Disclaimer: machine translated by DeepL which may contain errors.
Can Quantum Mechanics Govern the Macroscopic World?
Kiyotaka Aikawa
(Associate Professor, Department of Physics)
Classical mechanics, invented by Newton, was long considered to be a theory that could explain the motion of objects very well. However, from the end of the 19th century to the beginning of the 20th century, detailed investigations of various phenomena involving electrons, atoms, and light, which constitute matter, revealed many phenomena that could not be understood by classical mechanics, and quantum mechanics was developed as a theory that could describe such behavior. In the 21st century, technologies that surpass classical methods based on quantum mechanics have been developed, and various technologies such as quantum computers, quantum communications, and quantum sensing are approaching practical application.
Although quantum mechanics has succeeded in accurately describing the behavior of microscopic particles, there are still some unresolved mysteries. One of these is the question of how large an object quantum mechanics can be applied to. Originally, quantum mechanics was created to understand the phenomena of microscopic particles that cannot be explained by classical mechanics, but there is no fundamental restriction on the size of particles to which quantum mechanics can be applied. There is no reason why macroscopic objects should not behave in a quantum mechanical manner.
However, the difference between quantum mechanics and classical mechanics becomes significant only when the motion of particles is very slow, and since collisions with other objects or gaseous atoms or molecules destroy the quantum state, only sufficiently slow-moving objects in a vacuum are expected to maintain quantum mechanical behavior. Such objects do not exist in everyday life, and it is only under carefully arranged conditions in the laboratory that it is possible to investigate whether macroscopic objects exhibit quantum mechanical behavior.
The most prominent example of quantum mechanical behavior is the phenomenon of interference, in which an object behaves as a wave. Wave interference has long been known in light, but recent advances in quantum mechanics research have shown that even particles such as atoms and molecules can behave as waves and interfere with each other. The largest particle interference observed so far is that of a molecule containing about 104 atoms. The question of whether quantum mechanical interference can be observed in objects larger than this is an important topic for future research.
(upper) Particles trapped by the laser (lower) Motion spectra of the particles (peaks around 75 kHz, 80 kHz, and 200 kHz indicate the three-dimensional motion of the particles. (Red indicates the uncooled state, blue indicates the state with feedback cooling, and the motion especially around 200 kHz is cooled to the ground state.)Optomechanics is a research field in which the motion of objects much larger than atoms and molecules is controlled mainly by light to investigate quantum mechanical motion. For example, a microscopic cantilever attached to a substrate or a mirror suspended by a thread are well-known systems. Very recently, a new experimental system has been developed in which microparticles are trapped by a focused laser beam in a vacuum. It is called levitated optomechanics because the particles are levitated. In this system, the generation and maintenance of quantum states is expected to be easier than in other systems with contact. Recently, the Novotny group in Switzerland, the Aspelmeyer group in Austria, and the Aikawa group at the University of Tokyo have succeeded in feedback cooling of the motion of a single microparticle to the lowest energy state, the quantum ground state. By carefully observing and controlling the motion of fine particles, we may soon be able to see the quantum nature of their motion and their interference behavior.