The interface between quantum mechanics and general relativity is a century old problem. On the one hand, the endeavour to combine the two into a grand unified theory provides a puzzle that has occupied theoretical physicists for decades and produced an astounding number of competing theories. Recent ideas pose that entanglement and quantum correlations may give rise to spacetime itself. On the other hand, there is a possibility that gravity may provide the means to restore a classical reality from the quantum one, for example through a gravitationally induced collapse of the wavefunction.
The first key question is then: is gravity quantum? Can spacetime itself be in a superposition? The renown physicist Richard Feynman came up with a thought-experiment that could answer this question and due to the tremendous progress in our control of quantum systems, what Feynman considered a thought-experiment, may now be within reach in a realistic future laboratory.
The key idea is this: by bringing two massive objects into a spatial quantum superpositions, their mutual gravitational interaction can bring them into an quantum entangled state. If we can demonstrate experimentally that the masses have become entangled, while they interact only through gravity, we arrive at the conclusion that, yes, the gravitational field must be considered a quantum system that supports superpositions of metrics. Only then can entanglement be created. Intuitively, because the only interaction between the masses was through gravity, and formally, because entanglement between systems cannot increase under local operations and classical communication. If, on the other hand, no entanglement is created, even though the experiment is carried out correctly (making sure the superposition states have sufficient coherence times, the masses and distances are sufficient to result in observable entanglement), we arrive at a perhaps even more profound conclusion: either quantum theory or general relativity is unsuitable to describe the experiment.
So far, no experimental platform exists that can meet the challenging central requirement for such a test: A picogram-scale mass in a micrometre-scale spatial superposition with a second-scale coherence time. The core objective of this ERC starting grant proposal is the development, characterization and demonstration of an experimental platform that is compatible with the requirements to generate entanglement through the gravitational field.
