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Sub-Poissonian atom number fluctuations by three-body loss in mesoscopic ensembles

We show that three-body loss of trapped atoms leads to sub-Poissonian atom number fluctuations. We prepare hundreds of dense ultracold ensembles in an array of magnetic microtraps which undergo rapid three-body decay. The shot-to-shot fluctuations of the number of atoms per trap are sub-Poissonian, for ensembles comprising 50--300 atoms. The measured relative variance or Fano factor $F=0.53\pm 0.22$ agrees very well with the prediction by an analytic theory ($F=3/5$) and numerical calculations. These results will facilitate studies of quantum information science with mesoscopic ensembles.