Fiber-based quantum secure direct communication without active polarization compensation

Quantum secure direct communication (QSDC) that allows people to directly transmit confidential information through insecure channels is an important branch of quantum communication. The widespread adoption of the QSDC demands the development of simple and stable systems. However, most of the existent QSDC systems involve a complex self-alignment process at the initial stage and additional hardware to compensate environmental disturbance. In this study, we present a fiber-based QSDC system without active polarization compensation. Our system comprises a stable transmitter and a novel Sagnac-Mach-Zehnder interferometer for security detection. This robust system simplifies the self-alignment and is immune to environmental disturbance. The robustness of the system was theoretically and experimentally verified, and low bit error rates in a 12 min continuous operation with an active polarization scrambler were attained. In addition, we performed a proof-of-principle QSDC demonstration, and a secrecy capacity of 3.43 kbps over a 5 km fiber with a detection bit error rate of 0.85% +/- 0.07% and a quantum bit error rate of 0.42% +/- 0.05% were achieved. Experimental results confirm the viability of the proposed QSDC system for practical applications.