A Flexible Improvement for Ping-Pong Protocol

A deterministic quantum communication protocol, called the ping-pong protocol, has been represented by Boström and Felbinger Phys. Rev. Lett. 89 (2002) 187902. Based on an entangled pair of photons, it allows asymptotically secure key distribution and quasi-secure direct communication. However, it was concluded from our previous research that the experiment realization of this protocol requires two optic paths strictly equivalent for interferometry. Here we propose an improved protocol, which is more flexible and secure for conceivable applications. Its feasibility and security are also discussed.