Tailoring quantum walks in integrated photonic lattices

TL;DR

This paper compares linear and nonlinear integrated photonic waveguide arrays for quantum walks, experimentally validates their behaviors, and demonstrates inverse design of arrays to generate maximally entangled states, advancing high-dimensional quantum photonics.

Contribution

It provides a systematic comparison of linear and nonlinear quantum walks in integrated photonics and introduces inverse design techniques for creating entangled states.

Findings

Validated the emergence of non-classicality in nonlinear waveguide arrays.

Demonstrated tuning of quantum walk depth over an order of magnitude.

Engineered aperiodic arrays to generate maximally entangled states.

Abstract

Unlike discrete photonic circuits, which manipulate photons step-by-step using a series of optical elements, arrays of coupled waveguides enable photons to interfere continuously across the entire structure. When composed of a nonlinear material, such arrays can also directly generate quantum states of light within the circuit. To clarify the similarities and distinctions between these two approaches of quantum walks, we conduct here a systematic comparison between linear waveguide arrays, injected with photons produced externally, and nonlinear arrays, where photon pairs are continuously generated via parametric down-conversion. We experimentally validate these predictions using III-V semiconductor nonlinear waveguide lattices with varied geometries, enabling us to tune the depth of the quantum walks over an order of magnitude and reveal the gradual emergence of non-classicality in the output state. Finally, we demonstrate an inverse-design approach to engineer \textit{aperiodic} waveguide arrays, whose optimized coupling profiles generate maximally entangled states such as the biphoton W-state. These results highlight the potential of continuously-coupled photonic systems to harness high-dimensional entanglement within compact architectures.