Compact and programmable large-scale optical processor in free space

TL;DR

This paper introduces a scalable, programmable free-space optical platform capable of high-dimensional unitary transformations with only three layers, enabling complex quantum walk simulations and quantum information processing.

Contribution

The authors develop a novel free-space photonic system that performs high-dimensional unitaries efficiently, supporting diverse quantum walk dynamics with minimal layers.

Findings

Implemented quantum walks over 30 time steps in 1D and 2D lattices.

Distributed a single input mode across over 7,000 outputs.

Demonstrated compatibility with quantum optics protocols using heralded single photons.

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 performs 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. Despite being restricted to translationally-invariant systems, 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 establish free-space optical processors as promising resources for high-dimensional quantum simulation and scalable optical information processing.