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Single-Spin Devices and the Foundations of Quantum Mechanics

Alan M. Kadin

Published Sep 1, 2014• 0 citations
OpenAlex

Abstract

It is well known that electron spin is quantized, and is measured to be either spin up or spin down in a magnetic field, as was first demonstrated in the classic Stern-Gerlach experiment almost 100 years ago. However, it is also believed that a quantum spin may be indeterminate until it is measured, being in a quantum superposition of the two quantum states. On the contrary, I propose (based on a locally realistic spin-quantized picture of quantum waves) that an electron quantum state is always either spin up or spin down, but is never in a superposition of the two. This concept should be directly testable using a two-stage Stern-Gerlach experiment, similar to that presented in standard quantum textbooks, but apparently never carried out experimentally. This experiment should be straightforward using modern atomic beam laboratory equipment. If successful, this could lead to a serious review of quantum foundations, as well as a new set of practical applications. In particular, a binary storage or logic element based on quantum spin should continue to work down to the atomic scale. This should enable computer memories with orders of magnitude greater density than those based on conventional magnetic memories. 2

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Abstract For nearly one hundred years most quantum-mechanics texts have depicted a continuous, spin-direction superposition of the wavefunction of the silver atom traversing a Stern–Gerlach (S–G) magnet. But there are sound scientific arguments which deny that understanding. Schrodinger’s equation for that continuous wavefunction development cannot describe the transfer of energy from the magnetic field to that atom. Modern micro-fabrication techniques now make it possible to implement a simple, accessible experiment testing whether a spin superposition does continue in the magnet. Because the S–G experiment is the prototype for quantum measurement, that observation is crucial to implementing a realistic quantum theory of measurement.

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The nature of electron spin has presented an enigma right from the beginning of quantum mechanics. We suggest that a simple realistic picture of a real coherently rotating vector field can account for both the Schrodinger equation and electron spin in a consistent manner. Such a rotating field carries distributed angular momentum and energy in the same way as a circularly polarized electromagnetic wave. We derive the Schrodinger equation from the relativistic Klein-Gordon Equation, where the complex wave function maps onto a fixed-axis real rotating vector. Such a realistic picture can also explain the Stern-Gerlach experiment which first identified electron spin. Remarkably, the predictions of a two-stage Stern-Gerlach experiment within this realistic picture differ from those of the orthodox quantum superposition approach. This two-stage experiment has not actually been done, and could provide insights into the limits of realistic models. This realistic picture also avoids quantum paradoxes and enables realistic explanations for a variety of quantum phenomena.

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A Century Ago the Stern–Gerlach Experiment Ruled Unequivocally in Favor of Quantum Mechanics

Břetislav Friedrich

Abstract In 1921, Otto Stern conceived the idea for an experiment that would decide between a classical and a quantum description of atomic behavior, as epitomized by the Bohr–Sommerfeld–Debye model of the atom. This model entailed not only the quantization of the magnitude of the orbital electronic angular momentum but also of the projection of the angular momentum on an external magnetic field – the so‐called space quantization. Stern recognized that space quantization would have observable consequences: namely, that the magnetic dipole moment due to the orbital angular momentum would be space quantized as well, taking two opposite values for atoms whose only unpaired electron has just one quantum of orbital angular momentum. When acted upon by a suitable inhomogeneous magnetic field, a beam of such atoms would be split into two beams consisting of deflected atoms with opposite projections of the orbital angular momentum on the magnetic field. In contradistinction, if atoms behaved classically, the atomic beam would only broaden along the field gradient and have maximum intensity at zero deflection, i. e., where there would be a minimum or no intensity for a beam split due to space quantization. Stern anticipated that, although simple in principle, the experiment would be difficult to carry out – and invited Walther Gerlach to team up with him. Gerlach's realism and experimental skills together with his sometimes stubborn determination to make things work proved invaluable for the success of the Stern–Gerlach experiment (SGE). After a long struggle, Gerlach finally saw, on 8 February 1922, the splitting of a beam of silver atoms in a magnetic field. The absence of the concept of electron spin confused and confounded the interpretation of the SGE, as the silver atoms were, in fact, in a 2S state, with zero orbital and spin angular momentum. However, a key quantum feature whose existence the SGE was designed to test – namely space quantization of electronic angular momentum – was robust enough to transpire independent of whether the electronic angular momentum was orbital or due to spin. The SGE entails other key aspects of quantum mechanics such as quantum measurement, state preparation, coherence, and entanglement. Confronted with the outcome of the SGE, Stern noted: “I still have objections to the idea of beauty of quantum mechanics. But she is correct.”

