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. 2015 Oct 9;6:8491. doi: 10.1038/ncomms9491

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(a) Measured microwave PSD S(ω) with red-sideband driving (top to bottom: C=324, 785, 3,810 and 94,500). Black dashed line: noise floor n_add of the amplifier chain. As the cooperativity C of the cooling tone is increased, S(ω₀) is reduced and the mechanical linewdith is broadened. Initially, the thermal noise of the membrane appears as a peak with decreasing height. At higher C (lower two panels), the noise floor outside of the mechanical linewidth (green dashed line) begins to increase above n_add, indicating the presence of cavity noise. In the lowest panel, the output noise of the cavity is suppressed at ω₀ due to correlations of the mechanical and cavity fluctuations. From the thermal peak and the cavity noise background level (green dashed line), we extract both n_m and n_c for all cooling powers (b). As a function of C, the mechanical occupancy n_m (red circle) drops, closely following (blue line) until the onset of cavity noise n_c (green squares) limits the final occupancy to a minimum of n_m=5.2, corresponding to T_m=34 μK. The error bars indicate the uncertainty of the data points and are calculated with the errors from the Lorentzian fit. The green line in b shows the expected n_c from the measured carrier sideband noise of the microwave generator.

Inline graphic — (a) Measured microwave PSD S(ω) with red-sideband driving (top to bottom: C=324, 785, 3,810 and 94,500). Black dashed line: noise floor n_add of the amplifier chain. As the cooperativity C of the cooling tone is increased, S(ω₀) is reduced and the mechanical linewdith is broadened. Initially, the thermal noise of the membrane appears as a peak with decreasing height. At higher C (lower two panels), the noise floor outside of the mechanical linewidth (green dashed line) begins to increase above n_add, indicating the presence of cavity noise. In the lowest panel, the output noise of the cavity is suppressed at ω₀ due to correlations of the mechanical and cavity fluctuations. From the thermal peak and the cavity noise background level (green dashed line), we extract both n_m and n_c for all cooling powers (b). As a function of C, the mechanical occupancy n_m (red circle) drops, closely following (blue line) until the onset of cavity noise n_c (green squares) limits the final occupancy to a minimum of n_m=5.2, corresponding to T_m=34 μK. The error bars indicate the uncertainty of the data points and are calculated with the errors from the Lorentzian fit. The green line in b shows the expected n_c from the measured carrier sideband noise of the microwave generator.