  Maintenance-free wavelength-division-multiplexing quantum key distribution
for 30 days was achieved through a 22-km field fiber. Using
polarization-independent interferometers and stabilization techniques, we
attained a quantum bit error rate as low as 1.70% and a key rate as high as
229.8 kbps, making the record of total secure key of 595.6 Gbits accumulated
over an uninterrupted operation period.
