Measuring the dispersive frequency shift of a rectangular microwave cavity induced by an ensemble of Rydberg atoms

TL;DR

This paper measures the dispersive frequency shift caused by Rydberg atoms in a microwave cavity, demonstrating a method to quantify atom numbers nondestructively, with implications for quantum information and scattering studies.

Contribution

It provides experimental validation of dispersive interactions between Rydberg atoms and microwave cavities, aligning with the Tavis-Cummings model, and achieves high collective coupling strengths.

Findings

Dispersive shift inversely proportional to atom-cavity detuning

Maximal collective coupling strength of 1 MHz

Nondestructive measurement of Rydberg atom number

Abstract

In recent years the interest in studying interactions of Rydberg atoms or ensembles thereof with optical and microwave frequency fields has steadily increased, both in the context of basic research and for potential applications in quantum information processing. We present measurements of the dispersive interaction between an ensemble of helium atoms in the 37s Rydberg state and a single resonator mode by extracting the amplitude and phase change of a weak microwave probe tone transmitted through the cavity. The results are in quantitative agreement with predictions made on the basis of the dispersive Tavis-Cummings Hamiltonian. We study this system with the goal of realizing a hybrid between superconducting circuits and Rydberg atoms. We measure maximal collective coupling strengths of 1 MHz, corresponding to 3*10^3 Rydberg atoms coupled to the cavity. As expected, the dispersive shift is found to be inversely proportional to the atom-cavity detuning and proportional to the number of Rydberg atoms. This possibility of measuring the number of Rydberg atoms in a nondestructive manner is relevant for quantitatively evaluating scattering cross sections in experiments with Rydberg atoms.