High‐Performance Quantum Secure Direct Communication Based on Variable‐Length Encoding with Hyperentangled States

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.