Multi-photon entanglement in high dimensions

TL;DR

This paper reports the first experimental creation of multi-photon entangled states with both the number of particles and their dimensions exceeding two, revealing complex asymmetric entanglement structures useful for advanced quantum communication.

Contribution

The authors demonstrate a novel method to generate high-dimensional, multi-particle entangled states with asymmetric structures, expanding the capabilities of quantum entanglement beyond previous two-dimensional, two-particle systems.

Findings

Successfully generated multi-photon entangled states with high dimensions and multiple particles.

Demonstrated asymmetric entanglement structure with different dimensional spaces for photons.

Proposed a layered quantum cryptographic protocol utilizing these states.

Abstract

Entanglement lies at the heart of quantum mechanics $-$ as a fundamental tool for testing its deep rift with classical physics, while also providing a key resource for quantum technologies such as quantum computation and cryptography. In 1987 Greenberger, Horne, and Zeilinger realized that the entanglement of more than two particles implies a non-statistical conflict between local realism and quantum mechanics. The resulting predictions were experimentally confirmed by entangling three photons in their polarization. Experimental efforts since have singularly focused on increasing the number of particles entangled, while remaining in a two-dimensional space for each particle. Here we show the experimental generation of the first multi-photon entangled state where both $-$ the number of particles and the number of dimensions $-$ are greater than two. Interestingly, our state exhibits an asymmetric entanglement structure that is only possible when one considers multi-particle entangled states in high dimensions. Two photons in our state reside in a three-dimensional space, while the third lives in two dimensions. Our method relies on combining two pairs of photons, high-dimensionally entangled in their orbital angular momentum, in such a way that information about their origin is erased. Additionally, we show how this state enables a new type of "layered" quantum cryptographic protocol where two parties share an additional layer of secure information over that already shared by all three parties. In addition to their application in novel quantum communication protocols, such asymmetric entangled states serve as a manifestation of the complex dance of correlations that can exist within quantum mechanics.