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arXiv:2205.15333 · Scientific Reports

Quantum correlations beyond entanglement in a classical-channel model of gravity

Federico Roccati, Benedetto Militello, Emilio Fiordilino, R. Iaria, L. Burderi, T. Di Salvo, Francesco Ciccarello

Published Oct 21, 2022• 4 citations
arXivHTMLPDF

Abstract

A direct quantization of the Newtonian interaction between two masses is known to establish entanglement, which if detected would witness the quantum nature of the gravitational field. Gravitational interaction is yet compatible also with gravitational decoherence models relying on classical channels, hence unable to create entanglement. Here, we show in paradigmatic cases that, despite the absence of entanglement, a classical-channel model of gravity can still establish quantum correlations in the form of quantum discord between two masses. This is demonstrated for the Kafri-Taylor-Milburn (KTM) model and a recently proposed dissipative extension of this. In both cases, starting from an uncorrelated state, a significant amount of discord is generally created. This eventually decays in the KTM model, while it converges to a small stationary value in its dissipative extension. We also find that initial local squeezing on the state of the masses can significanlty enhance the generated discord.

DOI
10.1038/s41598-022-22212-1

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A direct quantization of the Newtonian interaction between two masses is known to establish entanglement, which if detected would witness the quantum nature of the gravitational field. Gravitational interaction is yet compatible also with gravitational decoherence models relying on classical channels, hence unable to create entanglement. Here, we show in paradigmatic cases that, despite the absence of entanglement, a classical-channel model of gravity can still establish quantum correlations in the form of quantum discord between two masses. This is demonstrated for the Kafri-Taylor-Milburn (KTM) model and a recently proposed dissipative extension of this. In both cases, starting from an uncorrelated state, a significant amount of discord is generally created. This eventually decays in the KTM model, while it converges to a small stationary value in its dissipative extension. We also find that initial local squeezing on the state of the masses can significanlty enhance the generated discord.
Abstract Experiments witnessing the entanglement between two particles interacting only via the gravitational field have been proposed as a test whether gravity must be quantized. In the language of quantum information, a non-quantum gravitational force would be modeled by local operations with classical communication (LOCC), which cannot generate entanglement in an initially unentangled state. This idea is criticized as too constraining on possible alternatives to quantum gravity. We present a parametrized model for the gravitational interaction of quantum matter on a classical spacetime, inspired by the de Broglie–Bohm formulation of quantum mechanics, which results in entanglement and thereby provides an explicit counterexample to the claim that only a quantized gravitational field possesses this capability.
The paradigm in which gravity can generate entanglement between distant masses represents a profound shift in our understanding of quantum-gravitational interfaces and is still among the active and debated topics. Recent work [A. ur Rahman , ] established gravitational cat states as robust carriers of quantum correlations under thermal and classical fields. In our work, we demonstrate their capacity to stabilize quantum resources under a dissipative environment, which is the dominant decoherence mechanism in engineered platforms such as superconducting qubits. By incorporating both gravitational-like and spin-orbit couplings in a Markovian open system, we analytically demonstrate that quantum correlations attain nonvanishing steady-state values, and the stationary fidelity is enhanced by increasing the gravitational-like coupling. The spin-orbit interaction drives Rabi-like oscillations that counteract monotonic decay, and the gravitational coupling mimics non-Markovian memory, enabling long-term information stability. These mechanisms position gravity-inspired interactions not merely as entanglement generators, but as active stabilizers of quantumness in realistic dissipative environments.
A recent paper by Aziz and Howl (Nature 2025) argues that, once quantum matter is described at the level of quantum field theory and coupled to a classical gravitational field, higher order processes can generate entanglement between two spatially separated masses. A contemporaneous critical note (Marletto, Oppenheim, Vedral, Wilson, arXiv:2511.07348v1) shows that, in the actual nonrelativistic limit employed there, the interaction becomes ultra local, the total unitary factorizes, and no entanglement is generated from a product input. In this comment we (i) restate the core point of that critique, (ii) give a channel theoretic reformulation that makes the conclusion model independent, and (iii) clarify the distinction between activation of entanglement in already quantum matter and genuine mediation of entanglement by a classical field. Once these clarifications are in place, the standard BMV inference that observation of gravity mediated entanglement strongly indicates nonclassical gravitational degrees of freedom remains intact.
Recently, table-top experiments involving massive quantum systems have been proposed to test the interface of quantum theory and gravity. In particular, the crucial point of the debate is whether it is possible to conclude anything on the quantum nature of the gravitational field, provided that two quantum systems become entangled solely due to the gravitational interaction. Typically, this question has been addressed by assuming a specific physical theory to describe the gravitational interaction, but no systematic approach to characterise the set of possible gravitational theories which are compatible with the observation of entanglement has been proposed. Here, we remedy this by introducing the framework of Generalised Probabilistic Theories (GPTs) to the study of the nature of the gravitational field. This framework enables us to systematically study all theories compatible with the detection of entanglement generated via the gravitational interaction between two systems. We prove a no-go theorem stating that the following statements are incompatible: i) gravity is able to generate entanglement; ii) gravity mediates the interaction between the systems; iii) gravity is classical. We analyse the violation of each condition, in particular with respect to alternative non-linear models such as the Schrödinger-Newton equation and Collapse Models.
Abstract It is currently believed that there is no experimental evidence on possibly quantum features of gravity or gravity-motivated modifications of quantum mechanics. Here we show that single-atom interference experiments achieving large spatial superpositions can rule out a framework where the Newtonian gravitational interaction is fundamentally classical in the information-theoretic sense: it cannot convey entanglement. Specifically, in this framework gravity acts pairwise between massive particles via classical channels, which effectively induce approximately Newtonian forces between the masses. The experiments indicate that if gravity does reduce to the pairwise Newtonian interaction between atoms at low energies, this interaction cannot arise from the exchange of just classical information, and in principle has the capacity to create entanglement. We clarify that, contrary to current belief, the classical-channel description of gravity differs from the model of Diosi and Penrose, which is not constrained by the same data.
What if gravity is classical? If true, a consistent coexistence of classical gravity and quantum matter requires that gravity exhibit irreducible fluctuations. These fluctuations can mediate classical correlations, but not quantum entanglement, between the quantized motion of the gravitationally interacting matter. We use a consistent theory of quantum-classical dynamics in the Newtonian limit of gravity to show that experimentally relevant observables can conclusively test the hypothesis that gravity is classical. This can be done, for example, by letting highly coherent source masses interact with each other gravitationally, and performing precise measurements of the cross-correlation of their motion. Theory predicts a characteristic phase response that distinguishes classical gravity from quantum gravity, and from naive sources of decoherence. Such experiments are imminently viable.