Ultra-long quantum walks via spin-orbit photonics

TL;DR

This paper demonstrates ultra-long photonic quantum walks across hundreds of modes using spin-orbit effects in liquid-crystal metasurfaces, enabling large-scale quantum simulations with minimal optical losses.

Contribution

It introduces a novel method for implementing long-range couplings in photonic quantum walks using a few metasurfaces, surpassing previous experimental limits.

Findings

Achieved quantum walks up to 320 steps without amplification.

Demonstrated maximal entanglement generation under dynamical disorder.

Enabled large-scale unitary evolutions with low optical losses.

Abstract

The possibility of fine-tuning the couplings between optical modes is a key requirement in photonic circuits for quantum simulations. In these architectures, emulating the long-time evolution of particles across large lattices requires sophisticated setups, that are often intrinsically lossy. Here we report ultra-long photonic quantum walks across several hundred optical modes, obtained by propagating a light beam through very few closely-stacked liquid-crystal metasurfaces. By exploiting spin-orbit effects, these implement space-dependent polarization transformations that mix circularly polarized optical modes carrying quantized transverse momentum. As each metasurface implements long-range couplings between distant modes, by using only a few of them we simulate quantum walks up to 320 discrete steps without any optical amplification, far beyond state-of-the-art experiments. To showcase the potential of this method, we experimentally demonstrate that in the long-time limit a quantum walk affected by dynamical disorder generates maximal entanglement between two system partitions. Our platform grants experimental access to large-scale unitary evolutions while keeping optical losses at a minimum, thereby paving the way to massive multi-photon multi-mode quantum simulations.