  We propose a quantum cryptographic system based on a planar lightwave circuit
(PLC) and report on optical interference experiments using PLC-based unbalanced
Mach-Zehnder interferometers (MZIs). The interferometers exhibited
high-visibility (>0.98) interference even when the polarisation in the optical
fibre connecting the two MZIs was randomly modulated. The results demonstrate
that a PLC-based setup is suitable for achieving a polarisation-insensitive
phase-coding cryptographic system.
