  We developed a one-way quantum key distribution (QKD) system based upon a
planar lightwave circuit (PLC) interferometer. This interferometer is expected
to be free from the backscattering inherent in commercially available two-way
QKD systems and phase drift without active compensation. A key distribution
experiment with spools of standard telecom fiber showed that the bit error rate
was as low as 6% for a 100-km key distribution using an attenuated laser pulse
with a mean photon number of 0.1 and was determined solely by the detector
noise. This clearly demonstrates the advantages of our PLC-based one-way QKD
system over two-way QKD systems for long distance key distribution.
