Fig. 3: Frequency and population standard deviations.

Main: standard deviation of the phonon population extracted from Fig. 2b as a function of the average population. The black line is a \(\propto \sqrt{n}\) guide to the eye. Top inset: frequency shift standard deviation calculated over 10 h averaging as a function of cryostat temperature. The line is a power-law fit. The magenta point is inferred from data taken in the self-oscillating regime; in both graphs, the orange dot is obtained with a measuring input power twice smaller (see text). Error bars correspond to type A uncertainties extracted from the fitting process (Supplementary Note 3).