  Measurement-based control, utilizing an active feedback loop, is a standard
tool in technology. Feedback control is also emerging as a useful and
fundamental tool in quantum technology and in related fundamental studies,
where it can be used to prepare and stabilize pure quantum states in various
quantum systems. Feedback-cooling of center-of-mass micromechanical
oscillators, which typically exhibit a high thermal noise far above the quantum
regime has been particularly actively studied and has recently been shown to
allow for ground-state cooling using optical measurements. Here, we realize
measurement-based feedback operations in an electromechanical system, cooling
the mechanical thermal noise down to 3 quanta, limited by added amplifier
noise. Counter-intuitively, we also obtain significant cooling when the system
is pumped at the blue optomechanical sideband, where the system is unstable
without feedback.
