Eavesdropping on the "ping-pong" quantum communication protocol.

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.