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 http://arxiv.org/abs/0707.3880 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, https://www.physicsforums.com/showpost.php?p=1407933&postcount=52 Chad Orzel has some helpful intuitive discussion of it at his blog. http://scienceblogs.com/principles/2007/09/watching_wavefunctions_collaps.php It was published in Nature, available as pay-per-view http://www.nature.com/nature/journal/v448/n7156/abs/nature06057.html 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.