An improved control mode for the ping-pong protocol operation in imperfect quantum channels

Quantum direct communication (QDC) can bring confidentiality of sensitive information without any encryption. Aping-pong protocol, a well-known example of entanglement-based QDC, offers asymptotic security in a perfect quantum channel. However, it has been shown (Wojcik in Phys Rev Lett 90(15):157901, 2003. doi: 10.1103/PhysRevLett.90.157901) that it is not secure in the presence of losses. Moreover, legitimate parities cannot rely on dense information coding due to possible undetectable eavesdropping even in the perfect setting (Pavicic in Phys Rev A 87(4): 042326, 2013. doi: 10.1103/PhysRevA.87.042326). We have identified the source of the above-mentioned weaknesses in the incomplete check of the EPR pair coherence. We propose an improved version of the control mode, and we discuss its relation to the already-known attacks that undermine the QDC security. It follows that the new control mode detects these attacks with high probability and independently on a quantum channel type. As a result, an asymptotic security of the QDC communication can be maintained for imperfect quantum channels, also in the regime of dense information coding.