On-chip generation of hybrid polarization-frequency entangled biphoton states

We demonstrate a chip-integrated semiconductor source that combines polarization and frequency entanglement, allowing the generation of entangled biphoton states in a hybrid degree of freedom without post-manipulation. Our AlGaAs device is based on type-II spontaneous parametric downconversion in a counterpropagating phase-matching scheme in which the modal birefringence lifts the degeneracy between the two possible nonlinear interactions. This allows the direct generation of polarization–frequency entangled photons at room temperature and telecom wavelength, and in two distinct spatial modes, offering enhanced flexibility for quantum information protocols. The state entanglement is quantified by a combined measurement of the joint spectrum and Hong–Ou–Mandel interference (raw visibility <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m1"> <mml:mrow> <mml:mn>70.1</mml:mn> <mml:mi>%</mml:mi> <mml:mo>±</mml:mo> <mml:mn>1.1</mml:mn> <mml:mi>%</mml:mi> </mml:mrow> </mml:math> ) of the biphotons, allowing to reconstruct a restricted density matrix in the hybrid polarization–frequency space.