Fig. 1: Common-path nonlinear interferometer (CP-NLI).
From: Entangling entanglement: coupling frequency and polarization of biphotons on demand
a General scheme of a CP-NLI consisting of two nonlinear media, pumped to generate polarization-entangled biphotons. The wavefunction of the biphotons from the first nonlinear medium \(\left\vert {\psi }_{1}\right\rangle\) undergoes a transformation \(\hat{R}\) and interferes with the biphoton wavefunction from the second nonlinear medium \(\left\vert {\psi }_{2}\right\rangle\). The transformation \(\hat{R}\) determines the output state \(\left\vert {\psi }_{{{{\rm{NLI}}}}}\right\rangle\). b A modified reflective CP-NLI. The biphoton amplitudes generated from the forward and backward paths interfere with each other. PC: polarization controller.
