  The violation of J. Bell's inequality with two entangled and spatially
separated quantum two- level systems (TLS) is often considered as the most
prominent demonstration that nature does not obey ?local realism?. Under
different but related assumptions of "macrorealism", plausible for macroscopic
systems, Leggett and Garg derived a similar inequality for a single degree of
freedom undergoing coherent oscillations and being measured at successive
times. Such a "Bell's inequality in time", which should be violated by a
quantum TLS, is tested here. In this work, the TLS is a superconducting quantum
circuit whose Rabi oscillations are continuously driven while it is
continuously and weakly measured. The time correlations present at the detector
output agree with quantum-mechanical predictions and violate the inequality by
5 standard deviations.
