  We report the generation of polarization entangled photon pairs in the
1550-nm wavelength band using spontaneous four-wave mixing in a
dispersion-shifted fiber loop. The use of the fiber-loop configuration made it
possible to generate polarization entangled states very stably. With accidental
coincidences subtracted, we obtained coincidence fringes with >90 %
visibilities, and observed a violation of Bell's inequality by seven standard
deviations. We also confirmed the preservation of the quantum correlation
between the photons even after they had been separated by 20 km of optical
fiber.
