Progressive field-state collapse and quantum non-demolition photon counting

• Published in: Nature (London) Vol. 448; no. 7156; pp. 889 - 893
• Main Authors: Bernu, Julien, Kuhr, Stefan, Brune, Michel, Haroche, Serge, Sayrin, Clément, Deléglise, Samuel, Guerlin, Christine, Gleyzes, Sébastien, Raimond, Jean-Michel
• Format: Journal Article
• Language: English
• Published: London Nature Publishing 23-08-2007; Nature Publishing Group
• Subjects: Atoms & subatomic particles; Classical and quantum physics: mechanics and fields; Exact sciences and technology; Foundations, theory of measurement, miscellaneous theories (including aharonov-bohm effect, bell inequalities, berry's phase); Fundamental areas of phenomenology (including applications); Light; Measurement techniques; Optics; Photon statistics and coherence theory; Physics; Quantum mechanics; Quantum optics; Quantum Physics; Quantum theory; Quantum nondemolition measurement; Photon counting; Rydberg states; Photon statistics; Quantum optics; Rubidium Atoms; Quantum theory; quantum measurement; quantum jump; QND; measurement
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature06057

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.