# Probing, Quantifying and Freezing Coherence in a Thermal Ensemble of Atoms

## TL;DR

This paper introduces a new quantifier for measuring and controlling quantum coherence in room-temperature atomic ensembles, demonstrating the transition from EIT to ATS and coherence freezing.

## Contribution

It proposes an experimental method to quantify and actively control coherence in thermal atomic ensembles, advancing room-temperature quantum system research.

## Key findings

- New coherence quantifier for atomic ensembles
- Demonstration of coherence control and freezing
- Observation of EIT to ATS transition

## Abstract

Creating stable superposed states of matter is one of the most intriguing aspects of quantum physics, leading to a variety of counter-intuitive scenarios along with a possibility of restructuring the way we understand, process and communicate information. Accordingly, there has been a major research thrust in understanding and quantifying such coherent superposed states. Here we propose and experimentally explore a quantifier that captures effective coherent superposition of states in an atomic ensemble at room-temperature. The quantifier provides a direct measure of ground state coherence for electromagnetically induced transparency (EIT) along with distinct signature of transition from EIT to Autler-Townes splitting (ATS) regime in the ensemble. Using the quantifier as an indicator, we further demonstrate a mechanism to coherently control and freeze coherence by introducing an active decay compensation channel. In the growing pursuit of quantum systems at room-temperature, our results provide a unique way to phenomenologically quantify and coherently control coherence in atom-like systems.