However, when photons are contained within structures that are smaller than their wavelength, these measures collapse into each other, and so the definition is of total angular momentum (TAM). It’s this feature, only occurring for photons confined in this way, that has now been entangled for the first time.

Researchers at Technion-Israel Institute of Technology used gratings to confine photons within a circular or spiral nanoscale platform and mapped their states, entangling the TAMs of pairs of photons before scattering them to free space. Entangling TAMs might seem like a minor development, seeing that SAMs and OAMs have each been entangled before, but the authors write: “We observe that entanglement in TAM leads to a completely different structure of quantum correlations of photon pairs, compared with entanglement related to the two constituent angular momenta.”

Quantum entanglement is considered key to quantum computing. The authors propose their work could lead to information processing conducted using the entangled TAMs of photons confined to chips. Entangling TAMs allows quantum processors based around photons to be smaller than would be possible if one of the properties that only emerges under less confined conditions was used. That potentially enables the miniaturization of future quantum computers.