Fiber-based frequency-degenerate polarization entangled photon pair sources for information encoding

In this paper, a generation scheme of telecom band frequency-degenerate polarization entangled photon pairs is proposed and demonstrated experimentally. It is based on the vector spontaneous four wave mixing (SFWM) process in a Sagnac fiber loop, in which two frequency-degenerate and polarization orthogonal biphoton states have equal probabilities for generating along the clockwise and counter-clockwise directions. The quantum interference between them at the 50:50 fiber coupler of the fiber loop separates the two frequency-degenerate photons in a pair, leading to the generation of polarization entanglement. The raw fringe visibilities of the two-photon interferences under two non-orthogonal polarization bases are 91% and 90%, respectively. Information can be encoded on the generated photon pairs using the polarization entangled Bell states, which requires the frequency-degenerate property. It is demonstrated by a simplified Bell state measurement with a fringe visibility of 83%.