  We estimate the channel characteristics of field-installed dark fibers at
single photon levels for quantum key distribution (QKD). Measured fibers are
telecom single-mode dark fibers over 45 km connecting a center and suburbs of
Tokyo. Their total losses are about 14dB, and 50% of the whole lengths are
aerial lines. We find that stray light from other public internet fibers is
dominant and crosstalk occurs at bending points in laying cables. These results
mean the crosstalk from public networks can increase the bit error rate in the
QKD system, and imply an underlying information leakage through an adjacent
covered-fiber.
