Improved QSDC Protocol over a Collective-Dephasing Noise Channel.

Recently, Ge and Liu (Chin. Phys. Lett. 24(10):2727-2729, ) proposed a quantum secure direct communication (QSDC) protocol using a decoherence-free subspace (DFS) against collective-dephasing noise. Users of their protocol can directly recover the secret message after quantum transmission without transmission of any additional classical information except for the eavesdropping check. This study points out a pitfall in Ge and Liu's scheme, in which an eavesdropper can deliberately modify the message without being detected. Furthermore, an enhanced scheme is proposed to avoid the modification attack and to improve the qubit efficiency from 8.3 % to 12.5 %. [ABSTRACT FROM AUTHOR]