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Upper bounds on spontaneous wave-function collapse models using millikelvin-cooled nanocantilevers

Collapse models predict a tiny violation of energy conservation, as a consequence of the spontaneous collapse of the wave function. This property allows to set experimental bounds on their parameters. We consider an ultrasoft magnetically tipped nanocantilever cooled to millikelvin temperature. The thermal noise of the cantilever fundamental mode has been accurately estimated in the range $0.03-1$ K, and any other excess noise is found to be negligible within the experimental uncertainty. From the measured data and the cantilever geometry, we estimate the upper bound on the Continuous Spontaneous Localization (CSL) collapse rate in a wide range of the correlation length $r_C$. Our upper bound improves significantly previous constraints for $r_C>10^{-6}$ m, and partially excludes the enhanced collapse rate suggested by Adler. We discuss future improvements.