Conversion of transverse momentum correlation of photon pairs into polarization entanglement by using wavefront-splitting interference

The transverse momentum correlation of photon pairs generated by noncollinear spontaneous parametric down-conversion with type-I phase matching is transformed into polarization entanglement. A robust wavefront-splitting interferometer scheme is used, due to which the phase of the entangled state does not require active stabilization. Frequency-degenerate signal and idler photons are separated and coupled into single-mode optical fibers. The concept is demonstrated in a compact and modular setup. A straightforward alignment process for producing any of the Bell states is presented. The entangled source has an output photon flux of 8.33 +/- 1.27 kcps/mW normalized to pump power (150 mW), and 17 +/- 3% heralding ratio. The measured state fidelity with respect to the negative anticorrelated Bell state is 0.951 +/- 0.004.