Challenging spontaneous collapse models with ultracold layered force sensors

Despite the unquestionable empirical success of quantum theory, witnessed by the recent uprising of quantum technologies, the debate on how to reconcile the theory with the macroscopic classical world is still open. Spontaneous collapse models, based on stochastic and nonlinear modifications of the Schrodinger equation, are the only experimentally testable solution so far proposed. Here, we describe a new experiment based on monitoring a high quality factor microcantilever loaded by a layered test mass, specifically designed to test the Continuous Spontaneous Localization (CSL) model. The measurements are in good agreement with pure thermal motion for temperatures down to $100$ mK. From the absence of excess noise we infer a new bound on the collapse rate at the characteristic length $r_c=10^{-7}$ m, which improves over previous mechanical experiments by more than one order of magnitude. Our results are explicitly challenging a well-motivated region of the CSL parameter space proposed by Adler.