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 are one of the few testable solutions so far proposed. In particular, the continuous spontaneous localization (CSL) model has become subject of intense experimental research. Experiments looking for the universal force noise predicted by CSL in ultrasensitive mechanical resonators have recently set the strongest unambiguous bounds on CSL. Further improving these experiments by direct reduction of mechanical noise is technically challenging. Here, we implement a recently proposed alternative strategy that aims at enhancing the CSL noise by exploiting a multilayer test mass attached on a high quality factor microcantilever. The test mass is specifically designed to enhance the effect of CSL noise at the characteristic length r_{c}=10^{-7} m. 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 1 order of magnitude. Our results explicitly challenge a well-motivated region of the CSL parameter space proposed by Adler.
Spontaneous collapse models predict that a weak force noise acts on any mechanical system, as a consequence of the collapse of the wave function. Significant upper limits on the collapse rate have been recently inferred from precision mechanical experiments, such as ultracold cantilevers and the space mission LISA Pathfinder. Here, we report new results from an experiment based on a high-Q cantilever cooled to millikelvin temperatures, which is potentially able to improve the current bounds on the continuous spontaneous localization (CSL) model by 1 order of magnitude. High accuracy measurements of the cantilever thermal fluctuations reveal a nonthermal force noise of unknown origin. This excess noise is compatible with the CSL heating predicted by Adler. Several physical mechanisms able to explain the observed noise have been ruled out.
The continuous spontaneous localization (CSL) model is the most studied among collapse models, which describes the breakdown of the superposition principle for macroscopic systems. Here, we derive an upper bound on the parameters of the model by applying it to the rotational noise measured in a recent short-distance gravity experiment [Lee et al., Phys. Rev. Lett. 124, 101101 (2020)]. Specifically, considering the noise affecting the rotational motion, we found that despite being a tabletop experiment the bound is only one order of magnitude weaker than that from LIGO for the relevant values of the collapse parameter. Further, we analyze possible ways to optimize the shape of the test mass to enhance the collapse noise by several orders of magnitude and eventually derive stronger bounds that can address the unexplored region of the CSL parameters space.
Abstract Spontaneous collapse models (CM) have been proposed as a possible solution of the quantum measurement problem. In CM the unitary evolution of the wave function is modified by stochastic corrections which lead to a dynamical reduction at macroscopic level. Unlike interpretations of quantum mechanics, CM predict departures from standard theory that can be experimentally tested. Here, we focus on one of these predictions, a universal force noise acting on any mechanical system. In particular, we discuss recent bounds on collapse models set by ultracold cantilevers experiments and gravitational wave detectors. First notable results are a partial exclusion of the CSL parameters proposed by Adler and a full exclusion of an early model based on quantum gravity ideas.
Spontaneous collapse models have been proposed as a possible realist solution of the quantum measurement problem. They assume that the standard unitary evolution is valid only in the microscopic limit, with stochastic corrections leading to a dynamical reduction of the wave function at macroscopic level. A remarkable feature of collapse models is that, unlike interpretations of quantum mechanics, they predict departures from standard theory that can be experimentally tested. A common prediction is a violation of energy conservation, which manifests as an universal force noise acting on mechanical systems. Here, we will discuss recent bounds on collapse models set by mechanical experiments, focusing on ultracold cantilevers and gravitational wave detectors. First notable results are a partial exclusion of the CSL parameters proposed by Adler and a full exclusion of an early model based on quantum gravity arguments.
Collapse models predict a tiny violation of energy conservation, as a consequence of the spontaneous collapse of the wave function. This property allows us 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 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.
Quantum mechanics, which governs all microscopic phenomena, encounters challenges when applied to macroscopic objects that exhibit classical behavior. To address this micro-macro disparity, collapse models such as the Continuous Spontaneous Localization (CSL) and Diosi-Penrose (DP) models have been proposed. These models phenomenologically modify quantum theory to reconcile its predictions with the observed classical behavior of macroscopic systems. Based on previous works\,([Phys.\,Rev\,D,\,95(8):084054\,(2017)] and [Phys.\,Rev.\,D,\,94:124036,\,(2016)]), an improved bound on the collapse model parameters is given using the updated acceleration noise data released from LISA Pathfinder\,([Phys.\,Rev.\,D, 110(4):042004,\,(2024)]). The CSL collapse rate is bounded to be at most
λCSL≤8.3×10−11\,
s−1 at the mili-Hertz band when
rCSL=10−7m, and the DP model's regularization cut-off scale is constraint to be
σDP∼285.5\,fm. Furthermore, we discuss the potential advantages of using deep-underground laboratories to test these quantum collapse models. Our results show the quiet seismic condition of the current deep-underground laboratory has the potential to further constrain the CSL collapse model to
λCSL≤3×10−11s−1 when
rCSL=10−7m.