Israel Journal of Chemistry•Jun 28, 2023•6 citations

Stern-Gerlach Effect for Electron Beams

Herman Batelaan, T. J. Gay, J. J. Schwendiman

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.

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Quantum Mechanics

Alexander R. Pruss

Since all experiments are subjected to the laws of quantum mechanics, the invalidity of the law of causality is definitively proved by quantum mechanics. (Heisenberg, 1927, p. 197, trans. in Stern, 1969, p. 132) THE PROBLEM OF INDETERMINISM If quantum mechanics is correct, on whatever interpretation, there are experimental setups such that no amount of observational knowledge of the initial system could possibly predict the outcome. An electron goes through a magnetic field in the Stern-Gerlach experiment in a superposed spin state | up > + | down >. It will take an upward or downward path through the magnetic field, and if quantum mechanics is correct, no observation of its physical configuration prior to the performance of this kind of experiment can possibly reveal which it will be. The standard indeterministic interpretation of quantum mechanics has it that not only can no observation of the prior physical properties of the system, including of the electron, allow us to predict the outcome of the experiment, but that the system lacks any physical properties, observable or not, such that were they known ahead of time, we would know what the outcome is. In other words, on this interpretation, two quantum mechanical systems that start with exactly the same intrinsic properties can behave in different ways. This is indeterminism. It is seen as challenging the PSR for exactly the reason libertarian free will is, but more deeply because the defender of libertarian free will can allow for indeterministic explanations involving the reasons for one action rather than another.

Cambridge University Press eBooks•Mar 20, 2006•0 citations

Spin Quantization in a Classical Model

David Lloyd Selke

The origins of quantum theory lie in a failure to explain certain phenomena using classical theory. But despite its adoption, quantum theory abounds with concepts described as “weird” or “spooky.” Even Feynman said that no one understands quantum theory. We revisit the Stern-Gerlach experiment that gave rise to the concept of spin and its quantization from a classical point of view. We present a simplified model of the classical electron which has two stable states which separate into two beams in the presence of an inhomogenous magnetic field.

