Standard Polarization-Maintaining Fibers as a Source of Polarization-Entangled Photons

Entangled photons are a crucial resource for quantum communication, quantum computation and fundamental tests of quantum mechanics, which often require the distribution or processing of entangled photons through single-mode fiber (SMF) networks. However, coupling into SMFs has been a challenge due to the spatial mode mismatch between the created photons and the guided mode in the SMF. Recently, it was demonstrated that efficient generation of photon-pairs at visible wavelengths is possible using standard, commercially available polarization-maintaining fibers (PMFs), with high coupling efficiency into SMFs. Here we demonstrate the capability of the source to generate polarization-entangled photon-pairs by inserting a PMF source into a Sagnac interferometer. With a total pump power of 10mW of $\sim$200fs pulses at 715nm and 80MHz, we obtain 350 coincidences/s of photons at 850nm and 620nm. We perform a quantum state tomography to reconstruct the density matrix, yielding, without background subtraction, a tangle of $T=0.629\pm 0.022$, a linear entropy of $S=0.264\pm 0.014$, and a fidelity with a maximally entangled state of $90.40\pm 0.56\%$, clearly exhibiting non-classical entanglement. We expect this source to be useful for fiber-based quantum communication protocols.