  We demonstrate quantum key distribution (QKD) with bidirectional 10 Gb/s
classical data channels in a single fiber using dense wavelength division
multiplexing. Record secure key rates of 2.38 Mbps and fiber distances up to 70
km are achieved. Data channels are simultaneously monitored for error-free
operation. The robustness of QKD is further demonstrated with a secure key rate
of 445 kbps over 25 km, obtained in the presence of data lasers launching
conventional 0 dBm power. We discuss the fundamental limit for the QKD
performance in the multiplexing environment.
