Direct measurement of the van der Waals interaction between two Rydberg atoms

TL;DR

This paper presents a direct experimental measurement of the van der Waals interaction between two Rydberg atoms, confirming the $C_6/R^6$ dependence and aligning with theoretical predictions, with implications for quantum technologies.

Contribution

The study provides the first direct measurement of Rydberg-Rydberg van der Waals interactions and demonstrates precise control over atomic interactions for quantum applications.

Findings

Measured $C_6$ coefficient agrees with ab-initio calculations

Observed $C_6$ increases with principal quantum number $n$

Demonstrated coherent partial Rydberg blockade

Abstract

We report on the direct measurement of the van der Waals interaction between two isolated, single Rydberg atoms separated by a controlled distance of a few micrometers. By working in a regime where the single-atom Rabi frequency of the laser used for excitation to the Rydberg state is comparable to the interaction energy, we observe a \emph{partial} Rydberg blockade, whereby the time-dependent populations of the various two-atom states exhibit coherent oscillations with several frequencies. A quantitative comparison of the data with a simple model based on the optical Bloch equations allows us to extract the van der Waals energy, and to observe its characteristic $C_6/R^6$ dependence. The magnitude of the measured $C_6$ coefficient agrees well with an \emph{ab-initio} theoretical calculation, and we observe its dramatic increase with the principal quantum number $n$ of the Rydberg state. Our results not only allow to test an important physical law, but also demonstrate a degree of experimental control which opens new perspectives in quantum information processing and quantum simulation using long-range interactions between the atoms.