Flat-optics generation of broadband photon pairs with tunable polarization entanglement

TL;DR

This paper demonstrates the generation of broadband photon pairs with tunable polarization entanglement using flat optics in a GaP film, enabling control over entanglement states and broadband quantum effects.

Contribution

It introduces a flat-optics platform for tunable polarization entanglement in photon pairs via SPDC, which was not previously achieved in flat photonic devices.

Findings

Tunable polarization entanglement from maximally entangled to nearly disentangled.

Generation of broadband photon pairs with a 12 fs Hong-Ou-Mandel dip.

Potential for creating hyperentangled photon states.

Abstract

The concept of `flat optics' is quickly conquering different fields of photonics, but its implementation in quantum optics is still at infancy. In particular, polarization entanglement, which is central to quantum photonics due to the simplicity of polarization qubit encoding and control, is so far not realized on flat platforms. Meanwhile, relaxed phase matching of flat nonlinear optical sources enables enormous freedom in tailoring their polarization properties. Here we use this freedom to generate photon pairs with tunable polarization entanglement via spontaneous parametric down-conversion (SPDC) in a $400$ nm GaP film. By changing the pump polarization, we tune the polarization state of photon pairs from maximally entangled to almost disentangled, which is impossible in a single bulk source of SPDC. Polarization entanglement, in combination with the broadband frequency spectrum, results in an ultranarrow (12 fs) Hong-Ou-Mandel effect and promises extensions to hyperentanglement.