Speaker: Dr. Gautam Satishchandran (University of Chicago)
Title: Gravitationally Mediated Entanglement: Newtonian Field, Gravitons and Black Holes
12 July 2022 16:00 (Paris), ZOOM
Recently proposed "low-energy" tabletop experiments in quantum gravity aim to experimentally verify quantum entanglement mediated by a Newtonian gravitational field. We argue that if the Newtonian gravitational field of a body can mediate entanglement with another body, then it should also be possible for the body producing the Newtonian field to entangle directly with on-shell gravitons. Our arguments are made by revisiting a gedankenexperiment previously analyzed by Belenchia et al., which showed that a quantum superposition of a massive body requires both quantized gravitational radiation and local vacuum fluctuations of the spacetime metric in order to avoid contradictions with complementarity and causality. We provide a precise and rigorous description of the entanglement and decoherence effects occurring in this gedankenexperiment, thereby significantly improving upon the back-of-the-envelope estimates given in the analysis of Belenchia et al. and also showing that their conclusions are valid in much more general circumstances. As a by-product of our analysis, we show that under the protocols of the gedankenexperiment, there is no clear distinction between entanglement mediated by the Newtonian gravitational field of a body and entanglement mediated by on-shell gravitons emitted by the body. This suggests that Newtonian entanglement implies the existence of graviton entanglement and supports the view that the experimental discovery of Newtonian entanglement may be viewed as implying the existence of the graviton. Furthermore, our analysis sheds light on the relationship between the coherence of a quantum spatial superposition and the causal structure of spacetime. By performing a similar analysis of such a quantum superposition in the exterior of a black hole we prove that the mere presence of a black hole will eventually destroy the coherence of any superposition. We provide estimates of the decoherence time for such quantum superpositions. We believe that the fact that a black hole will eventually decohere any quantum superposition may be of fundamental significance for our understanding of the nature of black holes in a quantum theory of gravity.
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