In 1926, right after writing his famous wave equation, Schrödinger introduces the concept of coherent states to resolve the outstanding puzzle how to correctly describe the quantum dynamics of a mechanical harmonic oscillator. Today, 100 years later, mechanical quantum systems are an experimental reality in laboratories all over the world - enabled by the development of quantum optomechanics, a new paradigm for light-matter interaction that allows quantum optical control of solid-state mechanical objects (Curiously, one of the first ideas along this line already appeared in a letter from Schrödinger to Sommerfeld in 1931). Devices currently being studied cover a mass range of more than 17 orders of magnitude - from nanomechanical waveguides of some picograms to macroscopic, kilogram-weight mirrors of gravitational wave detectors.
The fast progress in controlling ever increasing masses in the quantum regime creates new and unexpected opportunities to address one of the outstanding questions at the interface between quantum physics and gravity, namely “does gravity require a quantum description?”. Concretely, quantum optomechanics enables experiments that directly probe the phenomenology of quantum states of gravitational source masses. This can lead to experimental outcomes that are inconsistent with the predictions of a purely classical field theory of gravity. Such 'Quantum Cavendish' experiments will rely on delocalized motional quantum states of sufficiently massive objects and gravity experiments on the micrometer scale. I review the current status in the lab and the challenges to be overcome for future experiments.