Non-Destructive Probing of Rabi Oscillations on the Cesium Clock   Transition near the Standard Quantum Limit
  We report on non-destructive observation of Rabi oscillations on the Cs clock transition. The internal atomic state evolution of a dipole-trapped ensemble of cold atoms is inferred from the phase shift of a probe laser beam as measured using a Mach-Zehnder interferometer. We describe a single color as well as a two-color probing scheme. Using the latter, measurements of the collective pseudo-spin projection of atoms in a superposition of the clock states are performed and the observed spin fluctuations are shown to be close to the standard quantum limit. 
P. J. Windpassinger, D. Oblak, P. G. Petrov, M. Kubasik, M. Saffman,   C. L. Garrido Alzar, J. Appel, J. H. Mueller, N. Kjae...