  We demonstrated ultra fast BB84 quantum key distribution (QKD) transmission
at 625 MHz clock rate through a 97 km field-installed fiber using practical
clock synchronization based on wavelength-division multiplexing (WDM). We
succeeded in over-one-hour stable key generation at a high sifted key rate of
2.4 kbps and a low quantum bit error rate (QBER) of 2.9%. The asymptotic secure
key rate was estimated to be 0.78-0.82 kbps from the transmission data with the
decoy method of average photon numbers 0, 0.15, and 0.4 photons/pulse.
