Entanglement is one of the most characteristic phenomena of quantum mechanics: two or more particles become linked in such a way that measuring the state of one instantly determines the state of the other, even if they are separated by an enormous distance, defying the laws of classical physics.

Creating this type of link between light and the motion of a material object has long been one of the goals of physics research, particularly in the field known as quantum optomechanics. It is towards this goal that the research carried out by scientists from the Department of Physics and Astronomy at the University of Florence and the National Institute of Optics (CNR-INO) is directed, with the support of the National Institute of Nuclear Physics (INFN) and the European Laboratory for Non-linear Spectroscopy (LENS). The team has succeeded in demonstrating the observation of stationary entanglement between the motion of a glass nanosphere and the light transmitted through an optical cavity.

The research findings have been published in *Science* in an article (doi: 10.1126/science.aeh1375) with Francesco Marin, a lecturer in Experimental Physics of Matter at the University of Florence, as corresponding author, and Quentin Deplano (University of Florence), Antonio Pontin (CNR-INO) and Francesco Marino (CNR-INO) as co-authors.

In the experiment, a glass sphere approximately 100 nanometres in diameter is held suspended in a vacuum by means of an optical tweezers (a highly focused laser beam), and placed inside an optical cavity formed by a pair of mirrors, maintained at room temperature. Two different lasers play complementary roles: one cools and stabilises the particle’s motion, whilst the other generates the quantum correlations responsible for entanglement.

“The most significant aspect is that the correlations are not confined to the cavity: they are transferred to the light that emerges from it and propagates through space. In this way, the motion of the nanospher, a localised quantum system, is linked to a light field that can carry information to another location,” says Marin.

Evidence of entanglement comes from the analysis of the separability parameter. Whilst for classical correlations this value cannot fall below the threshold of unity, in the experiment it dropped to a minimum value of 0.918 ± 0.029, confirming the quantum nature of the link. The phenomenon also demonstrated remarkable stability across a frequency band exceeding 40 kilohertz.

“Another important aspect of the research,” explains the Unifi lecturer, “is that the experiment operates in a steady state at room temperature, without requiring complex cryogenic conditions. This makes levitated optomechanical systems particularly promising for the transfer and storage of quantum states.”

“Connecting a stationary material object with a ‘floating’ light field travelling through space opens up revolutionary possibilities,” adds Deplano. “The motion of the nanospheres could act as a quantum memory, capable of storing the information carried by light and then releasing it in future computers and quantum networks.”

“In addition to the technological implications,” he concludes, “the ability to control quantum states in massive objects at room temperature allows us to explore the fine line between the quantum microcosm and the macroscopic world, paving the way for future tests of quantum gravity.”

The research was funded by PNRR funds, through the National Quantum Science and Technology Institute (Unifi) and Integrated Infrastructure Initiative in Photonic and Quantum Sciences (CNR-INO) projects.

Link to the article: Stationary entanglement of a levitated oscillator with an optical field | Science