Hyperentanglement in structured quantum light

TL;DR

This paper presents a novel source of high-dimensional, hyperentangled photon pairs in structured light modes, enabling advanced quantum information protocols with increased resilience and complexity.

Contribution

It introduces the first demonstration of hyperentanglement involving spectrally and spatially structured light in a single biphoton state, combining multiple entanglement forms.

Findings

Achieved near-unity visibilities and fidelities in entangled photon pairs.

Generated high-dimensional hyperentangled states in time-frequency and vector-vortex modes.

Demonstrated coexistence of three different entanglement forms in one biphoton state.

Abstract

Entanglement in high-dimensional quantum systems, where one or more degrees of freedom of light are involved, offers increased information capacities and enables new quantum protocols. Here, we demonstrate a functional source of high-dimensional, noise-resilient hyperentangled states encoded in time-frequency and vector-vortex structured modes, which in turn carry single-particle entanglement between polarisation and orbital angular momentum. Pairing nonlinearity-engineered parametric downconversion in an interferometric scheme with spin-to-orbital-angular-momentum conversion, we generate highly entangled photon pairs at telecom wavelength that we characterise via two-photon interference and quantum state tomography, achieving near-unity visibilities and fidelities. While hyperentanglement has been demonstrated before in photonic qubits, this is the first instance of such a rich entanglement structure involving spectrally and spatially structured light, where three different forms of entanglement coexist in the same biphoton state.