# A self-interfering clock as a „which path“ witness

### Friday, 13 May. 2016, 14:00 - 15:30

**Presenter:** Ron Folman

**Host:** Caslav Brukner

**Where:** IQOQI Seminar Room, 2nd Floor, Boltzmanngasse 3, 1090 Vienna

In Einstein’s general theory of relativity, time depends locally on gravity; in standard

quantum theory, time is global—all clocks “tick” uniformly. We demonstrate a new tool for investigating time in the overlap of these two theories: a self-interfering clock, comprising two atomic spin states. We prepare the clock in a spatial superposition of quantum wave packets, which evolve coherently along two paths into a stable interference pattern. If we make the clock wave packets “tick” at different rates, to simulate a gravitational time lag, the clock time along each path yields “which path” information, degrading the pattern’s visibility.

In contrast, in standard interferometry, time cannot yield “which path” information.

This proof-of-principle experiment may have implications for the study of time and general relativity and their impact on fundamental effects such as decoherence and the emergence of a classical world.