Nonlinear optical spectra from Rydberg-mediated photon-photon interactions

TL;DR

This study investigates how Rydberg interactions induce nonlinear spectral effects in cold-atom EIT, revealing different behaviors in three- and four-level systems and informing atomic sensor development.

Contribution

It provides experimental insights into Rydberg-mediated nonlinearities in EIT, comparing models and clarifying conditions for microwave sensing without bias.

Findings

Nonlinear spectral broadening observed with increasing photon interactions.

Resonance shifts occur in three-level systems, but not in four-level systems.

Simple models can effectively describe the observed nonlinearities.

Abstract

While Rydberg-Rydberg interactions are essential for quantum nonlinear optics and quantum information processing, their role in microwave and radio-frequency sensing remains poorly understood. Here we experimentally investigate Rydberg interaction-induced nonlinearity in cold-atom Rydberg electromagnetically induced transparency (EIT). In a three-level EIT system, increasing photon-photon interactions produces nonlinear spectral broadening accompanied by resonance shifts, while a microwave-dressed four-level system exhibits pronounced nonlinear broadening without detectable spectral shifts. Our three-level data can be explained by a conditional superatom model, whereas our four-level observations are surprisingly captured by a simple dephasing model. Comparisons with three representative models provide key insights to the role of many-body interactions in Rydberg EIT spectroscopy. Furthermore, our results clarify the conditions under which microwave field characterization can be performed in the nonlinear regime without introducing systematic bias. Our study advances both fundamental understanding of many-body physics and practical development of atomic sensors.