Ping-pong protocol eavesdropping in almost perfect channels

Abstract

An undetectable eavesdropping of the entanglement based quantum direct communication in lossy quantum channels has already been demonstrated by Zhang et al. (Phys Lett A 333(12):46–50, 2004). The circuit proposed therein induces losses at a constant 25% rate. Skipping of some protocol cycles is advised in situations when the induced loss rate is too high. However, such policy leads to a reduction in information gain proportional to the number of skipped cycles. The entangling transformation, parametrized by the induced loss ratio, is proposed. The new method permits fine-tuning of the loss ratio by a modification of coupling coefficients. The proposed method significantly improves efficiency of the attack operated in the low loss regime. The other properties of the attack remain the same.