  We experimentally demonstrate a proposal [Phys. Rev. A 87, 052325 (2013)] of
a scheme for robust distribution of polarization entangled photon pairs over
collective noisy channels having the reciprocity. Although the scheme employs
the robustness of two qubit decoherence-free subspace, by utilizing the forward
propagation of one half of the entangled photons and the backward propagation
of a classical reference light, it achieves an entanglement-sharing rate
proportional to the transmittance of the quantum channel for the signal photon.
We experimentally observed the efficient sharing rate while keeping a highly
entangled state after the transmission. We also show that the protection method
is applicable to transmission of arbitrary polarization state of a single
photon.
