The sky of quantum light is not flat

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 attempting to harness their unique characteristics in quantum technologies, particularly quantum computers and ultra-high-precision sensors. However, these non-classical properties have remained poorly understood until now, given their intrinsic complexity.

In a recent study, researchers from the MPQ and the department of Mathematics at ENS Paris have shown that these properties can be better described using a new geometric representation. Instead of describing light in the conventional way, they consider its phase reference—usually assumed to be a number, embodying the invariance of electromagnetism under time translation—to be an integral part of the quantum system. This conceptually simple modification transforms the standard mathematical description—defined on a plane—into a representation on the surface of a sphere. On this sphere, specific points appear, which the researchers have called “stars.” These points play a very important role in describing multiphoton states, as they reveal the extremely complex way in which photons are linked to one another through quantum entanglement, and how the nonclassical properties of light are directly related to this entanglement.

The flat representation typically used in quantum optics thus appears as a local approximation of this sphere, just as the Earth’s surface appears flat to us on a human scale. In this approximation, only a small portion of the “stars” is visible, while the rest remain hidden. These results reveal a richer structure than previously thought and offer a unified view of two fundamental features of quantum physics: the nonclassical states of light and the ability of similar particles to become entangled. They have been published in Physical Review Letters.

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Contacts: Pérola Milman