Entangling entanglement: coupling frequency and polarization of biphotons on demand

Quantum information is often carried in the frequency and polarization degrees of freedom (DoFs) in single photons and entangled photons. We demonstrate an approach to couple and decouple the frequency and polarization DoFs of broadband biphotons. Our approach is based on a nonlinear interferometer consisting of a linear dispersive medium and a polarization controller in between the two biphoton sources (nonlinear media). When the two DoFs are decoupled, maximally polarization-entangled biphotons are observed in the polarization DoF, while interference fringes are observed in the spectrum of the biphotons. When the two DoFs are coupled, by adjusting the polarization controller, interference fringes disappear from the spectrum and instead appear in the degree of polarization entanglement, varying between 0 and 1, depending on the signal and idler frequencies. Our approach offers a convenient means of tuning the polarization entanglement and can be employed for arbitrary biphoton polarization state generation.