Quantum Direct Communication Protocol Strengthening Against Pavicic'S Attack

A quantum circuit providing an undetectable eavesdropping of information in message mode, which compromises all two-state psi-phi quantum direct communication (QDC) protocols, has been recently proposed by Pavicic [Phys. Rev. A 87 (2013) 042326]. A modification of the protocol's control mode is proposed, which improves users' 25% detection probability of Eve to 50% at best, as that in ping-pong protocol. The modification also improves the detection probability of Wojcik's attack [Phys. Rev. Lett 90 (2003) 157901] to 75% at best. The resistance against man-in-the-middle (MITM) attack as well as the discussion of security for four Bell state protocols is presented. As a result, the protocol security is strengthened both theoretically and practically, and quantum advantage of superdense coding is restored.