  Quantum secure direct communication (QSDC) has attracted a lot of attention,
which exploits deep-rooted quantum physical principles to guarantee
unconditional security of communication in the face of eavesdropping. We first
briefly review the fundamentals of QSDC, and then present its evolution,
including its security proof, its performance improvement techniques, and
practical implementation. Finally, we discuss the future directions of QSDC.
