I will talk about experimental progress towards cooling a macroscopic oscillator to its quantum mechanical ground state. Such oscillator can, in principle, be used for limiting the parameters of certain wavefunction collapse theories as well as for tests of quantumness of gravity. It can also be used as an ultrasensitive force probe.

The experimental challenge is extreme isolation of the milligram-scale oscillator from the environment, while retaining control over its motion. We use a dilution refrigerator with 3.5 mK base temperature and advanced vibration isolation. We have achieved a coherence time of over 100 hours, corresponding to less than a microhertz resonant linewidth at the resonant frequency of a few hertz, indicating a high degree of isolation. These numbers can be significantly improved by using nuclear demagnetization and taking the temperature of the surrounding bath to below 100 microkelvins.