Interplay of classical and quantum dynamics in a thermal ensemble of atoms

TL;DR

This paper experimentally investigates how quantum superposition and classical optical pumping dynamics compete and interplay in a thermal atomic ensemble driven by coherent fields, revealing that optical pumping primarily drives the steady state formation.

Contribution

It provides the first experimental exploration of the temporal evolution of quantum and classical dynamics in a thermal atomic ensemble using a stroboscopic probing technique.

Findings

Optical pumping drives atoms to a steady state over longer timescales.

Observation of half-cycle Rabi flops in a thermal ensemble.

Identification of signatures distinguishing closed and open atomic systems.

Abstract

In a thermal ensemble of atoms driven by coherent fields, how does evolution of quantum superposition compete with classical dynamics of optical pumping and atomic diffusion? Is it optical pumping that first prepares a thermal ensemble, with coherent superposition developing subsequently or is it the other way round: coherently superposed atoms driven to steady state via optical pumping? Using a stroboscopic probing technique, here we experimentally explore these questions. A 100 ns pulse is used to probe an experimentally simulated, closed three-level, lambda-like configuration in rubidium atoms, driven by strong coherent control and incoherent fields. Temporal evolution of probe transmission shows an initial overshoot with turn-on of control, resulting in a scenario akin to lasing without inversion (LWI). The corresponding rise time is dictated by coherent dynamics, with a distinct experimental signature of half-cycle Rabi flop in a thermal ensemble of atoms. Our results indicate that, in fact, optical pumping drives the atoms to a steady state in a significantly longer time-scale that sustains superposed dark states. Eventual control turn-off leads to a sudden fall in transmission with an ubiquitous signature for identifying closed and open systems. Numerical simulations and toy-model predictions confirm our claims. These studies reveal new insights into a rich and complex dynamics associated with atoms in thermal ensemble, which are otherwise absent in state-prepared, cold atomic ensembles.