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Physical Review A

Observing the spin of a free electron

B. M. Garraway, Stig Stenholm

Published Jul 1, 1999• 45 citations
Publisher

Abstract

Long ago, Bohr, Pauli, and Mott argued that it is not, in principle, possible to measure the spin components of a free electron. One can try to use a Stern-Gerlach type of device, but the finite size of the beam results in an uncertainty of the splitting force that is comparable with the gradient force. The result is that no definite spin measurement can be made. Recently there has been a revival of interest in this problem, and we will present our own analysis and quantum-mechanical wave-packet calculations which suggest that a spin measurement is possible for a careful choice of initial conditions.

DOI
10.1103/physreva.60.63

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•Jun 29, 2015•0 citations
Long ago, Bohr, Pauli, and Mott argued that it is not, in principle, possible to measure the spin components of a free electron. One can try to use a Stern-Gerlach type of device, but the finite size of the beam results in an uncertainty of the splitting force that is comparable with the gradient force. The result is that no definite spin measurement can be made. Recently there has been a revival of interest in this problem, and we will present our own analysis and quantum-mechanical wave-packet calculations which suggest that a spin measurement is possible for a careful choice of initial conditions.
In 1932 Pauli and Bohr declared that electromagnetic fields cannot be used to separate electrons according to their spin. A Stern–Gerlach magnet can split atomic beams into beams with different polarizations, but the Lorentz force blurs the splitting for charged particles such as electrons. Brillouin suggested that electrons could be separated by applying magnetic gradients along the electron beam, but Pauli disagreed. He wrote in 1958: "It is impossible to observe the spin of the electron, separated fully from its orbital momentum, by means of experiments based on the concept of classical particle trajectories."
The measurement of spin-1/2 observables using a Stern-Gerlach type device is studied. A magnetic field with a dominant dipole component leads to a measurement of spin in one direction. The measurement is shown to be feasible (in principle) for electrons as well as for neutral particles. A quadrupole field leads to a joint measurement of two incompatible spin observables. Again, both the electron and the neutral case are presented. The accuracy of the joint measurement is compared to the limit imposed by the uncertainty principle.
The conflict between Bohr's assertion that the magnetic moment of the electron cannot be measured with experiments based on the concept of classical trajectories, and the measurement of the magnetic moment of electrons in a modified Penning trap by Dehmelt et al. has led us to reevaluate other implications of Bohr's assertion. We show that, contrary to the analysis of Bohr and Pauli, the assumption of classical trajectories in a Stern-Gerlach--like device can result in a high degree of spin separation for an electron beam. This effect may persist within a fully quantum-mechanical analysis. The magnetic fields considered are such that a tabletop Stern-Gerlach electron spin filter is feasible.
Stern-Gerlach experiment by free electron is very important experiment because it answered some questions that remain unanswered for almost a century. Bohr and Pauli considered its objective observation as impossible while some other scientists considered such observation as possible. The experiment on free electrons has not been conducted so far because the high magnetic field gradient predicted there was thought as impossible to generate. This paper proves that it is not only possible but also observable using a high vacuum lamp which is deionized well. To obtain a high magnetic field gradient, it is not necessary to have a very strong magnetic field and it is possible to observe the phenomenon using a very sharp pointed magnet and adjusting the voltage in a certain distance from free electron beams. that objective observation requires your consideration of some technical points simultaneously.In this experiment, no electric field and no magnetic field does not change with time.
In the late 1920s Niels Bohr propagated the idea that the magnetic moment of a free electron could not be observed. This derived from the idea that the spin degree of freedom characterized the electron only when it is bound in an atom. This view initiated a lively discussion, which involved many of the most prominent theoreticians of the time. The independent existence of the electron spin became an issue of principle. In particular it was deemed that quantum effects would destroy the separated classical trajectories in a Stern-- Gerlach type of experiment. We review these discussions and some later developments. Quantum effects do prove to be essential, but they do not overwhelm the magnetic effects of spin. In addition to these arguments, it has been possible experimentally to determine the electron g -factor with high accuracy in electromagnetic traps. In fact no principle seems to prevent the observation of the magnetic moment of the free electron.
But no good and simple criterion of entanglement for more complicated cases is known. This situation is rather strange since everybody talks about entanglement. How to produce and how to measure the entanglement are also interesting issues. When one measures the spin of the electron, for example, there are three major operators one can think of [1] a) Wigner spin, b) Dirac spin, c) Magnetic moment. The notion of chirality is important to distinguish these operators. Incidentally, many people talk about the Stern-Gerlach apparatus to measure the spin polarization of the electron. This is also strange since N. Bohr and N. F. Mott and other prominent people have shown many years ago that one cannot measure the electron spin polarization by a Stern-Gerlach apparatus, since the inhomogeneous magnetic field disturbs the motion of the electron so much and it is impossible to detect the spin orientation. The Stern-Gerlach particles are electrically neutral. One may ask if novel features appear in high energy physics? Chiral symmetry (including the use of weak interaction) can become important such as in a neutrino-antineutrino pair in Z decay