Prof. Jun Nishimura of KEK Theory Center and his collaborator pointed out that the quantum decoherence phenomenon, which is unique to quantum mechanics, can be reproduced from complex solutions of the classical equations of motion. Their result, along with numerical calculations based on this, was published in Physical Review Letters on May 27, 2025 (US Eastern Time).
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.134.210401

The paper is available in preprint format.
https://arxiv.org/pdf/2408.16627

In quantum mechanics, there are characteristic states that can be expressed as a. ʹʹsuperpositionʹʹ of multiple different states, such as Schrödingerʹs cat. However, when there is an environment that affects the system in question, it is known that this state breaks down within a short time and the system makes a transition to a classical situation in which multiple different states can exist with certain probabilities. This phenomenon is called ʹʹquantum decoherenceʹʹ. The present work showed for the first time that this phenomenon can be reproduced from complex solutions of the classical equations of motion. (In general, a complex number is obtained by adding a fictitious number that becomes a negative real number when squared, to an ordinary real number.) In particular, for a simple model of quantum decoherence, accurate numerical calculations were performed without any approximations or assumptions, unlike conventional methods. In recent years, with the progress of quantum technologies such as quantum computers, it is getting increasingly important to understand quantum decoherence and accurately evaluate its effects. This work provides a new and powerful theoretical framework for achieving this. This research is based on a collaboration with Hiromasa Watanabe, a postdoctoral researcher of Department of Physics, and Research and Education Center for Natural Sciences, Keio University.