  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.
