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Schematic experimental setup. The oscillator’s angular position is measured by pointing laser-1 to the center of the oscillator and detecting the reflected light beam with a low-noise position detector in combination with a high sensitivity lock-in detector (LIA). Two capacitive feedback electrodes on the left side are used for dynamic position control. Laser-2 exerts a radiation pressure force on one arm of the balance, used for calibrating the angular deflection. (b) Flow of thermal fluctuation energy.
The oscillator’s position is cooled in three consecutive steps, shown in the measured power spectral density functions [(a)–(c)]. The proportional control is switched off during measurement. Freely running (a), the equivalent noise temperature is , due to microseismic excitation. For a higher damping rate (b), this temperature is reduced to , and finally in (c) to , which corresponds to a rms angular uncertainty. (d) An artificial electronic impulse applied to the feedback electrodes excites the oscillator to an equivalent temperature of . In this case, the damping force is first disabled. The total measurement time is always , except for in (c) (with respect to the replaced figure). Shown in the inset is the measured dependence of the inverse equivalent noise temperature on the artificial damping applied to the system. This behavior agrees well to the theoretical relation given in Eq. (3).
Dynamic control of the oscillator’s natural frequency. (a) Uncontrolled oscillator, , (b) positive , , and (c) negative , . The oscillator is excited externally for measurement and the amplitudes in resonance of curves (b) and (c) are scaled to that of (a). Above the oscillator’s adjusted resonance frequency, seismic disturbances will be effectively suppressed from coupling to the oscillator system. For an appropriate measurement period, the oscillator becomes stationary.
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