Atomic clocks are so precise they can measure the gravitational time dilation difference within atoms one millimeter away from each other.

Just listened to this podcast with Ivette Fuentes where she is devising experiments to probe quantum gravity effects using atomic clocks and entanglement. Interesting stuff.

https://podcasts.apple.com/ca/podcast/theories-of-everything-with-curt-jaimungal/id1521758802?i=1000666383456