Broadband Characterization of Polarization Mode Dispersion for Quantum Communication Channels
We present a method for characterizing polarization fiber channels carrying broadband quantum signals, where narrowband filtering would waste photon flux. Wavelength-dependent polarization mode dispersion (PMD) maps each input state to a trajectory on the Poincar\'e sphere; we show that the singular value decomposition of the band-averaged rotation matrix yields, in closed form, the optimal input states, the mutually unbiased measurement bases, and their infidelities. The three singular values provide a compact, bandwidth-dependent channel signature that separates first- from higher-order PMD, and the resulting 5%-infidelity bandwidth gives a practical filtering budget. We characterize deployed fiber links in Masdar City and demonstrate PMD mitigation by concatenating two channels through a single polarization controller.