# Experimental Two-dimensional Quantum Walk on a Photonic Chip

## TL;DR

This paper demonstrates a large-scale two-dimensional quantum walk on a photonic chip using femtosecond laser writing, revealing unique quantum transport features and paving the way for advanced quantum computing applications.

## Contribution

It introduces a novel 2D continuous-time quantum walk implementation on a photonic chip with 49x49 nodes, using external geometry instead of photon internal degrees of freedom.

## Key findings

- Quantum walks exhibit ballistic evolution and variance profiles consistent with simulations.
- The experiment reveals the transient nature of 2D quantum walks, a feature beyond 1D.
- Large-scale 2D quantum walk architecture enables complex quantum simulations.

## Abstract

Quantum walks, in virtue of the coherent superposition and quantum interference, possess exponential superiority over its classical counterpart in applications of quantum searching and quantum simulation. The quantum enhanced power is highly related to the state space of quantum walks, which can be expanded by enlarging the photon number and/or the dimensions of the evolution network, but the former is considerably challenging due to probabilistic generation of single photons and multiplicative loss. Here we demonstrate a two-dimensional continuous-time quantum walk by using the external geometry of photonic waveguide arrays, rather than the inner degree of freedoms of photons. Using femtosecond laser direct writing, we construct a large-scale three-dimensional structure which forms a two-dimensional lattice with up to 49X49 nodes on a photonic chip. We demonstrate spatial two-dimensional quantum walks using heralded single photons and single-photon-level imaging. We analyze the quantum transport properties via observing the ballistic evolution pattern and the variance profile, which agree well with simulation results. We further reveal the transient nature that is the unique feature for quantum walks of beyond one dimension. An architecture that allows a walk to freely evolve in all directions and a large scale, combining with defect and disorder control, may bring up powerful and versatile quantum walk machines for classically intractable problems.