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.

To account for the dark matter content in our Universe, post-inflationary scenarios predict for the QCD axion a mass in the range (10 − 10 3 ) µeV. Searches with haloscope experiments in this mass range require the monitoring of resonant cavity modes with frequency above 5 GHz, where several experimental limitations occur due to linear amplifiers, small volumes, and low quality factors of Cu resonant cavities. In this paper we deal with the last issue, presenting the result of a search for galactic axions using a haloscope based on a 36 cm 3 NbTi superconducting cavity. The cavity worked at T = 4 K in a 2 T magnetic field and exhibited a quality factor Q0 = 4.5 × 10 5 for the TM010 mode at 9 GHz. With such values of Q the axion signal is significantly increased with respect to copper cavity haloscopes. Operating this setup we set the limit gaγγ < 1.03 × 10 −12 GeV −1 on the axion photon coupling for a mass of about 37 µeV. A comprehensive study of the NbTi cavity at different magnetic fields, temperatures, and frequencies is also presented.

Different approaches to quantum gravity, such as string theory 1,2 and loop quantum gravity, as well as doubly special relativity 3 and gedanken experiments in black-hole physics 4-6 , all indicate the existence of a minimal measurable length 7,8 of the order of the Planck length, L p = √h G/c 3 = 1.6 × 10 −35 m. This observation has motivated the proposal of generalized uncertainty relations, which imply changes in the energy spectrum of quantum systems. As a consequence, quantum gravitational effects could be revealed by experiments able to test deviations from standard quantum mechanics 9-11 , such as those recently proposed on macroscopic mechanical oscillators 12. Here we exploit the sub-millikelvin cooling of the normal modes of the ton-scale gravitational wave detector AURIGA, to place an upper limit for possible Planck-scale modifications on the ground-state energy of an oscillator. Our analysis calls for the development of a satisfactory treatment of multi-particle states in the framework of quantum gravity models. General relativity and quantum physics are expected to merge at the Planck scale, defined by distances of the order of ∼ L p and/or extremely high energies of the order of ∼ E p = ch/L p = 1.2 × 10 19 GeV. Therefore, present approaches to test quantum gravitational effects are mainly focused on highenergy astronomical events 13-15 , which allowed stringent limits to the predicted breaking of Lorentz invariance at the Planck scale to be put in place 16. On the other hand, the emergence of a minimal length scale can result in relevant consequences also for low-energy quantum mechanics experiments. The Heisenberg relation states that the uncertainties in the measurements of a position x and its conjugate momentum p are related by x p ≥h/2; that is, the position and the momentum of a particle cannot be determined simultaneously with arbitrarily high accuracy. However, an arbitrarily precise measurement of only one of the two observables, say position, is still possible at the cost of our knowledge about the other (momentum), a fact that is obviously incompatible with the existence of a minimal observable distance. This consideration motivates the introduction of generalized Heisenberg uncertainty principles 1-7. As a consequence, an alternative way to check quantum gravitational effects would be to perform high-sensitivity measurements of the uncertainty relation,