In physics experiments, noise is usually a hindrance that interferes with precise measurements, but noise can sometimes carry important information. A typical example is "shot noise," which arises from statistical fluctuations in the number of particles responsible for some kind of flow in a physical system.
Magnets possess magnetic force when electron spins, which correspond to tiny magnets held by electrons, align in the same direction. However, electron spins do not have continuous values, and magnetization, which is the physical quantity representing the direction and strength of magnets, also has non-continuous values. In physics, having non-continuous, discrete values is called quantization. Recently, proposals have been made to observe shot noise in magnetization by measuring changes in current that flows when microwaves are applied to magnets, but this has not been realized due to experimental difficulties.
A research team led by Associate Professor Takeo Kato from the Institute for Solid State Physics at the University of Tokyo focused on the fact that recent advances in optical measurement technology enable efficient and high-speed measurement of magnetization. The team proposed a new method for measuring shot noise in magnetization using optical technology. The team theoretically formulated the shot noise observed as fluctuations around the average value of magnetization when pulse laser light is applied to magnets in a stable state within a magnetic field, disturbing the direction of electron spins before they return to their original state. The team demonstrated that by measuring this shot noise, it is possible to determine the magnitude of magnetization quantization.
Through this achievement, changes in individual electron spins can now be measured through shot noise in magnetization. This is important not only for the development of quantum devices that store information by manipulating electron spins in magnets, but also as a promising application of laser technology.
(Article: Masanori Nakajo)
The intensity of fluctuations around the average value of magnetization measured at this time contains information about magnetization quantization (right).