Programmable pixel-mode linear interferometers using multi-plane light conversion

TL;DR

This paper introduces a scalable, free-space, pixel-mode linear interferometer architecture using multi-plane light conversion, demonstrating high-fidelity programmable transformations on up to 16 modes with potential applications in quantum and classical photonics.

Contribution

The authors propose and experimentally validate a novel MPLC-based architecture for programmable linear optical transformations that scales linearly with the number of modes, unlike traditional methods.

Findings

Achieved high-fidelity unitaries on up to 16 modes

Demonstrated various optical transformations including beamsplitters and Hadamard gates

Showed linear scaling of phase planes with mode number

Abstract

Programmable linear optical interferometers are a core primitive in optical signal processing, quantum information processing, and photonic computing. Existing photonic-integrated implementations realize arbitrary $M$-mode unitaries using Mach--Zehnder-interferometer meshes whose footprint and accumulated loss scale with $O(M^2)$ optical components. Here we analyze and experimentally demonstrate a programmable architecture for implementing linear optical transformations directly on spatially tiled free-space pixel modes using multi-plane light conversion (MPLC). In this architecture, $M$ spatial modes arranged on a transverse lattice undergo a unitary transformation and are mapped to $M$ output modes of identical geometry through a sequence of programmable phase masks separated by free-space propagation segments. Numerical simulations show that arbitrary $M$-mode unitaries can be compiled to a desired high fidelity using a number of phase planes that scales approximately linearly with $M$. Using a spatial-light-modulator-based MPLC, we experimentally demonstrate programmable interferometers acting on up to $16$ spatial pixel modes, including tunable beamsplitters, Hadamard unitaries, spatial permutations, boosted-Bell-measurement unitaries, and partial unitaries on select subsets of modes. These results establish MPLC-based pixel-mode interferometers as a promising architecture for programmable linear optics with applications in classical and quantum optical interconnects, photonic switching, and quantum information processing.