  We proposed a scheme of continuous-variable quantum key distribution, in
which the bright Einstein-Podolsky-Rosen entangled optical beams are utilized.
The source of the entangled beams is placed inside the receiving station, where
half of the entangled beams are transmitted with round trip and the other half
are retained by the receiver. The amplitude and phase signals modulated on the
signal beam by the sender are simultaneously extracted by the authorized
receiver with the scheme of the dense-coding correlation measurement for
continuous quantum variables, thus the channel capacity is significantly
improved. Two kinds of possible eavesdropping are discussed. The mutual
information and the secret key rates are calculated and compared with those of
unidirectional transmission schemes.
