  Phase-sensitive quantum communication has received considerable attention to
overcome the distance limitation of quantum communication. A fundamental
problem in phase-sensitive quantum communication is to compensate for phase
drift in an optical fiber channel. A combination of time-, wavelength-, and
space-division multiplexing can improve the phase stability of the optical
fiber. However, the existing phase compensations have used only time- and
wavelength-division multiplexing. Here, we demonstrate space-division
multiplexed phase compensation in the Osaka metropolitan networks. Our
compensation scheme uses two neighboring fibers, one for quantum communication
and the other for sensing and compensating the phase drift. Our field
investigations confirm the correlation of the phase drift patterns between the
two neighboring fibers. Thanks to the correlation, our space-division
multiplexed phase compensation significantly reduces the phase drift and
improves the quantum bit error rate. Our phase compensation is scalable to a
large number of fibers and can be implemented with simple instruments. Our
study on space-multiplex phase compensation will support the field deployment
of phase-sensitive quantum communication.
