# Compact and programmable large-scale optical processor in free space

## TL;DR

A new free-space optical platform uses three layers to process light efficiently, enabling large-scale quantum simulations and scalable information processing.

## Contribution

The platform introduces a compact, programmable free-space photonic system that achieves high-dimensional unitary transformations with minimal layers.

## Key findings

- The platform distributes a single input mode across over 7,000 outputs using only three layers.
- It supports quantum walks with up to 30 time steps in one- and two-dimensional lattices.
- The system is compatible with quantum optics protocols and can simulate topological effects and synthetic gauge fields.

## Abstract

Photonic circuits are central to classical and quantum information processing. While integrated technologies dominate, free-space architectures are emerging as attractive alternatives, offering broad bandwidth and direct manipulation of optical modes without confinement in waveguides. A key challenge for scalability lies in circuit depth, as the number of layers manipulating the optical field typically grows with the system size. Here, we introduce a programmable free-space photonic platform that implements translation-invariant, high-dimensional unitary transformations using only three layers. Information is encoded in structured light modes defined by circular polarization and quantized transverse momenta, and processed with spatial light modulators interleaved with half-wave plates. We implement unitaries that are equivalent to quantum walks over up to 30 time steps, in one- and two-dimensional lattices, distributing a single input mode across more than 7,000 outputs, where conventional approaches would require tens or hundreds of layers. The platform supports diverse quantum walk dynamics, including disorder, synthetic gauge fields, and topological effects, previously explored only in separate experiments. Using coincidence detection with a time-tagging camera, we show compatibility with quantum optics protocols and provide examples of quantum walks of heralded single photons. These results contribute to establishing free-space optical processors as promising resources for high-dimensional quantum simulation and scalable optical information processing.

A programmable three-layer photonic platform implements large-scale unitary transformations, distributing the input field into thousands of output modes for scalable photonic information processing in free space.