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Progress of Theoretical Physics Supplement

Probing Quantum Mechanics towards the Everyday World

A. J. Leggett

Published Jan 1, 2007• 9 citations
Publisher

Abstract

I present the motivation for experiments which attempt to generate, and verify the existence of, quantum superpositions of two or more states which are by some reasonable criterion “macroscopically” distinct, and show that various a priori objections to this program made in the literature are flawed. I review the extent to which such experiments currently exist in the areas of free-space molecular diffraction, magnetic biomolecules, quantum optics and Josephson devices, and sketch possible future lines of development of the program.

DOI
10.1143/ptps.170.100

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I present the motivation for experiments which attempt to generate, and verify the existence of, quantum superpositions of two or more states which are by some reasonable criterion “macroscopically” distinct, and show that various a priori objections to this program made in the literature are flawed. I review the extent to which such experiments currently exist in the areas of free-space molecular diffraction, magnetic biomolecules, quantum optics and Josephson devices, and sketch possible future lines of development of the program.
I present the motivation for experiments which attempt to generate, and verify the existence of, quantum superpositions of two or more states which are by some reasonable criterion `macroscopically' distinct, and show that various a priori objections to this programme made in the literature are flawed. I review the extent to which such experiments currently exist in the areas of free-space molecular diffraction, magnetic biomolecules, quantum optics and Josephson devices, and sketch possible future lines of development of the programme.
I present the motivation for experiments which attempt to generate, and verify the existence of, quantum superpositions of two or more states which are by some reasonable criterion ‘macroscopically’ distinct, and show that various a priori objections to this programme made in the literature are flawed. I review the extent to which such experiments currently exist in the areas of free-space molecular diffraction, magnetic biomolecules, quantum optics and Josephson devices, and sketch possible future lines of development of the programme.
In this paper I first briefly review the apparently bizarre consequences of attempting to apply quantum mechanics consistently to macroscopic bodies in certain kinds of situations (the ‘Schrödinger's Cat’ paradox). The paradox results from the assumption that the linear superpositions of macroscopically distinct states, which are unambiguously predicted by the formalism, do indeed exist in nature. Until recently, this assumption was experimentally untested. However, recent advances in techniques of microfabrication, cryogenics and noise control have opened up the possibility of indirect and possibly even direct tests, in particular in superconducting systems incorporating Josephson devices. I describe some of these tests, review the current experimental and theoretical situation and discuss the implications of various possible outcomes to the experiments.
It is widely believed that the microscopic, that is, atomic-scale or smaller, world is governed by quantum mechanics while the macroscopic world, where our direct and daily experiences reside, is governed by classical mechanics. But why? Why do we not observe quantum phenomena such as quantum tunneling and quantum superposition in the macroscopic world? Where is the boundary between the microscopic and macroscopic worlds when it comes to the governing physical principles? Macroscopic quantum phenomena were first examined theoretically by A. Leggett in the 1970s. This boldly audacious idea was experimentally demonstrated in a series of groundbreaking works on Josephson junctions by John Clarke, Michel Devoret, and John Martinis in 1985, that have been celebrated by the 2025 Nobel Prize in Physics. The trio’s work spurred the development of various qubit types based on superconducting circuits, ultimately leading to the current advancement of superconducting quantum computers.
Lying in the core of the conceptual scheme of current quantum theory, the notion of quantum superposition is a consequence of the quantum-mechanical superposition principle. However, relying on precise but not attainable space or time coordinates, quantum superposition makes little sense physically if we use it to describe or explain experimental results obtained by measurement. Space or time is usually modeled by a metric space endowed with the usual distance function. Irrelevant to instrument precision and measurement accuracy, unattainability of precise coordinates is a well-established mathematical fact following from properties of the corresponding metric topology, and hence rules out completely, in principle, the possibility for us to obtain precise coordinates by measurement in actually performed experiments. In classical mechanics, unattainability of precise coordinates is hardly noticeable, and measurement results obtained in classical physics can still be reasonably explained even though precise coordinates are unattainable. However, when we explain measurement results obtained in quantum physics, unattainability of precise coordinates should not be omitted. In the conceptual scheme of current quantum theory, omission of unattainability of precise coordinates is an unfortunate flaw, which is largely responsible not only for the invalidity of quantum superposition in description of the physical world but also for various ineligible applications of quantum mechanics. Nevertheless, in no sense does the flaw imply quantum mechanics as a successful theory failing to be correct. As shown in this article, the flaw is repairable. By taking into account unattainability of precise coordinates, we can reinterpret the meaning of quantum superposition to repair the flaw without affecting eligible applications of quantum mechanics while avoiding all the ineligible applications.