Researchers at the Institute of Quantum Optics and Quantum Information (IQOQI), the University of Vienna, and the Universitat Autònoma de Barcelona have achieved a new milestone in quantum physics: they were able to entangle three particles of light in a high-dimensional quantum property related to the "twist" of their wavefront structure. Just like Schrödinger's famous cat that is simultaneously dead and alive, all previous demonstrations of multi-particle entanglement have been with quantum objects in two discrete levels, or dimensions. The twisted photons used in the Vienna experiment have no such limit to their dimensionality, and can simultaneously exist in three or more quantum states.
Ref: Multi-photon entanglement in high dimensions. Nature Photonics (29 February 2016) | DOI: 10.1038/nphoton.2016.12
Forming the backbone of quantum technologies today, entanglement, has been demonstrated in physical systems as diverse as photons, ions and superconducting circuits. Although steadily pushing the boundary of the number of particles entangled, these experiments have remained 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 dimensions are greater than two. Two photons in our state reside in a three-dimensional space, whereas the third lives in two dimensions. This asymmetric entanglement structure6 only appears in multiparticle entangled states with d > 26. Our method relies on combining two pairs of photons, high-dimensionally entangled in their orbital angular momentum. In addition, we show how this state enables a new type of ‘layered’ quantum communication protocol. Entangled states such as these serve as a manifestation of the complex dance of correlations that can exist within quantum mechanics.