A continuous-variable quantum secure direct communication protocol with squeezed states

Quantum secure direct communication (QSDC) is a recently developed practical solution, which transmits secret messages between legitimate parties, without setting up a cryptographic key. The QSDC protocols are mainly classified into entanglement-based and single photon-based approaches. Their security has been analyzed using Wyner's wiretap channel theory, and their secrecy capacity has been calculated [1]. Hitherto, QSDC protocols have been implemented on optical fiber and free-space channels, all in discrete-variable (DV) encoding that employs single-photon detectors [3]. Nevertheless, none of the CV-QSDC protocols employing either coherent or squeezed quantum states have been implemented yet.