An Efficient Bidirectional Semiquantum Secure Direct Communication Protocol Using Bell States

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.