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When high speed electrons are passed through non-homogeneous magnetic field, 50% of the electron will be deflected up, and the rest 50% will be deflected down (Stern Gerlach experiment).

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Suppose the electron beam that has previously been deflected up is now passed again through another non-homogeneous magnetic field. Will 100% of the electron be deflected upwards by the second Stern Gerlach apparatus, since the spin state has already been filtered previously by the first apparatus?

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  • $\begingroup$ The Wiki article explicitly mentions this type of sequential Stern-Gerlach experiment. $\endgroup$
    – ACuriousMind ♦
    Commented Nov 26, 2022 at 19:46
  • $\begingroup$ Also, please give proper attribution when you post pictures - where is the illustration in your post from (and why is it needed to understand your question)? $\endgroup$
    – ACuriousMind ♦
    Commented Nov 26, 2022 at 19:47
  • $\begingroup$ the precise illustration is from semanticscholar.org/paper/… . $\endgroup$
    – James
    Commented Nov 26, 2022 at 19:50

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That depends very much on the conditions. Idealized, if there are no additional fields and the electron beam moves through a perfect vacuum, then the spin will remain stable (as the Hamiltonian for the spin-part of the wave function is zero when there are no electromagnetic fields).

But in reality there will always be scatterers (because no vacuum is perfect), stray magnetic fields, radio waves, etc. and those will turn the spins (and turn the spins within different parts of the beam differently due to being inhomogeneous).

But if you have Stern-Gerlach apparatuses a few centimeters apart in a reasonable vacuum and screen external fields, then those influences will be negligible. (Such experiments have been done and have been important for understanding spin). The point is: It's not exactly 100% of the ions/electrons that are deflected in the same direction on the next apparatus, but 99.9% or so.

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  • $\begingroup$ this is also the same as with photon polarization, i suppose, that polarization of a photon is usually stable unless in the presence of strong influences? $\endgroup$
    – James
    Commented Nov 26, 2022 at 19:46
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    $\begingroup$ Regarding photons: In vacuum the polarization is stable. I some media the polarization remains stable (even in things like most types of glass that severly lower the phase velocity, so you could say they are a strong influence), but others (e.g. optical active ones, or birefringent ones, depending on the orientation) turn the polarization, despite being transparent and straight propagation through them. $\endgroup$
    – Sebastian Riese
    Commented Nov 26, 2022 at 20:17

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