European Journal of Applied Physics•Feb 6, 2021•0 citations
It is well known that electron spin is quantized, and is measured to be either spin up or spin down in a magnetic field, as was first demonstrated in the classic Stern-Gerlach experiment almost 100 years ago. However, it is also believed that a quantum spin may be indeterminate until it is measured, being in a quantum superposition of the two quantum states. On the contrary, I propose (based on a locally realistic spin-quantized picture of quantum waves) that an electron quantum state is always either spin up or spin down, but is never in a superposition of the two. This concept should be directly testable using a two-stage Stern-Gerlach experiment, similar to that presented in standard quantum textbooks, but apparently never carried out experimentally. This experiment should be straightforward using modern atomic beam laboratory equipment. If successful, this could lead to a serious review of quantum foundations, as well as a new set of practical applications. In particular, a binary storage or logic element based on quantum spin should continue to work down to the atomic scale. This should enable computer memories with orders of magnitude greater density than those based on conventional magnetic memories. 2
Abstract For nearly one hundred years most quantum-mechanics texts have depicted a continuous, spin-direction superposition of the wavefunction of the silver atom traversing a Stern–Gerlach (S–G) magnet. But there are sound scientific arguments which deny that understanding. Schrodinger’s equation for that continuous wavefunction development cannot describe the transfer of energy from the magnetic field to that atom. Modern micro-fabrication techniques now make it possible to implement a simple, accessible experiment testing whether a spin superposition does continue in the magnet. Because the S–G experiment is the prototype for quantum measurement, that observation is crucial to implementing a realistic quantum theory of measurement.
The nature of electron spin has presented an enigma right from the beginning of quantum mechanics. We suggest that a simple realistic picture of a real coherently rotating vector field can account for both the Schrodinger equation and electron spin in a consistent manner. Such a rotating field carries distributed angular momentum and energy in the same way as a circularly polarized electromagnetic wave. We derive the Schrodinger equation from the relativistic Klein-Gordon Equation, where the complex wave function maps onto a fixed-axis real rotating vector. Such a realistic picture can also explain the Stern-Gerlach experiment which first identified electron spin. Remarkably, the predictions of a two-stage Stern-Gerlach experiment within this realistic picture differ from those of the orthodox quantum superposition approach. This two-stage experiment has not actually been done, and could provide insights into the limits of realistic models. This realistic picture also avoids quantum paradoxes and enables realistic explanations for a variety of quantum phenomena.
Abstract In 1921, Otto Stern conceived the idea for an experiment that would decide between a classical and a quantum description of atomic behavior, as epitomized by the Bohr–Sommerfeld–Debye model of the atom. This model entailed not only the quantization of the magnitude of the orbital electronic angular momentum but also of the projection of the angular momentum on an external magnetic field – the so‐called space quantization. Stern recognized that space quantization would have observable consequences: namely, that the magnetic dipole moment due to the orbital angular momentum would be space quantized as well, taking two opposite values for atoms whose only unpaired electron has just one quantum of orbital angular momentum. When acted upon by a suitable inhomogeneous magnetic field, a beam of such atoms would be split into two beams consisting of deflected atoms with opposite projections of the orbital angular momentum on the magnetic field. In contradistinction, if atoms behaved classically, the atomic beam would only broaden along the field gradient and have maximum intensity at zero deflection, i. e., where there would be a minimum or no intensity for a beam split due to space quantization. Stern anticipated that, although simple in principle, the experiment would be difficult to carry out – and invited Walther Gerlach to team up with him. Gerlach's realism and experimental skills together with his sometimes stubborn determination to make things work proved invaluable for the success of the Stern–Gerlach experiment (SGE). After a long struggle, Gerlach finally saw, on 8 February 1922, the splitting of a beam of silver atoms in a magnetic field. The absence of the concept of electron spin confused and confounded the interpretation of the SGE, as the silver atoms were, in fact, in a 2S state, with zero orbital and spin angular momentum. However, a key quantum feature whose existence the SGE was designed to test – namely space quantization of electronic angular momentum – was robust enough to transpire independent of whether the electronic angular momentum was orbital or due to spin. The SGE entails other key aspects of quantum mechanics such as quantum measurement, state preparation, coherence, and entanglement. Confronted with the outcome of the SGE, Stern noted: “I still have objections to the idea of beauty of quantum mechanics. But she is correct.”
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.
Since all experiments are subjected to the laws of quantum mechanics, the invalidity of the law of causality is definitively proved by quantum mechanics. (Heisenberg, 1927, p. 197, trans. in Stern, 1969, p. 132) THE PROBLEM OF INDETERMINISM If quantum mechanics is correct, on whatever interpretation, there are experimental setups such that no amount of observational knowledge of the initial system could possibly predict the outcome. An electron goes through a magnetic field in the Stern-Gerlach experiment in a superposed spin state | up > + | down >. It will take an upward or downward path through the magnetic field, and if quantum mechanics is correct, no observation of its physical configuration prior to the performance of this kind of experiment can possibly reveal which it will be. The standard indeterministic interpretation of quantum mechanics has it that not only can no observation of the prior physical properties of the system, including of the electron, allow us to predict the outcome of the experiment, but that the system lacks any physical properties, observable or not, such that were they known ahead of time, we would know what the outcome is. In other words, on this interpretation, two quantum mechanical systems that start with exactly the same intrinsic properties can behave in different ways. This is indeterminism. It is seen as challenging the PSR for exactly the reason libertarian free will is, but more deeply because the defender of libertarian free will can allow for indeterministic explanations involving the reasons for one action rather than another.
The origins of quantum theory lie in a failure to explain certain phenomena using classical theory. But despite its adoption, quantum theory abounds with concepts described as “weird” or “spooky.” Even Feynman said that no one understands quantum theory. We revisit the Stern-Gerlach experiment that gave rise to the concept of spin and its quantization from a classical point of view. We present a simplified model of the classical electron which has two stable states which separate into two beams in the presence of an inhomogenous magnetic field.