Here's the [paper][1] on the arxiv.
I found it hard to believe that they accounted for other forces precisely enough that they could attribute the phase change to gravity, but this is beyond me so I trust the result.
At first I thought "they showed that you can measure a particle falling in gravity," which seemed dumb because we already know that particles fall in gravity. But they showed that you can measure a single (aggregate) particle falling in gravity, which is pretty cool because if gravity is quantum then that means that they observed an interaction between the graviton and their rubidium atom.
There's no evidence for gravitons.
> ...if gravity is quantum...
You don't need evidence to support a premise.
Their point is measuring gravity's influence on a single particle is not necessarily evidence of the existence of the graviton.
Nobody's claiming it is.
“ if gravity is quantum then that means that they observed an interaction between the graviton and their rubidium atom.”
Actually, we do need evidence for premises which are not definitions or basic assumptions (axioms). Graviton is still a hypothetical particle and falls under neither of two categories - Definitions or axioms.
That doesn't stop us from imagining what something could mean in the context of gravitons if evidence of them ever does turn up.
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