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  • Featured in Physics
  • Open Access

Quantum Effects in Gravity Beyond the Newton Potential from a Delocalized Quantum Source

Lin-Qing Chen1,2,3,* and Flaminia Giacomini1,†

  • 1Institute for Theoretical Physics, ETH Zürich, HIT, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland
  • 2Institute of Quantum Optics and Quantum Information Vienna (IQOQI-Vienna), Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austria
  • 3Faculty of Physics, University of Vienna, Boltzmanngasse 5, 1090 Vienna, Austria

  • *Contact author: linqing.nehc@gmail.com
  • †Contact author: flaminiagiacomini@gmail.com
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Phys. Rev. X 15, 031063 – Published 4 September, 2025

DOI: https://doi.org/10.1103/hl1c-t8z9

Abstract

Recent progress in tabletop experiments offers the opportunity to show, for the first time, that gravity is not compatible with a classical description. In all current experimental proposals, such as the generation of gravitationally induced entanglement between two quantum sources of gravity, gravitational effects can be explained solely with the Newton potential, namely, in a regime that is consistent with the weak-field limit of general relativity and does not probe the field nature of gravity. Hence, the Newtonian origin of the effects is a limitation to the conclusions on the nature of gravity that can be drawn from these experiments. Here, we identify two effects that overcome this limitation: They cannot be reproduced using the Newton potential, and they are independent of graviton emission. First, we show that the interaction between two generic quantum sources of gravity, e.g., in wide Gaussian states, cannot be reproduced with the Newton potential nor with a known classical theory of gravity. Second, we show that the quantum commutator between the gravitational field and its canonically conjugate momentum appears as an additional term in the relative phase of a generic quantum source interacting with a test particle. Observing these two effects would give further quantitative information and stronger evidence, compared to experiments only involving the Newton potential, to show that gravity is nonclassical. More broadly, identifying stronger quantum aspects of gravity than those reproducible with the Newton potential is crucial to prove the nonclassicality of gravity and to plan a new generation of experiments testing quantum aspects of gravity in a broader sense than what has been proposed so far.

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synopsis

Removing Classical Inputs from Quantum Gravity Tests

Published 4 September, 2025

Researchers suggest that unambiguous signals of quantum gravity could appear in future tabletop experiments with gravitationally interacting objects.

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Popular Summary

Testing whether gravity is fundamentally classical or quantum has long seemed out of reach, since quantum gravitational effects were believed to only manifest at extremely high energies or tiny length scales. However, advances in quantum technology now allow us to propose entirely new tests of gravity—specifically, by observing how a quantum object generates a gravitational field. Until now, most experimental proposals have been limited, as they test only the static Newtonian potential, without probing gravity as a dynamical field. In this work, we identify two key predictions that go beyond this limitation and offer stronger evidence that gravity may not be classical.

We base our analysis on linearized quantum gravity, the widely accepted low-energy limit that all quantum gravity theories are expected to match. The first prediction shows that when two systems are in highly delocalized quantum states, their gravitational interaction cannot be explained by any known classical model, nor the Newtonian potential. The second prediction reveals that the quantum commutation relations of the gravitational field leave a measurable imprint through the interaction between a test particle and a gravitation source. These features would be absent if gravity were purely classical.

These results chart new directions for experimental efforts to probe quantum aspects of gravity. Our work helps define a broader and more ambitious experimental agenda aimed at answering one of the deepest open questions in physics: Is gravity a quantum phenomenon?

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Phys. Rev. X 15, 031063– Published 4 September, 2025
  • Received 12 April 2024
  • Revised 4 April 2025
  • Accepted 8 July 2025
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