  The success of a future quantum internet will rest in part on the ability of
quantum and classical signals to coexist in the same optical fiber
infrastructure, a challenging endeavor given the orders of magnitude
differences in flux of single-photon-level quantum fields and bright classical
traffic. We theoretically describe and experimentally implement Procrustean
entanglement concentration for polarization-entangled states contaminated with
classical light, showing significant mitigation of crosstalk noise in dense
wavelength-division multiplexing. Our approach leverages a pair of
polarization-dependent loss emulators to attenuate highly polarized crosstalk
that results from imperfect isolation of conventional signals copropagating on
shared fiber links. We demonstrate our technique both on the tabletop and over
a deployed quantum local area network, finding a substantial improvement of
two-qubit entangled state fidelity from approximately 75\% to over 92\%. This
local filtering technique could be used as a preliminary step to reduce
asymmetric errors, potentially improving the overall efficiency when combined
with more complex error mitigation techniques in future quantum repeater
networks.
