Eavesdropping on Quantum Secure Direct Communication with W State in Noisy Channel

Security of the quantum secure direct communication protocol(i.e., the C-S QSDC protocol) recently proposed by Cao and Song[Chin. Phys. Lett. 23 (2006) 290] is analyzed in the case ofconsiderable quantum channel noise. The eavesdropping scheme is presented, which reveals that the C-S QSDC protocol is not secure if the quantum bit error rate (QBER) caused by quantum channel noise is higher than 4.17%. Our eavesdropping scheme inducesabout 4.17% QBER for those check qubits. However, such QBER canbe hidden in the counterpart induced by the noisy quantum channelif the eavesdropper Eve replaces the original noisy channel by anideal one. Furthermore, if the QBER induced by quantum channel noise islower than 4.17%, then in the eavesdropping scheme Eve still can eavesdrop part of the secret messages by safely attacking a fraction of the transmitted qubits. Finally, an improvement on the C-S QSDC protocol is put forward.