Large-scale free-space photonic circuits in two dimensions

TL;DR

This paper introduces a novel large-scale free-space photonic circuit technology that enables complex quantum walks with hundreds of modes in a compact, two-dimensional layout, advancing quantum photonics processing capabilities.

Contribution

The authors demonstrate a new free-space photonic circuit platform using liquid-crystal metasurfaces to implement large unitary maps for quantum walks, with potential for scalable, low-loss quantum photonic applications.

Findings

Implemented large-scale unitary maps in free space

Achieved coupling of a single input to hundreds of outputs

Maintained constant losses regardless of walk length

Abstract

Photonic circuits, engineered to couple optical modes according to a specific map, serve as processors for classical and quantum light. The number of components typically scales with that of processed modes, thus correlating system size, circuit complexity, and optical losses. Here we present a photonic-circuit technology implementing large-scale unitary maps in free space, coupling a single input to hundreds of output modes in a two-dimensional compact layout. The map corresponds to a quantum walk of structured photons, realized through light propagation in three liquid-crystal metasurfaces, having their optic axes artificially patterned. Theoretically, the walk length and the number of connected modes can be arbitrary, while keeping losses constant. The patterns can be designed to replicate multiple unitary maps. We also discuss limited reconfigurability by adjusting the overall birefringence and the relative displacement of the optical elements. These results lay the basis for the design of low-loss non-integrated photonic circuits, primarily for manipulating multi-photon states in quantum regimes.