Collective state measurement of mesoscopic ensembles with single-atom   resolution
  For mesoscopic ensembles containing 100 or more atoms we measure the total atom number and the number of atoms in a specific hyperfine state with single-atom resolution. The measurement detects the atom-induced shift of the resonance frequency of an optical cavity containing the ensemble. This work extends the range of cavity-based detection with single-atom resolution by more than an order of magnitude in atom number, and provides the readout capability necessary for Heisenberg-limited interferometry with atomic ensembles. 
Hao Zhang, Robert McConnell, Senka \'Cuk, Qian Lin, Monika H.   Schleier-Smith, Ian D. Leroux, Vladan Vuleti\'c
physics.atom-ph quant-ph