Shot-noise limited monitoring and phase locking of the motion of a   single trapped ion
  We perform high-resolution real-time read-out of the motion of a single trapped and laser-cooled Ba ion. By using an interferometric setup we demonstrate shot-noise limited measurement of thermal oscillations with resolution of 4 times the standard quantum limit. We apply the real-time monitoring for phase control of the ion motion through a feedback loop, suppressing the photon recoil-induced phase diffusion. Due to the spectral narrowing in phase-locked mode, the coherent ion oscillation is measured with resolution of about 0.3 times the standard quantum limit. 
P. Bushev, G. Hetet, L. Slodicka, D. Rotter, M. A. Wilson, F.   Schmidt-Kaler, J. Eschner and R. Blatt
quant-ph