title: Spectroscopic insensitivity to cold collisions in a two-state mixture of fermions.
abstract: We have experimentally demonstrated the absence of spectroscopic resonance shifts in a mixture of two interacting Fermi gases. This result is linked to observations in an ultracold gas of thermal bosons. There, the measured resonance shift due to interstate collisions is independent of the coherence in the system, and twice that expected from the equilibrium energy splitting between the two internal states in a fully decohered cloud. We give a simple theoretical explanation of these observations, which elucidates the effect of coherent radiation on an incoherent mixture of atoms.
authors.affiliation: Department of Physics, MIT-Harvard Center for Ultracold Atoms, and Research Laboratory of Electronics, MIT, Cambridge, Massachusetts 02139, USA.
authors.forename: Martin W
authors.initials: MW
authors.lastname: Zwierlein
authors.forename: Zoran
authors.initials: Z
authors.lastname: Hadzibabic
authors.forename: Subhadeep
authors.initials: S
authors.lastname: Gupta
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Ketterle