Two-dimensional double-quantum spectra reveal collective resonances in an atomic vapor

TL;DR

This paper demonstrates that double-quantum coherence signals in potassium vapor reveal collective atomic resonances caused by weak dipole-dipole interactions, highlighting the importance of many-body effects even at low densities.

Contribution

It provides the first experimental observation of collective resonances in an atomic vapor using two-dimensional double-quantum spectroscopy, emphasizing inter-atomic interactions.

Findings

Double-quantum signals at twice the frequency of one-quantum coherences

Collective resonances involve multiple atoms, not single atoms

Weak dipole-dipole interactions induce these collective effects

Abstract

We report the observation of double-quantum coherence signals in a gas of potassium atoms at twice the frequency of the one-quantum coherences. Since a single atom does not have a state at the corresponding energy, this observation must be attributed to a collective resonance involving multiple atoms. These resonances are induced by weak inter-atomic dipole-dipole interactions, which means that the atoms cannot be treated in isolation, even at a low density of $10^{12}$ cm$^{-3}$.