The spatial state of non-interacting photons

Juan P. Torres, Gabriel Molina‐Terriza, Lluís Torner

We address the spatial shape of entangled photon states generated in spontaneous parametric downconversion in non-collinear geometries, when the interacting waves exhibit Poynting vector walk-off. We discuss how such shape depends on the interplay between the state ellipticity caused by the non-collinear geometry, and the Poynting vector walk-off. In particular, we study the impact of the new features exhibited by walking entangled photons on the implementation of quantum protocols based on the spiral spectra of the entangled photons.

Thad G Walker

Sören Arlt, Mario Krenn, Xuemei Gu

Interactions between quantum systems enable quantum gates, the building blocks of quantum information processing. In photonics, direct photon-photon interactions are too weak to be practically useful, so effective interactions are engineered with linear optics and measurement. A central challenge is to realize such interactions non-locally, i.e., between photons that remain spatially separated. We present experimental proposals for several essential non-local multiphoton quantum gates that act on spatially separated photons, in both qubit and high-dimensional qudit systems. All solutions were discovered by the AI-driven discovery system called PyTheus. Rather than using pre-shared entanglement or Bell state measurements, our gates use as a resource quantum indistinguishability by path identity - a technique that exploits coherent superpositions of the photon pair origins. While analyzing these solutions, we uncovered a new mechanism that mimics much of the properties of quantum teleportation, without shared entanglement or Bell state measurements. Technically, our results establish path indistinguishability as a practical resource for distributed quantum information processing; conceptually, they demonstrate how automated discovery systems can contribute new ideas and techniques in physics.

Mohammad F. Maghrebi, Ofer Firstenberg, Alexey Vyacheslavovich Gorshkov

Andrea Tabacchini, Mathieu L. Juan, Gabriel Molina‐Terriza

We study the topic of non-linear light-matter interaction for the generation of multi-photon entangled states. In particular, we want the quantum correlations to be inherent in the Orbital Angular Momentum (OAM) degree of freedom. Our first proposal is to generate four photons, multidimensional, spatially-entangled states, while the final goal is to make them interact with nanostructures for quantum metrology applications.

I. K. Karpenko