'Quantum Tests of Gravity' by Markus Aspelmeyer

On April 26th 2018, Prof. Markus Aspelmeyer from the University of Vienna visited our Institute. He gave the colloquia "Quantum Tests of Gravity".

It is well known that quantum systems can serve as extremely sensitive probes of gravity. A series of pioneering experiments, including with neutrons, atom interferometers and atomic clocks, have established a firm basis of quantum experiments that test effects of weak gravitational fields. The last few years have seen a renewed interest accompanied by a dramatic increase of experiments and experimental proposals to explore the interface between quantum physics and gravity. On the one hand, quantum optics and cold atom experiments have been pushing the sensitivity of measurements of space and time to unprecedented regimes. On the other hand, the fast progress in macroscopic quantum experiments may soon allow to study large quantum superposition states involving clocks or increasingly massive objects.

[Achrononmaster]

All due respect to experimentalists, but this sort of work is ill-founded as a test of QG. It's a good experiment. But it will not prove spacetime is in superposition. It will only show matter can be placed in superposition and the gravity effects thereof measured without detecting which-way information in an interferometer set-up. This is superposing matter fields, not gravity. If you want to test quantum gravity you'll need gravitons, which you will not be able to find (Dyson's argument).
What it means is that in our universe gravity is classical. But we would describe it quantum mechanically if gravitons were detectable, since they're not, we do not need to quantize gravity. We quantize *_local_* matter fields only, not *_global_* spacetime.
Put in other words, if you can fabricate, generate or pull gravitons out of a hat, they'd be spacetime instantons essentially, and would be subject to entanglement and superposition. These are not superpositions of global spacetime though, they're GR instantons or solitons, or localised gravity waves. They'd be subject to QM just like photons. I think they'd disperse so rapidly into gravity waves we'll never detect them like a particle (local energy deposition), but that's just a guess. Maybe your group can do it?
Put in yet other words, if you want a proper test to tell if GR needs a quantization treatment, you'll need to superpose universes (the rep.s of the Poincaré symmetries), which is an impossible thing to do, so you might as well just assume gravity is classical and if some creature beyond spacetime can do the experiment wish them all the best, it won't mean a thing for us.