Resolving the gravitational redshift across a millimetre-scale atomic sample

Cover of Nature Feb. 17 2022 -- SEEING RED
Einstein’s general theory of relativity predicts that Earth’s gravity will sufficiently distort space-time so that clocks at different distances from the planet will tick at different rates — an effect called gravitational redshift. In this week’s issue, Tobias Bothwell, Jun Ye and their colleagues demonstrate this effect at a sub-millimetre scale. The researchers use a cloud of ultracold strontium atoms, effectively creating a series of atomic clocks. As illustrated on the cover, the atoms are trapped in pancake-like optical traps and then interrogated by a laser. The team was able to observe a linear change in frequency — the redshift — from one side of the cloud to the other, showing that each atomic clock was ticking at a slightly different rate.