High fidelity field stop collection for polarization-entangled photon pair sources

We present an experimental demonstration of a bright and high fidelity polarization entangled photon pair source. The source is constructed using two critically phase matched $\beta$-Barium Borate crystals with parallel optical axes and photon pairs are collected after filtering with a circular field-stop. Near unity fidelities are obtained with detected pair rates exceeding \SI{100000}{pairs/\s/\mW} approaching the brightness of practical quasi-phase matched entangled photon sources. We find that the brightness scales linearly with the crystal length. We present models supporting the experimental data and propose strategies for further improvement. The source design is a promising candidate for emerging quantum applications outside of laboratory environments.