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Boiling electrons off a hot filament constitutes a preparation providing a mixed ensemble with nearly Maxwellian distribution of momentum eigenstates. To further purify the ensemble, one can let the electrons enter an accelerating electric field, and then a transverse magnetic field. The desired momentum is then selected by filtering out all those undesired momenta having the wrong curvature in the magnetic field. Thus, it is possible to get a pure state of mometum say $p_0$.

This is the standard approach of most texbooks (including Heisenbergs in 1930) to explain the preparation of electrons in a pure momentum eigenstate (plane wave).

Question: Does someone know references in literature to real experiments for this kind of preparation and up to which precision the momentum of the plane wave can be obtained?

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    $\begingroup$ That's what an electron microscope does. Have you tried looking at that literature? An introductory textbook should be plenty. $\endgroup$
    – Emilio Pisanty
    Commented Oct 19, 2019 at 11:21

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Having a single frequency quantum particle or a plane wave is not experimentally realised but the current methods have succeeded in narrowing it down to a very small bandwidth. Even in the method used in 30's the obtained electrons also would have a spread in the momentum otherwise spatially seperating them from the rest would not be possible. https://en.wikipedia.org/wiki/Single-photon_source will give you a bunch of techniques that are currently used to get particles or photons within a very narrow bandwidth

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    $\begingroup$ Preparing electron and positron beams was the specialty of the large electron positron (LEP) collider. See home.cern/science/accelerators/large-electron-positron-collider. Any free electron laser, such as FELIX, also uses collimated electron bundles. $\endgroup$
    – my2cts
    Commented Oct 19, 2019 at 8:09
  • $\begingroup$ @Harsh I am not looking for single-particle preparations but rather on a beam of electrons which is approximately seperated from the rest. Photon sources are not what I am looking for. $\endgroup$
    – user56224
    Commented Oct 19, 2019 at 14:59
  • $\begingroup$ Sorry I missed that. I was thinking on how to obtain a very narrow band or a specific frequency entity in principle. $\endgroup$
    – NormTrotsky
    Commented Oct 19, 2019 at 17:00

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