Classical theories of gravity produce entanglement
Summary
This Nature paper shows that classical theories of gravity can produce quantum entanglement once matter is treated properly using quantum field theory (QFT). The authors revisit Feynman’s 1957 thought experiment and compare three descriptions: perturbative quantum gravity (with quantised gravitons), perturbative classical gravity (classical metric coupled to quantum matter), and the QFT treatment of electromagnetic interactions as an analogy. They demonstrate that even if the gravitational field itself is classical, virtual quantum matter propagators (arising in QFT) can mediate quantum communication between masses and generate entanglement through higher-order processes. This challenges the common assertion that observing gravitationally induced entanglement would be unambiguous proof of a quantised gravitational field.
The work provides explicit perturbative calculations for a realistic variant of Feynman’s experiment (two spherical masses in spatial superposition). In the quantum-gravity description entanglement appears at second order via virtual graviton exchange. In the classical-gravity-plus-QFT-matter description, entanglement can arise at fourth order via virtual matter exchange even though the gravitational field is not quantised. The strength and relevance of the classical vs quantum contributions depend strongly on experimental parameters (mass, interaction time, superposition size), so the presence of entanglement alone is not necessarily a smoking gun for quantum gravity.
Key Points
- When matter is modelled with quantum field theory, a classical gravitational field can still enable quantum communication via virtual matter propagators.
- In a tabletop Feynman-style experiment, entanglement arises in perturbative quantum gravity at second order (virtual graviton exchange) but can also appear in perturbative classical gravity at fourth order through virtual matter processes.
- The classical-induced entanglement scales differently with mass and time than the quantum-gravity contribution; for sufficiently large masses (approaching Planck mass regimes) the classical effect can dominate.
- Therefore, detecting entanglement between masses does not automatically prove gravity is quantised—the experiment must operate in a parameter regime where classical contributions are negligible.
- The paper provides formulae and Feynman-diagram analyses quantifying both effects and maps out the regions of parameter space where each effect is significant.
- The result is generic: a similar mechanism would operate if other fields (e.g. electromagnetism) were treated classically while matter remained quantum—virtual matter channels can still entangle systems.
Context and relevance
There is intense experimental interest in using small-scale, table-top setups to probe the quantum nature of gravity by looking for gravitationally mediated entanglement. Earlier theorems argued that purely classical, local gravitational channels (LOCC) cannot create entanglement, so observing entanglement would imply quantised gravity. Aziz & Howl show this conclusion is too quick: if matter is treated with QFT then classical gravity can still produce entanglement through virtual matter exchange. That means experimental proposals must carefully choose masses, separation distances and interaction times to ensure they rule out the classical QFT-mediated channel. This paper therefore has direct impact on both experimental design and the interpretation of future results in the field.
Why should I read this
Short version: if you’re tracking experimental tests of quantum gravity or designing one, this paper slaps a big caveat on the idea that entanglement = quantised gravity. It does the hard QFT legwork and maps when a classical gravity model could fake the quantum signature. Read it so you don’t mistake a neat result for a definitive discovery — and so you can plan experiments that actually discriminate between the possibilities.
Author style
Punchy and consequential — the authors don’t just nitpick prior arguments, they supply explicit perturbative calculations and parameter plots that make the implications concrete for experimentalists. If you care about claims that a table-top entanglement test would prove quantum gravity, the detailed maths here are worth the dive.