title: Two-dimensional double-quantum spectra reveal collective resonances in an atomic vapor.
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 interatomic dipole-dipole interactions, which means that the atoms cannot be treated in isolation, even at a low density of 10(12)  cm(-3).
authors.affiliation: JILA, University of Colorado and National Institute of Standards and Technology, Boulder, Colorado 80309-0440, USA.
authors.forename: Xingcan
authors.initials: X
authors.lastname: Dai
authors.forename: Marten
authors.initials: M
authors.lastname: Richter
authors.forename: Hebin
authors.initials: H
authors.lastname: Li
authors.forename: Alan D
authors.initials: AD
authors.lastname: Bristow
authors.forename: Cyril
authors.initials: C
authors.lastname: Falvo
authors.forename: Shaul
authors.initials: S
authors.lastname: Mukamel
authors.forename: Steven T
authors.initials: ST
authors.lastname: Cundiff