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I'm looking for some experimental evidence of quantum wave packets spread to propose to my students. I notice that a huge attention is devoted in books to the theoretical frame, while almost nothing is said about real experiments with electrons or other particles. Still we have quantitative predictions (mainly, on variance dependence on time) and I'm sure they have been tested. Actually the complex experiments on packet revival do the job, but I'm looking for conceptually simpler (possibly historical) cases.

Can you help me?

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  • $\begingroup$ Use light. Single and double slit diffraction have quantum explanations. $\endgroup$
    – mmesser314
    Commented Oct 30, 2017 at 13:38
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    $\begingroup$ By using atomic Bose-Einstein condensate (BEC), you could get a much more visual result, as the expanding BEC can be imaged with resonant light. Cornell, Wieman, and Ketterle got the Nobel prize for their work in this field. The seminal works took place in the mid 90's, so it could still be too difficult for an undergraduate level experiment. My background is theory, unfortunately! $\endgroup$
    – user154997
    Commented Oct 30, 2017 at 14:26
  • $\begingroup$ I have this classic article in mind where two expanding BEC interfere. So you are not interested in the interference but it strongly suggest the spreading can also be visualised. $\endgroup$
    – user154997
    Commented Oct 30, 2017 at 14:30
  • $\begingroup$ Thank you. All interesting comments. Although, a conceptual issue remains. The quantum "matter" wave packet (photons are excluded) expands in such a way that the variance grows with the square of time (the so called ballistic regime). In contrast, a classical particle subject to randon walk has a variance that grows as the square root of time. This signature is very strong. Yet, I cannot find one experimental paper that confirms it, escluding very complex and indirect works on particles subject to external time dependent forces. $\endgroup$
    – Umberto
    Commented Oct 30, 2017 at 15:54
  • $\begingroup$ Yes, a litterature search is dominated by exactly the opposite of what you want to see: revival, which you sited, localised state in random potentials, which is what you might have had in mind in your last comment. That's why I proposed cold atoms BEC. $\endgroup$
    – user154997
    Commented Oct 30, 2017 at 19:04

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In the edX MIT physics course 8.04.1x, Zwiebach said that wave packet spreading shows up in proton accelerators. I found this reference that might be useful: arXiv:hep-ph/0002230.

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A few years late but here's a realization of Gaussian wave packet spreading in continuous space that uses few Lithium atoms and an optical lattice: https://arxiv.org/abs/2404.05699

A brief summary of what's done:

The atoms are cooled using laser cooling methods (which I don't completely understand) and the Gaussian states are prepared using the optical lattice, as the local minima can be approximated using a quadratic harmonic oscillator potential wells.

For time evolution in continuous space, the depth of the wells are drastically reduced over a short time scale. The wave packets evolve for a variable amount of time before the lattice is quickly turned on again. The wave packet is now localized in some well in the vicinity of it's original position. This procedure is repeated.

Position imaging is done using spontaneous emission of photons from an excited state of the Lithium atom to its ground state.

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