Quantum secure direct communication (QSDC) is an innovative quantum communication paradigm that transmits information directly using quantum states. First proposed in 2000 [1], QSDC has evolved over more than two decades [2]. Before Bennett and Brassard invented the first quantum key distribution (QKD) protocol, they initially studied QSDC.

Unfortunately, their submission was rejected, leading them to focus on QKD instead due to its simplicity and robustness against loss. Their QSDC protocol was eventually published in 2014 [3]. Notably, this same QSDC protocol was proposed independently by the Tsinghua group earlier [4].

The most appealing feature of quantum communication is its unconditional security. However, due to imperfections in devices, achieving unconditional security in practice is challenging. Device-independent (DI) protocols have been developed to secure unconditional security despite defects in the devices. DI-QSDC guarantees security based only on the observation of Bell inequality violations, without relying on any detailed description or trust in the devices’ inner workings.

Compared with conventional QSDC, DI-QSDC has relatively low secure channel capacity. To increase DI-QSDC’s secure channel capacity, Zeng et al. proposed a high-capacity DI-QSDC protocol based on the hyper-encoding technique, which uses the polarization and path degrees of freedom of photons [5]. The total message leakage rate of the DI-QSDC protocol depends only on the most robust degree of freedom of hyper-entanglement. They performed numerical simulations of its secure channel capacity against the communication distance. When the fidelity of entanglement in the polarization (Fp) and path (Fs) degrees of freedom is below 0.925, no secure communication is possible. When Fp=Fs=1, the maximum communication distance is 2.511 km, and when Fp=Fs=0.99, it is 2.196 km. When Fp=0.97 and Fs=0.98, it is 1.65 km. Compared to DI-QSDC with only polarization entanglement, the communication distance is more than doubled. This work lays a solid foundation for future studies of DI-QSDC.