Wave function collapse models postulate a fundamental breakdown of the quantum superposition principle at the macroscale. Therefore, experimental tests of collapse models are also fundamental tests of quantum mechanics. Here, we compute the upper bounds on the collapse parameters, which can be inferred by the gravitational wave detectors LIGO, LISA Pathfinder, and AURIGA. We consider the most widely used collapse model, the continuous spontaneous localization (CSL) model. We show that these experiments exclude a huge portion of the CSL parameter space, the strongest bound being set by the recently launched space mission LISA Pathfinder. We also rule out a proposal for quantum-gravity-induced decoherence.

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.

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 $r_C>10^{-6}\mathrm{m}$, and partially excludes the enhanced collapse rate suggested by Adler. We discuss future improvements.

Levitated nanoparticles and microparticles are excellent candidates for the realization of extremely isolated mechanical systems, with a huge potential impact in sensing applications and in quantum physics. Magnetic levitation based on static fields is a particularly interesting approach, owing to the unique property of being completely passive and compatible with low temperatures. Here, we show experimentally that micromagnets levitated above type-I superconductors feature very low damping at low frequency and low temperature. In our experiment, we detect five out of six rigid body mechanical modes of a levitated ferromagnetic microsphere, using a dc superconducting quantum interference device with a single pick-up coil. The measured frequencies are in agreement with a finite-element simulation based on an ideal Meissner effect. For two specific modes, we find further substantial agreement with analytical predictions based on the image method. We measure damping times $\tau$ exceeding ${10}^4\mathrm{s}$ and quality factors $Q$ beyond ${10}^7$, an improvement of 2–3 orders of magnitude over previous experiments based on the same principle. We investigate the possible residual loss mechanisms besides gas collisions, and argue that a much longer damping time can be achieved with further effort and optimization. Our results open the way towards the development of ultrasensitive magnetomechanical sensors with potential applications to magnetometry and gravimetry, as well as to fundamental and quantum physics.

We experimentally demonstrate the stable trapping of a permanent magnet sphere above a lead superconductor, at vacuum pressures of $4\times {10}^{-8}$ mbar. The levitating magnet behaves as a harmonic oscillator, with frequencies in the 4–31 Hz range detected, and shows promise to be an ultrasensitive acceleration sensor. We directly apply an acceleration to the magnet with a current carrying wire, which we use to measure a background noise of $\sim {10}^{-10}\hspace{0.17em}\mathrm{m}/\sqrt{\text{Hz}}$ at 30.75 Hz frequency. With current experimental parameters, we find an acceleration sensitivity of $S_a^{1/2}=1.2\pm 0.2\times {10}^{-10}\hspace{0.17em}\mathrm{g}/\sqrt{\text{Hz}}$ , for a thermal noise limited system. By considering a 300 mK environment, at a background helium pressure of $1\times {10}^{-10}$ mbar, acceleration sensitivities of $S_a^{1/2}\sim 3\times {10}^{-15}\hspace{0.17em}\mathrm{g}/\sqrt{\text{Hz}}$ could be possible with ideal conditions and vibration isolation. To feasibly measure with such a sensitivity, feedback cooling must be implemented.