Experimental violation of a Bell's inequality in time with weak measurement

TL;DR

This paper demonstrates a violation of a Bell-like inequality in time using a superconducting quantum circuit under continuous weak measurement, confirming quantum predictions and challenging macrorealist assumptions.

Contribution

It provides the first experimental test of a Bell's inequality in time with a superconducting qubit under weak measurement, showing quantum correlations in a single system over time.

Findings

Time correlations violate the inequality by 5 standard deviations.

Results agree with quantum mechanical predictions.

Supports non-classical temporal correlations in quantum systems.

Abstract

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.