Nonlinearity symmetry breaking for generating tunable quantum entanglement in semiconductor metasurfaces

Tunable biphoton quantum entanglement generated from nonlinear flat optics is highly desirable for cutting-edge quantum technologies, yet its tunability is substantially constrained by the symmetry of material nonlinear tensors. Here, we overcome this constraint by introducing symmetry breaking in nonlinear polarization via resonant metasurfaces. While asymmetric optical responses have enabled breakthroughs in classical applications like nonreciprocal light transmission, we report the experimental demonstration of asymmetric nonlinear responses for biphoton entanglement. Using a structural-asymmetric semiconductor metasurface incorporating [110] InGaP nanoresonators, we realize continuous tuning of polarization entanglement from partially entangled states to a Bell state by adjusting the pump wavelength. We also observe pronounced spatial anti-correlations and theoretically confirm that this approach can extend to tailor hyperentanglement. Furthermore, our nanoscale entanglement source features an ultrahigh coincidence-to-accidental ratio of ≈7 × 104, outperforming existing semiconductor flat optics by two orders of magnitude. Introducing asymmetric nonlinear response in quantum metasurfaces opens directions for tailoring on-demand quantum states.