Free-Space Quantum Secure Direct Communication: Basics, Progress, and Outlook

Quantum secure direct communication (QSDC) leverages quantum states to transmit information securely and reliably over a noisy, lossy, and wiretapped quantum channel. It has been proven to be information-theoretically secure. Recent experiments show that QSDC can enable long-distance communication and large-scale networking with existing technologies. In particular, free-space QSDC has the unique advantages and prospects for practical large-scale application. Here, we review the QSDC basics and the progress of free-space QSDC. The feasibility of satellite-based QSDC and the future directions of free-space QSDC are discussed. We propose a practical and feasible route that can make immediate application of QSDC in real life.