Violation of a Leggett-Garg Inequality Using Ideal Negative Measurements in Neutron Interferometry

Leggett-Garg inequalities (LGIs) have been proposed in order to assess how far the predictions of quantum mechanics defy ``macroscopic realism.'' With LGIs, correlations of measurements performed on a single system at different times are described. We report on an experiment that demonstrates the violation of an LGI with neutrons. The final measured value of the Leggett-Garg correlator K=1.120±0ex0ex0.007(stat)±0.019(sys), obtained in a neutron interferometric experiment, is clearly above the limit $K=1$ predicted by macrorealistic theories. The experimental results are analyzed within the framework of dynamical theory of neutron diffraction, evidently reproducing the obtained values.