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A system of one-dimensional quantum harmonic oscillators. The different angles are defined in the equation. (60). Note that for the most general 3D arrangement, the two dashed lines will not intersect. credit: *Physical review* (2024). doi: 10.1103/PhysRevX.14.021022

Gravity is part of our daily life. However, the force of gravity remains a mystery: to this day we do not understand whether its ultimate nature is geometric, as Einstein conceived it, or governed by the laws of quantum mechanics.

Until now, all experimental proposals to answer this question have relied on creating the quantum phenomenon of entanglement between heavy and macroscopic masses. But the heavier an object is, the more it tends to shed its quantum properties and become “classical,” making it very difficult to make a heavy mass behave like a quantum particle.

in a study published in *Physical review* This week, researchers from Amsterdam and Ulm propose an experiment that circumvents these problems.

## Classic or quantum?

Successfully combining quantum mechanics and gravitational physics is one of the major challenges of modern science. In general, progress in this field is hampered by the fact that we cannot yet perform experiments in systems in which quantum and gravitational effects are relevant.

And on a more fundamental level, as Nobel laureate Roger Penrose once said, we don't even know whether a combined theory of gravity and quantum mechanics would require “quantization of gravity” or “quantum gravity.”

In other words: Is gravity essentially a quantum force, whose properties are determined on the smallest possible scales, or is it a “classical” force whose large-scale geometric description suffices? Or is it something different yet?

It has always seemed that to answer these questions, quantum entanglement would play a central role. “The central question, initially posed by Richard Feynman in 1957, is to understand whether the gravitational field of a massive object could enter into a so-called quantum superposition, where it would be in several states,” says Ludovico Lami, a mathematical physicist at the University of Amsterdam and QuSoft. At the same time.

“Before our work, the main idea to solve this question experimentally was to look for gravitationally induced entanglement — the way in which distant but connected masses can exchange quantum information. The existence of such entanglement would refute the hypothesis that the gravitational field is purely local and classical.” “

## Different angle

The main problem with previous proposals is that creating distant but related massive objects – known as non-localized states – is very challenging. The heaviest object for which quantum delocalization has been observed so far is a large particle, much lighter than the smallest source mass whose gravitational field has been detected, which is just under 100 mg, more than a billion billion times heavier. This set aside any hope of experimental investigation for decades.

In the new work, Lammy and colleagues from Amsterdam and Ulm – interestingly, where Einstein was born – offer a possible way out of this impasse. They propose an experiment that would reveal the quantum of gravity without generating any entanglement.

“We have designed and studied a class of experiments involving a system of massive ‘harmonic oscillators’ – for example, a torsion pendulum, like the one used by Cavendish in his famous 1797 experiment to measure the strength of the gravitational force,” explains Lamy. “He set mathematically stringent limits on the signals of A certain empiricism of quantity, which local classical gravity should not be able to overcome.

“We carefully analyzed the experimental requirements needed to implement our proposal in an actual experiment, and found that although a certain degree of technological advancement is needed, such experiments could soon be within reach.”

## A shadow of tangle

Surprisingly, to analyze the experiment, researchers still need the mathematical mechanism of entanglement theory in quantum information science. how is that possible? According to Lamy, “The reason is that although entanglement does not exist physically, it still exists in the spirit, in the strict mathematical sense. It is enough that entanglement has been generated.”

The researchers hope that their paper is just the beginning, and that their proposal will help design experiments that may answer the fundamental question about the quantum of gravity much earlier than expected.

**more information:**

Ludovico Lami et al., Gravity quantum testing without entanglement, *Physical review* (2024). doi: 10.1103/PhysRevX.14.021022

**Magazine information:**

Physical review

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