On Sunday, I toured the 30 ft. tall atom fountain in the bowels of Stanford... an atom interferometer that will test the equivalence principle of Einstein’s theory of general relativity…. Does everything experience the same local gravitational pull form the Earth, regardless of mass?

It's the working end of Puzzle 94

From the bottom of this vertical tube, Rb85 and Rb87 atoms shoot up and then fall back by gravity in a race, like the hammer and feather on the moon, but with insane precision.

Their position is only resolved by observation on their return. At apogee near the top of this photo, they are in a cloud spread over 10 centimeters of superposition.

Jason Hogan and team hopes to detect differences in gravity to 15 or maybe 16 decimal places. If they see a difference, it could suggest a fifth fundamental force (beyond electromagnetism, gravity, strong and weak forces) that operates over long distances (meters to Earths).

Interestingly, this work started in Steven Chu’s group (now head of the DOE) using a 10x smaller cesium fountain.