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Quantum Engineering

An Efficient Bidirectional Semiquantum Secure Direct Communication Protocol Using Bell States

Chengyan Gao, Pengliang Guo, Jianwei Qiao

Published Jan 1, 2026• 0 citations
Publisher

Abstract

Semiquantum secure direct communication (SQSDC) allows a quantum user and a classical user to exchange confidential messages without preshared keys. However, existing SQSDC protocols are predominantly unidirectional, and those supporting bidirectional transmission often lack rigorous security checking in both directions. Here, we propose a bidirectional SQSDC protocol utilizing four‐photon states composed of two Bell states. The first Bell state facilitates forward transmission via Z‐basis encoding and decoding, while the second Bell state enables reverse transmission through bit‐flip operations. Security is ensured by Bell‐state measurements on checking photon pairs prior to each decoding phase, with photon reordering eliminating the need for preshared keys. Security analysis confirms robustness against intercept‐resend, entanglement‐measurement, and Trojan horse attacks. The protocol achieves a transmission efficiency of 30% and requires only currently available technologies, making it experimentally feasible for practical semiquantum communication networks.

DOI
10.1155/que2/9053534

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Semiquantum secure direct communication (SQSDC) allows a quantum user and a classical user to exchange confidential messages without preshared keys. However, existing SQSDC protocols are predominantly unidirectional, and those supporting bidirectional transmission often lack rigorous security checking in both directions. Here, we propose a bidirectional SQSDC protocol utilizing four‐photon states composed of two Bell states. The first Bell state facilitates forward transmission via Z‐basis encoding and decoding, while the second Bell state enables reverse transmission through bit‐flip operations. Security is ensured by Bell‐state measurements on checking photon pairs prior to each decoding phase, with photon reordering eliminating the need for preshared keys. Security analysis confirms robustness against intercept‐resend, entanglement‐measurement, and Trojan horse attacks. The protocol achieves a transmission efficiency of 30% and requires only currently available technologies, making it experimentally feasible for practical semiquantum communication networks.
In this paper, we propose the first semiquantum secure direct communication (SQSDC) protocol which is based on single photons in both polarization and spatial-mode degrees of freedom. This protocol doesn't require both communicants to possess quantum capabilities, and is feasible for the direct communication circumstance where one communicant is quantum and the other is classical. We validate the complete robustness of the transmissions of single photons between two communicants in Steps 1 and 2 of this protocol in detail. It turns out that during these transmissions, if Eve wants not to be detected by two communicants, she will obtain nothing useful. The security of the whole protocol is guaranteed by the complete robustness of the transmissions of single photons between two communicants in Steps 1 and 2 of this protocol and the subsequent classical one-time pad encryption process. In this protocol, one single photon with two degrees of freedom can carry two private bits from the message owner, so compared with the existing SQSDC protocols based on single photons in one degree of freedom, its quantum communication capacity is doubled.
Quantum secure direct communication (QSDC) can transmit secret messages directly from one user to another without first establishing a shared secret key, which is different from quantum key distribution. In this paper, we propose a novel quantum secure direct communication protocol based on signal photons and Bell states. Before the execution of the proposed protocol, two participants Alice and Bob exchange their corresponding identity IDA and IDB through quantum key distribution and keep them secret, respectively. Then the message sender, Alice, encodes each secret message bit into two single photons (| 01〉or|10〉) or a Bell state , and composes an ordered secret message sequence. To insure the security of communication, Alice also prepares the decoy photons and inserts them into secret message sequence on the basis of the values of IDA and IDB. By the secret identity IDA and IDB, both sides of the communication can check eavesdropping and identify each other. The proposed protocol not only completes secure direct communication, but also realizes the mutual authentication. The security analysis of the proposed protocol is presented in the paper. The analysis results show that this protocol is secure against some common attacks, and no secret message leaks even if the messages are broken. Compared with the two-way QSDC protocols, the presented protocol is a one-way quantum communication protocol which has the immunity to Trojan horse attack. Furthermore, our proposed protocol can be realized without quantum memory.
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.
Semiquantum cryptography has drawn much attention in recent years since it simplifies the implementation of quantum cryptographic protocols. In this paper, we study semiquantum secure direct communication (SQSDC). Based on single-photons, we propose a two-step SQSDC protocol with identity authentication, which can be used to prevent impersonation and the man-in-the-middle attack. Different from previous protocols based on quantum entanglement or quantum memory, our protocol removes there restrictions for simplifying its physical implementation. We prove that our two-step SQSDC protocol is completely robust. That is, any adversarial attempting to learn some information on the secure message or identity strings will inevitably induce some errors that can be detected by communicators.
Abstract This paper presents a groundbreaking protocol for controlled bidirectional quantum secure direct communication based on hyperentangled Bell states, involving two longitudinal momentum and polarization degrees of freedom. In this protocol, the controller Charlie governs bidirectional communication between two legitimate participants. Participants encode their secret messages using 64 unitary operations. However, without Charlie’s permission, specifically without publishing the final measurement results, participants cannot exchange secret messages. Security analysis are conducted to prevent eavesdropper Eve from obtaining users’ secret messages through illegitimate means. Compared with previously proposed controlled bidirectional quantum secure direct communication, the proposed protocol significantly improves transmission efficiency because each pair of photons can transmit 12 bits of classical information. The protocol achieves a remarkable qubit efficiency, reaching up to 85.7%. However, when we don’t take into account the number of qubits consumed by the decoy particles, we can theoretically achieve an efficiency of 200%.
Quantum secure direct communication (QSDC) enables the message sender to directly transmit messages to the message receiver through quantum channel without keys. Environmental noise is the main obstacle for QSDC's practicality. For enhancing QSDC's noise robustness, we introduce the quantum error correction (QEC) code into QSDC and propose the QSDC protocol based on the redundancy code. This QSDC protocol correlates atomic state with the electron–photon entangled pairs and transmits photons in quantum channels for two rounds. The parties can construct the remote atomic logical entanglement channel and decode messages with the heralded photonic Bell state measurement (BSM) and single electron measurement. This QSDC protocol is unconditionally secure in theory and has some advantages. First, benefiting from the heralded photonic BSM, it can eliminate the influence from photon transmission loss and has the potential to realize long-distance secure message transmission. Second, taking use of the error correction function of the repetition code, the error rate caused by the decoherence during the second round of photon transmission can be reduced, which can reduce the message error and increase the secret message capacity. Third, the whole protocol is feasible under current experimental condition. Our QSDC protocol can be extended to use other stronger QEC code. It provides a promising method to promote QSDC's practicality in the future.