Interference experiments give us much of our information about fundamental physics, and now they can be done with a new twist: self-interfering clocks.

Ron Folman of Ben-Gurion University of the Negev in Beer-Sheva, Israel, and colleagues demonstrated the idea using Bose–Einstein condensates of about 10,000 87Rb atoms prepared in a superposition of two Zeeman states as clocks. The clock time serves as a "which-way" witness. Because in standard quantum mechanics, time is a global parameter, but in general relativity it is local proper time that matters, this opens new avenues to understanding the interplay between quantum mechanics and gravity.