A self interfering clock as a which path witness

In Einstein s general theory of relativity time depends locally on gravity in standard quantum theory time is global all clocks tick uniformly In the talk I will describe 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