Security of the "ping-pong" quantum communication protocol recently proposed by Boström and Felbinger [Phys. Rev. Lett. 89, 187902 (2002)]] is analyzed in the case of considerable quantum channel losses. The eavesdropping scheme is presented, which reveals that the ping-pong protocol is not secure for transmission efficiencies lower than 60%. Our scheme induces 50% losses, which, however, can be hidden in the channel losses if one replaces the original lossy channel with a less lossy one. Finally, a possible improvement of the ping-pong protocol security is proposed.
Ping-Pong quantum communication protocol may be employed to transmit deterministically information, so it has instrigued mang researchers. However, there are drawbacks on the security due to the symmetry of the employed two-way quantum channel. In this paper we propose a new attack strategy to eavesdrop the initially transmitted information based on the techniques of quantum error correction code (QECC). In addition, an attack strategy of Ping-Pong scheme based either entanglement state or nonorthogonal state is proofed and demonstrated, respectively. Furthermore, information obtained by Eve and the possible probability detected by the legitimate communicators are analyzed by employing information theorems. Detail proofs and analysis illustrate all previous Ping-Pong quantum communication schemes are insecure, i.e., the eavesdropper may obtain all available information without being detected.
The eavesdropping scheme proposed by Wójcik [Phys. Rev. Lett. {\bf 90},157901(2003)] on the quantum communication protocol of Boström and Felbinger [Phys. Rev. Lett. {\bf 89}, 187902(2002)] is improved by constituting a new set of attack operations. The improved scheme only induces half of the eavesdropping losses in Wójcik's scheme, therefore, in a larger domain of the quantum channel transmission efficiency
η, i.e., [0,75%], the eavesdropper Eve can attack all the transmitted bits. Comparing to Wójcik's scheme, in the improved scheme the eavesdropping (legitimate) information gain does not vary in the
η domain of [0, 50%], while in the
η domain of (50%, 75%] the less eavesdropping losses induce more eavesdropping information gains, for Eve can attack {\it all} the transmitted bits and accordingly eavesdropping information gains do {\it not} decrease. Moreover, for the Boström-Felbinger protocol, the insecurity upper bound of
η presented by Wójcik is pushed up in the this paper, that is, according to Wójcik's eavesdropping scheme, the Boström-Felbinger protocol is not secure for transmission efficiencies lower than almost 60%, while according to the improved scheme, it is not secure for transmission efficiencies lower than almost 80%.