# One-Photon-Interference Quantum Secure Direct Communication
## 1 Introduction  
Quantum communication uses physical principles to ensure the security of communication. [PERSON] and [PERSON] proposed the quantum key distribution (QKD) protocol in 1984 (BB84) [1] which provides a secure way for key agreement. [PERSON] and [PERSON] proposed quantum secure direct communication (QSDC) [2] in 2000, which provides secure and reliable communication in a channel with both noise and eavesdropping [2]. Rapid inevitable developments have been made in the fields of QKD and QSDC [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]. QSDC and QKD are designed for different application scenarios: QKD uses quantum states for key negotiation, which is used in classical processes to transmit information. QSDC does not require the use of keys and can directly load information onto quantum states for secure transmission, eliminating some processes and the related hidden security vulnerabilities. QSDC is communication in the traditional communication sense, namely it is the transmission of information from one place to another place. Therefore, QSDC can perform the distribution of a predetermined key in classical cryptography. Because QSDC is a type of classical communication, it is compatible with the existing Internet naturally. Therefore, the secure repeater network formed by QSDC leverages QSDC to transmit classical encrypted messages and relay them using classical repeaters [11; 12], can construct quantum networks with end-to-end security using existing technologies, and thus can accelerate the development of quantum networks. Another advantage of QSDC is that it can perform stealth communication, offering secrecy and protection of the users because quantum signals are weak and difficult to be found, and furthermore the users can perceive their exposure by inspecting a sudden change in their communication error rate. QSDC transmits information directly through a quantum channel, using entanglement [2; 4; 5; 13; 14] or single photons [3; 13]. The security of these protocols has been completed [15; 16; 17] based on the quantum wiretap channel theory [18; 19; 20]. Due to imperfections in measurement devices, practical systems possess security loopholes [21; 22; 23]. In order to fix these loopholes, measurement-device-independent QSDC (MDI-QSDC) [24; 25] protocols have been developed. In 2021, [PERSON] et al. proposed a simple and powerful method to increase channel capacity using masking (INCUM) [26], which increases the channel capacity and communication distance of QSDC. There have been several successful demonstrations of QSDC in fiber [15; 27; 28] and in free space [29; 30]. In 2020, a secure-classical repeater QSDC network have been proposed and experimentally demonstrated [11; 12]. A 15-user QSDC network with direct links has been reported [31]. In 2022, [PERSON] et al. reported a QSDC system with mixed coding of phase and time-bin states and achieved a communication distance of 102.2 km in fiber, setting up a new world record [32]. Traditional QSDC protocols [2; 3; 24; 25] require two-way transmission, which leads to double loss of signal in the channel and also limits the transmission distance by half compared to one-way protocols in principle. Remarkably, with the help of the quantum-memory-free (QMF) technique, one-way QSDC protocols have been proposed [33; 34] and QMF-QSDC with sophisticated coding has also been designed [35; 36]. In addition, there are some one-way QSDC protocols using hyperentanglement [37; 38; 39]. However, the MDI-QSDC [24; 25] protocols face several practical limitations, including reliance on immature quantum memory, the assumption of ideal entangled or single-photon light sources, limited transmission distances and low secrecy rates. Single-photon-memory MDI-QSDC [40] utilizes QMF technique to eliminate the dependency of MDI-QSDC protocol on high-performance quantum memory. To further address other issues, we propose a one-photon-interference MDI QSDC (OPI-QSDC) protocol in this paper.