  We introduce a generalization of one-way superdense coding to two-way
communication protocols for transmitting classical bits by using entangled
quantum pairs. The proposed protocol caters for provision of entangled pairs,
which is assumed to be given in one-way superdense coding. The proposed
protocol gives a 50% increase in both data rate and energy efficiency compared
to the classical protocol. Even when decoherence is taken into consideration,
the quantum protocol performs better as long as the decoherence time is not
extremely short.
