degree of freedom of the photon can effectively carry two bits of information. Each round of transmission by a photon enables the four legitimate participants to obtain six classical bits of information. Notably, when compared to other photons based single-degree-of-freedom QSDC network protocols, the capacity of proposed QSDC protocol is tripled. Therefore, it significantly enhances the information transmission capability. Furthermore, comprehensive security analysis shows that our QSDC network protocol can withstand various attacks from external eavesdroppers.
Quantum secure direct communication (QSDC) is one of the most important branches of quantum communication. In contrast to the quantum key distribution (QKD) which distributes a secure key between distant parties, QSDC directly transmits secret message instead of sharing key in advance. To establish a secure QSDC protocol, on the one hand, the security of the quantum channel should be confirmed before the exchange of the secret message. On the other hand, the quantum state should be transmitted in a quantum data block since the security of QSDC is based on the error rate analysis in the theories on statistics. Compared with the deterministic quantum key distribution (DQKD) which can also be used to transmit deterministic information, QSDC schemes do not need extra classical bits to read the secret message except for public discussion. In this article, we introduce the basic principles of QSDC and review the development in this field by introducing typical QSDC protocols chronologically. The first QSDC protocol was proposed by Long and Liu, which can be used to establish a common key between distant parties. In their scheme, the method for transmitting quantum states in a block by block way and in multiple steps was proposed and the information leakage before eavesdropping detection was solved. Subsequently, Deng et al. presented two pioneering QSDC schemes, an entangled-state-based two-step QSDC scheme and a single-photon-state-based quantum one-time pad scheme, in which the basic principle and criteria for QSDC were pointed out. From then on, many interesting QSDC schemes have been proposed, including the high-dimension QSDC scheme based on quantum superdense coding, multi-step QSDC scheme based on Greenberger-Horne-Zeilinger states, QSDC scheme based on quantum encryption with practical non-maximally entangled quantum channel, and so on. We also introduce the anti-noise QSDC schemes which were designed for coping with the collective-dephasing noise and the collective-rotation noise, respectively. In 2011, Wang et al. presented the first QSDC which exploited the hyperentangled state as the information carrier and several QSDC schemes based on the spatial degree of freedom (DOF) of photon, single-photon multi-DOF state and hyperentanglement were proposed subsequently. In addition to the point-to-point QSDC schemes, we also review the QSDC networks. Finally, a perspective of QSDC research is given in the last section.
Abstract Quantum secure direct communication (QSDC) enables the direct transmission of secret messages without prior sharing of keys among the communicating parties. However, in practical situations, an eavesdropper can compromise the measurement device and potentially steal secret messages. To enhance the reliability of QSDC systems, measurement-device-independent (MDI) QSDC protocols have been developed. In this paper, we propose a high-capacity MDI-QSDC protocol with authentication by utilizing hyperentangled Bell states. The protocol allows the sender to transmit six classical bit messages to the receiver using one quantum state. Our protocol has a higher coding capacity compared to other MDI-QSDC protocols. Additionally, we achieve mutual authentication of the identities of the communicating parties, and our scheme has significant advantages over the traditional QSDC protocols, where the legitimacy of the communicating parties has to be assumed. Theoretical analysis proves that our protocol can effectively defend against common attacks.
Abstract Quantum secure direct communication (QSDC) enables the direct transmission of secret messages without prior sharing of keys among the communicating parties. However, in practical situations, an eavesdropper can compromise the measurement device and potentially steal secret messages. To enhance the reliability of QSDC systems, measurement-device-independent (MDI) QSDC protocols have been developed. In this paper, we propose a high-capacity MDI-QSDC protocol with authentication by utilizing hyperentangled Bell states. The protocol allows the sender to transmit six classical bit messages to the receiver using one quantum state. Our protocol has a higher coding capacity compared to other MDI-QSDC protocols. Additionally, we achieve mutual authentication of the identities of the communicating parties, and our scheme has significant advantages over the traditional QSDC protocols, where the legitimacy of the communicating parties has to be assumed. Theoretical analysis proves that our protocol can effectively defend against common attacks.
Quantum secure direct communication (QSDC) is a recently developed practical solution, which transmits secret messages between legitimate parties, without setting up a cryptographic key. The QSDC protocols are mainly classified into entanglement-based and single photon-based approaches. Their security has been analyzed using Wyner's wiretap channel theory, and their secrecy capacity has been calculated [1]. Hitherto, QSDC protocols have been implemented on optical fiber and free-space channels, all in discrete-variable (DV) encoding that employs single-photon detectors [1]–[2]. However, the implementation of continuous-variable (CV) QSDC, in which the keys are encoded into the quadratures of quantized electromagnetic fields (e.g., coherent states) and detected via homodyne techniques, is preferred due to: its lower costs, excellent integrability with existing optical communication systems, easy implementation from state preparation to measurement, and fast experimental realization. Furthermore, it has been proved that CV-QSDC protocols using squeezed states show higher tolerance in purely lossy channels and enhanced robustness against highly noisy ones, as well [3]. Nevertheless, none of the CV-QSDC protocols employing either coherent or squeezed quantum states have been implemented yet.
Quantum secure direct communication (QSDC) is a powerful technique of transmitting confidential information directly and securely based on the pre-established secure quantum channel and block transmission. Here we propose an efficient QSDC protocol using the complete Bell-state measurement (CBSM) resorting to linear optical elements and temporal-polarization hyperentanglement. In this protocol, the polarized entangled photons are utilized as the information carrier, and all the detection events of CBSM can be identified with common single-photon detectors instead of photon number resolving detectors due to the introduction of temporal degree of freedom (DOF). Moreover, since all the two-photon detection events in CBSM are effective and can be preserved with the efficiency of 100% rather than 50% in the previous QSDC, the quantum efficiency of QSDC can be doubled by encoding more messages on entangled photon pairs.