The quantum light sky is not flat
In a recent study, researchers have defined the relevant mathematical framework for coherently describing quantum states involving multiple photons, and in particular their entanglement – a property that underpins the effectiveness of quantum computing.
References:
Nicolas Moulonguet, Eloi Descamps, José Lorgeré, Astghik Saharyan, Arne Keller, Pérola Milman, Non-Gaussianity from Superselection Rules, Physical Review Letters 137, 050203 – Published 29 July, 2026.
DOI: 10.1103/5fl9-89j4
Open access: arXiv
Light can exhibit quantum properties that have no equivalent in our everyday experience. These ‘multiphoton’ states of light are being studied by physicists, who are seeking to harness their unique characteristics in quantum technologies, notably quantum computers and ultra-high-precision sensors. However, these non-classical properties remain poorly understood, given their intrinsic complexity.
This research was carried out in the following CNRS laboratory:
Laboratoire Matériaux et Phénomènes Quantiques (MPQ, CNRS / Université Paris Cité)
In a recent study, researchers have shown that these properties can be better understood using a new geometric representation. Instead of describing light in the conventional way, they treat its phase reference (usually assumed to be a number, embodying the time-translation invariance of electromagnetism) as an integral part of the quantum system. This conceptually simple modification transforms the usual mathematical description, defined on a plane, into a representation on the surface of a sphere. On this sphere, particular points appear, which the researchers call ‘stars’. These points play a very important role in the description of multiphoton states, as they reveal the extremely complex way in which photons are linked to one another through quantum entanglement, and how the non-classical properties of light are directly linked to this entanglement.
The flat representation usually employed in quantum optics thus appears as a local approximation of this sphere, in the same way that the Earth’s surface appears flat to us on a human scale. In this approximation, only a small proportion of the ‘stars’ are visible, whilst the rest remain hidden. These results highlight a richer structure than previously thought and offer a unified view of two fundamental characteristics of quantum physics: the non-classical states of light and the ability of identical particles to become entangled. They also provide a better understanding of the specific properties of photons that make a light-based quantum computer more powerful than its classical counterpart. They have been published in the Physical Review Letters.
