Improving Wojcik's eavesdropping attack on the ping-pong protocol

The eavesdropping scheme proposed by Wojcik [Phys. Rev. Lett. 90 (2003) 157901] on the ping-pong protocol [Phys. Rev. Lett. 89 (2002) 187902] is improved by constituting a new set of attack operations. The improved scheme only induces half of the eavesdropping-induced channel loss in Wojcik's scheme, therefore, in a larger domain of the quantum channel transmission efficiency eta, i.e., [0, 75%], the eavesdropper Eve can attack all the transmitted bits. The improvement of the ping-pong protocol security produced by Wojcik's is also suitable for our eavesdropping attack. (C) 2004 Elsevier B.V. All rights reserved.