Extended Data Fig. 6: Sideband cross-correlations in out-of-loop heterodyne measurements.

a, b, Real (a) and imaginary (b) parts of cross-spectra, at different electronic feedback gains. Each pair is simultaneously fitted to a theoretical model and the results are shown as black lines. The grey line marks the zero as a reference. c, d, Fitted mechanical resonance frequency (c) and effective linewidth (d) at different electronic gains. e, Extracted mechanical occupations as a function of fitted effective linewidths. The black line is a theoretical model based on an ideal delay filter and on parameters estimated from the in-loop spectra. The error bars are obtained by the fit uncertainties (1 s.d.).