Improving the capacity of the ping-pong protocol

We present a quantum communication protocol which keeps all the properties of the ping-pong protocol [Phys. Rev. Lett. 89, 187902 (2002)] but improves the capacity doubly as the ping-pong protocol. Alice and Bob can use the variable measurement basises in control mode to detect Eve's eavesdropping attack. In message mode, Alice can use one unitary operations to encode two bits information. Bob only needs to perform a Bell type measurement to decode Alice's information. A classical message authentification method can protect this protocol against the eavesdropping hiding in the quantum channel losses and the denial-of-service (DoS) attack.