Testing quantum gravity with interactive information sensing
The quantum nature of the gravitational field has been an open question for nearly 100 years. The simplest question one could ask is whether the gravitational field itself is a quantum degree of freedom. Here we propose a test of this idea. Specifically, if the field is quantum-mechanical, it should be a medium capable of communicating quantum information between a pair of masses. We suggest an experiment in which one could test this gravitational quantum communication using an atom interferometer to interrogate a mechanical oscillator. With state-of-the-art interferometry techniques and good environment isolation, this experiment appears to be feasible in the near term.
This result is extremely exciting for two main reasons. One is that it relies on a new idea about detection of quantum entanglement--a protocol we dub "interactive quantum information sensing". Unlike a more typical entanglement test like a Bell violation experiment, our proposal is highly robust to thermal noise and appears to be capable of sensing much weaker entanglement than often considered. Moreover, it requires only measurements on a single system in the entangled pair. This protocol has many possible applications besides the particular gravitational experiment we considered here. The other cause for excitement is that this test, if successfully implemented in the gravitational context, would constitute the first direct test capable of showing affirmative evidence for the quantization of gravity--a first step to answering the simplest and most fundamental question about the quantum theory of gravity.