Problems of a quantum secure direct communication scheme based on intermediate-basis

Quantum secure direct communication is a significant branch of quantum cryptography. Recently, a quantum secure direct communication scheme based on intermediate-basis was proposed (Liang et al. in Front Phys 18(5):51301, 2023 https://doi.org/10.1007/s11467-023-1284-4). It is believed that the scheme can improve the efficiencies of security detection and information encoding by utilizing the intermediate-basis. However, this paper points out two serious problems with the aforementioned scheme. Firstly, it fails to utilize two distinct bases since the intermediate-basis is merely a variant of the Bell basis. Accordingly, the quantum secure direct communication scheme based on intermediate-basis cannot achieve the promised high efficiency. Secondly, even if the intermediate-basis is substituted with another quantum basis, the modified quantum secure direct communication scheme cannot achieve the promised high efficiency because the receiver cannot correctly decode the secret message from the final entangled pairs. It is crucial to design correct and efficient quantum secure direct communication protocols. This work can remind the developers of quantum secure direct communication to avoid such pitfalls when they enhance the robustness and efficacy of their schemes.