The irreversible evolution of a microscopic system under measurement is a central feature of quantum theory. From an initial state generally exhibiting quantum uncertainty in the measured observable, the system is projected into a state in which this observable becomes precisely known. Its value is random, with a probability determined by the initial system’s state. The evolution induced by measurement (known as ‘state collapse’) can be progressive, accumulating the effects of elementary state changes. Here we report the observation of such a step-by-step collapse by non-destructively measuring the photon number of a field stored in a cavity. Atoms behaving as microscopic clocks cross the cavity successively. By measuring the light-induced alterations of the clock rate, information is progressively extracted, until the initially uncertain photon number converges to an integer. The suppression of the photon number spread is demonstrated by correlations between repeated measurements. The procedure illustrates all the postulates of quantum measurement (state collapse, statistical results and repeatability) and should facilitate studies of non-classical fields trapped in cavities.
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This work was supported by the Agence Nationale pour la Recherche (ANR), by the Japan Science and Technology Agency (JST), and by the EU under the IP projects SCALA and CONQUEST. C.G. and S.D. were funded by the Délégation Générale à l’Armement (DGA). J.-M.R. is a member of the Institut Universitaire de France (IUF).
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
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Guerlin, C., Bernu, J., Deléglise, S. et al. Progressive field-state collapse and quantum non-demolition photon counting. Nature 448, 889–893 (2007). https://doi.org/10.1038/nature06057
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