We demonstrate quantum walks of correlated photons in a 2D network of directly laser written waveguides coupled in a 'swiss cross' arrangement. The correlated detection events show high-visibility quantum interference and unique composite behaviour: strong correlation and independence of the quantum walkers, between and within the planes of the cross. Violations of a classically defined inequality, for photons injected in the same plane and in orthogonal planes, reveal non-classical behaviour in a non-planar structure.
@article{arxiv.1308.2554,
title = {Quantum walks of correlated photon pairs in two-dimensional waveguide arrays},
author = {Konstantinos Poulios and Robert Keil and Daniel Fry and Jasmin D. A. Meinecke and Jonathan C. F. Matthews and Alberto Politi and Mirko Lobino and Markus Gräfe and Matthias Heinrich and Stefan Nolte and Alexander Szameit and Jeremy L. O'Brien},
journal= {arXiv preprint arXiv:1308.2554},
year = {2014}
}