  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$^{-3}$.

Atomic Physics;Chemical Physics;Optics