Gravitationally-induced entanglement between two massive particles is sufficient evidence of quantum effects in gravity (1707.06036v2)

Abstract: All existing quantum gravity proposals share the same deep problem. Their predictions are extremely hard to test in practice. Quantum effects in the gravitational field are exceptionally small, unlike those in the electromagnetic field. The fundamental reason is that the gravitational coupling constant is about 43 orders of magnitude smaller than the fine structure constant, which governs light-matter interactions. For example, the detection of gravitons -- the hypothetical quanta of energy of the gravitational field predicted by certain quantum-gravity proposals -- is deemed to be practically impossible. In this letter we adopt a radically different, quantum-information-theoretic approach which circumvents the problem that quantum gravity is hard to test. We propose an experiment to witness quantum-like features in the gravitational field, by probing it with two masses each in a superposition of two locations. First, we prove the fact that any system (e.g. a field) capable of mediating entanglement between two quantum systems must itself be quantum. This argument is general and does not rely on any specific dynamics. Then, we propose an experiment to detect the entanglement generated between two masses via gravitational interaction. By our argument, the degree of entanglement between the masses is an indirect witness of the quantisation of the field mediating the interaction. Remarkably, this experiment does not require any quantum control over gravity itself. It is also closer to realisation than other proposals, such as detecting gravitons or detecting quantum gravitational vacuum fluctuations.

• The paper demonstrates that gravitationally-induced entanglement between two massive particles is a sufficient indicator of quantum effects in gravity.
• It employs a quantum information-theoretic framework with a Mach-Zehnder interferometer setup to test gravity's quantum nature via spatial superposition.
• The proposed experiment, achievable with current matter-wave interferometry and nano-mechanical oscillators, lays the groundwork for empirical validation of quantum gravitational theories.

Gravitationally-Induced Entanglement as an Experimental Test of Quantum Gravity

The intersection of quantum mechanics and general relativity presents compelling theoretical challenges, as efforts to formulate a unified theory of quantum gravity confront both technical and empirical difficulties. The paper by Marletto and Vedral takes a conceptual leap by proposing a quantum-information-theoretic approach to detecting quantum effects in gravity through gravitationally-induced entanglement between two massive particles. This experiment aims to fill the empirical void that has so far impeded our progress toward evidencing quantum-like features in gravitational interaction.

The paper establishes a crucial theoretical foundation by arguing that any mediator producing entanglement between two quantum systems must itself possess quantum characteristics. Applied to gravity, this principle posits that the observation of entanglement between two masses interacting solely via gravitational fields would serve as an indirect witness to gravity’s quantization. This argument is grounded in quantum information theory and is presented independent of any specific dynamical model, increasing its theoretical robustness across potential frameworks of quantum gravity.

The proposed experimental setup involves two masses, each placed in a superposition of spatial locations within separate Mach-Zehnder interferometers. The experiment would detect the entanglement between these masses as a function of the gravitational interaction mediated by the field, revealing quantum features intrinsic to the gravitational field without necessitating direct quantum manipulation of gravity itself. The theoretical analysis presented suggests that gravitational interactions, even at the Newtonian level, could induce entanglement detectable within currently achievable technological capabilities, such as matter-wave interferometry or nano-mechanical oscillators.

The paper's implications extend to several fronts. Practically, this experiment offers a potential empirical breakthrough by providing observable quantization evidence of the gravitational field, accessible through current experimental methodologies. Theoretically, the results could yield insights into the non-commuting nature of gravitational observables, contributing a significant piece to the puzzle of quantum gravity. By establishing a method of empirical inquiry into gravitational quantization, the authors provide a pivotal step toward resolving debates over the necessity and form of quantum gravity.

Future research inspired by this work could dive into exploring other forms of gravitational interactions or entanglement mediators, pushing the boundaries of what constitutes detectable quantum phenomena. Further, comparative analysis of the predictions of various quantum gravity models in light of these findings could enrich our understanding of gravity's quantum aspects.

In summary, Marletto and Vedral offer a novel approach to probing the quantum nature of gravity, proposing a feasible experiment that may bridge a critical gap in the experimental validation of quantum gravitational theories. Their work not only stands as a testament to the innovative cross-pollination of quantum information theory with traditional physics domains but also sets a course for subsequent exploration and discovery in the field of quantum gravity.