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Watching a wavefunction gradually collapse

  1. Sep 12, 2007 #1


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    some very beautiful experimental work, observing the progressive collapse of a a wavefunction.

    beautiful illustrations too

    I didn't see this discussed here so decided to start a thread on it

    Here's the abstract

    Progressive field-state collapse and quantum non-demolition photon counting
    Christine Guerlin (LKB - Lhomond), Julien Bernu (LKB - Lhomond), Samuel Deléglise (LKB - Lhomond), Clément Sayrin (LKB - Lhomond), Sébastien Gleyzes (LKB - Lhomond), Stefan Kuhr (LKB - Lhomond), Michel Brune (LKB - Lhomond), Jean-Michel Raimond (LKB - Lhomond), Serge Haroche (LKB - Lhomond)
    (Submitted on 26 Jul 2007)

    "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 measuring non-destructively 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."

    ZapperZ has listed this paper in the excellent "Recent Noteworthy" bibliography thread,

    Chad Orzel has some helpful intuitive discussion of it at his blog.

    It was published in Nature, available as pay-per-view

    If you have never seen a quantum wavefunction gradually collapsing down to a single classical value during the duration of a protracted measurment process then this will be an eye-opener.
  2. jcsd
  3. May 15, 2008 #2
    Is this consistent with unitary time evolution?

    I'm surprised this paper hasn't generated more discussion here -- I was amazed when I first saw it. I have only just now joined Physics Forums or I would have commented last year. If it is discussed on another thread, someone please direct me there. I am eager to hear others' opinions about this beautiful experiment, particularly about its interpretation.

    The question that first strikes me is something along the lines of this: Are we watching some behavior that can be explained without an explicit measurement process? Or does it require some non-unitary time behavior to explain?

    In other words, could we construct some wave function model (perhaps like the one in the paper by Allahverdyan et. al., arXiv:quant-ph/0702135v2) where enough interactions with a complicated (but still fully quantum mechanical!) measuring apparatus would show the critical behavior of one photon number state growing more probable than the others? Or is there no way that we will see this behavior with wave functions unless we have some wave function collapse (or "projection" or "measurement event" or whatever word you'd use) included in the model?
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