# High-Dimensional and Multi-Intensity One-Photon-Interference Quantum Secure Direct Communication  
[PERSON]  
1 State Key Laboratory of Low-Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, China 14  
[PERSON]  
2 Future Research Lab, China Mobile Research Institute, Beijing 100053, China 24 Frontier Science Center for Quantum Information, Beijing 100084, China 4  
[PERSON]  
1 State Key Laboratory of Low-Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, China 16  
[PERSON]  
1 State Key Laboratory of Low-Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, China 1  
[PERSON]  
1 State Key Laboratory of Low-Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, China 1345  
risks associated with measurement-device imperfections in legitimate users. On the other hand, quantum secure direct communication (QSDC) has rapidly become a key paradigm of quantum cryptography. It originates from the seminal work by [PERSON] and [PERSON] [13], which demonstrated the possibility of direct secret transmission in quantum channels, while subsequent protocols have extended their framework to incorporate various quantum resources, including polarizations in back-and-forth single photons [14]; orbital angular momentum states [15]; hyperentangled states [16]; high-dimensional optical degrees of freedom in both time and phase [17]; quadrature components, which are commonly used in continuous-variable (CV) protocols [18]; and so on. In facing the threats posed by attacks targeting experimental devices, the advent of MDI [19; 20; 21] and device-independent (DI) [22; 23; 24] techniques has further enhanced QSDC's security by incorporating realistic and imperfect implementations into its theoretical framework. QSDC also has the advantage of compatibility with existing Internet infrastructure [25], and simplifies its deployment by trimming the need for the management of pre-distributed keys. Numerous experimental demonstrations in recent years have proved the feasibility of these QKD [26; 27; 28; 29; 30] and QSDC [31; 32; 33; 34; 35] protocols, thereby increasing their potential for application in future scenarios requiring high levels of security. The security of QSDC is based on the quantum wiretap channel theory [36; 37], taking advantage of channel parameters such as the yield and error rate in transmission. As long as the secrecy channel capacity is non-zero, then there must exist a classical encoding scheme that ensures the secure and reliable transmission of information over a noisy and eavesdropping channel, according to [PERSON]'s theory [38; 39; 40]. To further increase the key generation rate and extend the distance of communication, [PERSON] et al. put forward the twin-field QKD (TF-QKD) [41], which replaces the two-photon Bell state measurements in MDI-QKD with single-photon interferences. This allows the key rate to scale with the square root of the channel transmittance, effectively doubling the secure transmission distance compared to prior protocols, and can break the Pirandola-Laurenza-Ottaviani-Banchi (PLOB) bound [42], which was once considered to be unfeasible without quantum repeaters. Thus, this novel feature has led to many research endeavors [43; 44; 45; 46; 47; 48]. The essential mechanism behind TF-QKD is to exploit the one-photon interference. Inspired by this, one-photon-interference QSDC (OPI-QSDC) [49] is proposed to enhance the practicality and performance of QSDC protocols, while forgoing the need for either ideal single-photon sources, entangled light sources, or quantum memory. Meanwhile, it also possesses the MDI characteristic that mitigates the vulnerabilities in measurement devices. However, OPI-QSDC employs only two phases for encoding secret information onto weak coherent pulses, leaving room for additional performance enhancement.