# Optical Density-Enhanced Squeezed Light Generation without Optical Cavities

## TL;DR

This paper proposes a cavity-free method for generating highly squeezed light using coherent population trapping in high optical density atomic media, achieving over 10 dB of noise reduction with practical experimental parameters.

## Contribution

It introduces a novel cavity-free scheme utilizing CPT and high OD to produce strongly squeezed light, simplifying experimental setups and enhancing robustness.

## Key findings

- Over 10 dB noise squeezing achievable at OD of 1000
- Squeezing degree increases with higher optical density
- The scheme is flexible with respect to two-photon detuning and light intensity

## Abstract

To achieve high degree of quantum noise squeezing, an optical cavity is often employed to enhance the interaction time between light and matter. Here, we propose to utilize the effect of coherent population trapping (CPT) to directly generate squeezed light without any optical cavity. Combined with the slow propagation speed of light in a CPT medium, a coherent state passing through an atomic ensemble with a high optical density (OD) can evolve into a highly squeezed state even in a single passage. Our study reveals that noise squeezing of more than $10$ dB can be achieved with an OD of 1,000, which is currently available in experiments. A larger OD can further increase the degree of squeezing. As the light intensity and two-photon detuning are key factors in the CPT interaction, we also demonstrate that the minimum variance at a given OD can be reached for a wide range of these two factors, showing the proposed scheme is flexible and robust. Furthermore, there is no need to consider the phase-matching condition in the CPT scheme. Our introduction of high OD in atomic media not only brings a long light-matter interaction time comparable to optical cavities, but also opens new avenue in the generation of squeezed light for quantum interface.