  We report recent progress towards the realization of a sub-mK, low-vibration
environment at the bottom stage of a dry dilution refrigerator for use in
mechanical tests of quantum mechanics. Using adiabatic nuclear demagnetization,
we have cooled a silicon cantilever force sensor to $T\approx 1$ mK. The
temperature of the tip-holder of the cantilever chip was determined via a
primary magnetic flux noise thermometer. The quality factor of the cantilever
continues to increase with decreasing temperature, reaching $Q\approx 4\cdot
10^4$ at $2$ mK. To demonstrate that the vibration isolation is not
compromised, we report the detection of the thermal motion of the cantilever
down to $T \approx 20$ mK, only limited by the coupling to the SQUID readout
circuit. We discuss feasible improvements that will allow us to probe
unexplored regions of the parameter space of continuous spontaneous
localization models.
