Precision minimally-destructive detection of ultra-cold atomic ensembles

Over the last two decades the cold-atom physics has matured from proof-of-principle demonstrations to a versatile platform for precision measurements and study of quantum phenomena. Ultra-cold atomic ensembles have been used both for technological and fundamental science applications. To fully exploit their potential, a precise measurement and control of the atom number in the ensemble is crucial. We report on a precise, minimally destructive measurement technique that can be used to prepare an atomic ensemble with a desired atom number. The measurement relies on the dispersive light-atom interaction; thus it is expected to have a negligible effect on the ensemble temperature and to induce minimal decoherence in the atomic quantum state. As a result, it can be used to perform quantum-enhanced measurements and prepare the atom-number state at the start of an interferometer sequence.