Merging quantum mechanics with gravitational physics remains a daunting challenge. Experiments combining quantum and gravitational effects are scarce, and the theoretical understanding is incomplete. Nobel laureate Roger Penrose questioned whether a unified theory would necessitate a 'quantisation of gravity' or a 'gravitisation of quantum mechanics'. Is gravity inherently a quantum force affecting the minutest scales, or is it classical with a macroscopic geometric description?

The cornerstone for resolving these questions has been the quantum phenomenon of entanglement. Ludovico Lami of the University of Amsterdam and QuSoft explains, "The central question, initially posed by Richard Feynman in 1957, is to understand whether the gravitational field of a massive object can enter a so-called quantum superposition, where it would be in several states at the same time. Prior to our work, the main idea to decide this question experimentally was to look for gravitationally induced entanglement - a way in which distant but related masses could share quantum information. The existence of such entanglement would falsify the hypothesis that the gravitational field is purely local and classical."

Previously, attempts to create 'delocalised states' with heavy objects have been impractical. The heaviest object observed in quantum delocalisation is a large molecule, significantly lighter than any mass with a detectable gravitational field. This has delayed experimental progress by decades.

Lami and his team propose a novel approach that does not rely on generating entanglement. They suggest using massive 'harmonic oscillators', like those in Cavendish's 1797 experiment, to demonstrate gravitational quantumness. "We design and investigate a class of experiments involving a system of massive 'harmonic oscillators' - for example, torsion pendula, essentially like the one that Cavendish used in his famous 1797 experiment to measure the strength of the gravitational force. We establish mathematically rigorous bounds on certain experimental signals for quantumness that a local classical gravity should not be able to overcome. We have carefully analysed the experimental requirements needed to implement our proposal in an actual experiments, and find that even though some degree of technological progress is still needed, such experiments could really be within reach soon," says Lami.

To interpret the experiment, the researchers still use entanglement theory mathematically. "The reason is that, although entanglement is not physically there, it is still there in spirit - in a precise mathematical sense. It is enough that entanglement could have been generated," Lami notes.

Their findings, detailed in Physical Review X, lay the groundwork for future experiments that could address the quantum nature of gravity sooner than anticipated.

Research Report:Testing the Quantumness of Gravity without Entanglement

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