Discrete-time quadrature feedback cooling of a radio-frequency   mechanical resonator
  We have employed a feedback cooling scheme, which combines high-frequency mixing with digital signal processing. The frequency and damping rate of a 2 MHz micromechanical resonator embedded in a dc SQUID are adjusted with the feedback, and active cooling to a temperature of 14.3 mK is demonstrated. This technique can be applied to GHz resonators and allows for flexible control strategies. 
M. Poot, S. Etaki, H. Yamaguchi, and H. S. J. van der Zant
cond-mat.mes-hall