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In quantum mechanics, the concept of spin is fascinating.

My question revolves around the experimental aspect of this phenomenon. Specifically, I'm curious to know if we have ever measured the quantum spin of a single, individual particle twice in succession.

To clarify, by 'single particle,' I mean the exact same particle, not two identical particles of the same type. I'm not looking for theoretical explanations or predictions, but rather concrete experimental evidence or procedures that have been carried out to address this question.

Has there been any experiment where the spin of the same quantum particle has been measured more than once?

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  • $\begingroup$ Yes: en.wikipedia.org/wiki/… $\endgroup$
    – hft
    Aug 6, 2023 at 2:21
  • $\begingroup$ @hft I think the question asks if there has been an experimental realization of at least one of the scenarios described in the link. As in, has anyone actually put multiple SG devices in a row? $\endgroup$ Aug 6, 2023 at 7:32
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    $\begingroup$ Successive spin measurements. (using Stern–Gerlach devices) are described in many books. $\endgroup$
    – kludg
    Aug 6, 2023 at 9:04
  • $\begingroup$ The quantum computing folks are doing this sort of thing all the time. $\endgroup$
    – hft
    Aug 9, 2023 at 0:52

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Keep in mind that when interacting or measuring a particle, it undergoes alterations, sometimes in multiple ways. For instance, consider an electron passing through a vertically positioned Stern–Gerlach device; its spin will align with the field. Subsequently, passing through a horizontally positioned Stern–Gerlach device will cause the electron's spin to rotate 90 degrees to match the new field. Although it might seem like you measured the electron's spin on both occasions, you essentially reset its spin each time.

The same principle applies to photons. Unpolarized photons passing through a vertical polarizer will all emerge as vertically polarized. While you could say you measured them, you merely changed their polarization. This becomes evident in the "Three Polarizer Experiment," where a photon traverses three polarizers: the first set vertically, the second set at 45 degrees, and the third set horizontally. When the photon eventually reaches a detection device and appears to be horizontally polarized, it's not because you directly measured its horizontal polarization; it's because you know what the last polarizer altered it to be.

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