Tunable Quantum Interference in Free Space with a Liquid-Crystal Metagrating

TL;DR

This paper demonstrates electrically tunable liquid-crystal metagratings as beam splitters for free-space quantum optics, enabling control of photon interference and scalable, parallel measurements of optical modes.

Contribution

It introduces liquid-crystal metagratings as tunable, scalable beam splitters for quantum interference control in free space.

Findings

Voltage-controlled tuning of splitting ratio achieved.

Controlled two-photon interference demonstrated.

Scalable, parallel coincidence measurements enabled.

Abstract

Structured optical materials provide a promising platform for photonic quantum information processing in free space. Beam splitters, a fundamental building block of photonic circuits, have recently been demonstrated in free space using geometric-phase optical elements. These devices coherently mix circularly-polarized transverse modes of freely-propagating optical fields, including modes carrying orbital angular momentum. In this work, we investigate liquid-crystal metagratings as electrically tunable beam splitters for transverse-momentum optical modes. By exploiting the voltage-controlled birefringence of liquid-crystal metasurfaces, we experimentally tune the splitting ratio of the device and thereby control the degree of two-photon interference between indistinguishable photons. At the output, photons are spatially resolved on different regions of a time-resolved single-photon-sensitive detector, enabling the reconstruction of coincidence maps in the Fourier plane. This approach is readily scalable and enables highly parallel coincidence measurements across a large number of optical modes.