Transient Dynamics from Quantum to Classical- From the Developed Coherent State via Extreme Squeezing -

TL;DR

This paper investigates how quantum systems evolve into classical signals through transient dynamics, using the Schwinger-Keldysh formalism to connect quantum fluctuations with classical behavior in various physical processes.

Contribution

It introduces a framework applying the Schwinger-Keldysh formalism to describe the quantum-to-classical transition during transient phenomena across different systems.

Findings

Derivation of classical Langevin equations from quantum dynamics.

Identification of conditions for classicalization of quantum fluctuations.

Insights into irreversibility in quantum-to-classical transition.

Abstract

We explore the transient dynamics associated with the emergence of the classical signal in the full quantum system. We start our study from the instability which promotes the squeezing of the quantum system. This is often interpreted as the particle production though being reversible in time. We associate this state a non-dissipative classical fluctuations and study their trigger to develop the coherent state which can be classical if sufficiently developed. The Schwinger-Keldysh in-in formalism yields the classical Langevin equation including the fluctuation force which faithfully reflects the particle production property of the original quantum system. This formalism is applied to some transient process; the initiation of the spontaneous symmetry breaking, appearance of the off-diagonal long-range order in Bose-Einstein condensation, a transient process of the classicalization of the quantum fluctuations in the inflationary cosmology,... and gives some implications on the origin of the irreversibility associated with the transition from quantum to classical.