The Quantum Ratio

Abstract: The concept of the Quantum Ratio was born out of the efforts to find a simple but universal criterion if the center of mass (CM) of an isolated (microscopic or macroscopic) body behaves quantum mechanically or classically, and under which conditions. It is defined as the ratio between the quantum fluctuation range, which is the spatial extension of the pure-state CM wave function, and the linear size of the body (the space support of the internal, bound-state wave function). The two cases where the ratio is smaller than unity or much larger than unity, roughly correspond to the body's CM behaving classically or quantum mechanically, respectively. An important notion following from the introduction of quantum ratio is that the elementary particles (thus the electron and the photon) are quantum mechanical. This is so even when the environment-induced decoherence turns them into a mixed state. Decoherence (mixed state) and classical state should not be identified. This simple observation is further elaborated, by analyzing some atomic or molecular processes. It may have far-reaching implications on the way quantum mechanics works, e.g., in biological systems.