Watching a wavefunction gradually collapse

[Martin]

http://scienceblogs.com/principles/2007/09/watching_wavefunctions_collaps.php

 

This is some really nice experimental results in quantum optics which were published in NATURE in August but which are available to subscribers only. Happily a physicist blogger named Chad Orzel extracted the gist of it for us, together with some nice pictures, and put it on his blog.

 

Here is the abstract of the technical piece in Nature which is pay-per-view

http://www.nature.com/nature/journal/v448/n7156/abs/nature06057.html

 

Chad does such a good job of presenting and explaining the gist of it that one hardly misses the original article.

The lead author is Christine Guerlin. The authors are at a lab in Paris, except for one who is now in Mainz.

 

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.

 

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a preprint of the technical article also appears to be available free on arxiv

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."

 

even more pictures in the technical article preprint. beautiful work!

 

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UPDATE to reply to the next post:

Lockheed I think you understand it correctly. The ideal person to discuss this would likely be Swansont. The notable feature in this experiment is that the process of measurement is subtle and GRADUAL, instead of clunky and abrupt. So as they progressively measure, the wavefunction gradually narrows down to a spike at some definite value

 

the ordinary way people have imagined quantum mechanics for 80 years is that the act of measuring ABRUPTLY puts the system in a new state where the variable measured is no longer uncertain and no longer subject to quantum fluctuation. Other things about the system may remain undetermined so there is still a quantum state but it simply no longer involves uncertainty about the thing that was measured. So the picture is that a bellcurve wavefunction abruptly narrows to a spike. that is one common oversimplified picture. What is nice here is they show the bellcurve GRADUALLY narrowing down. And it is not just some computer simulation, what you see in the figures is what they actually measured in the lab by repeatedly performing an experiment with some photons in a box----where they gradually over time extracted the information, gradually shrinking the wavefunction to a spike. It is clever how they did it, no?

[Reaper]

Interesting. I like the blog it has a very good explanation. So, basically what happens is that when you shoot more atoms in a particular energy field, the data collected on the graph will converge toward a particular axis or a single value, and that is its current quantum state, if I'm understanding this correctly, right?

[swansont]

Oh, way cool. Sounds like they are measuring the Stark shift of the clock atoms — you shift the energy levels and the "tick" rate changes in proportion to the electric field, which is proportional to photon number. (if you're running the clock to measure time, this is something you try and avoid, and you can't, totally, because there are always blackbody photons around)

 

I'll have to read the Nature paper at work. Gotta look at the details.

 

I imagine the uncertainty in the measurement (if you're doing it one atom at a time) is one factor in why you can't quickly make the final determination of the photon number.