Feedback cooling of a single trapped ion
  Based on a real-time measurement of the motion of a single ion in a Paul trap, we demonstrate its electro-mechanical cooling below the Doppler limit by homodyne feedback control (cold damping). The feedback cooling results are well described by a model based on a quantum mechanical Master Equation. 
Pavel Bushev, Daniel Rotter, Alex Wilson, Francois Dubin, Christoph   Becher, Juergen Eschner, Rainer Blatt, Viktor Steixner...