Quasiparticles, coherence, and nonlinearity

We consider rf spectroscopy of ultracold Fermi gases by exact simulations of the many-body state and the coherent dynamics in one dimension. Deviations from the linear response sum rule result are found to suppress the pairing contribution to the rf line shifts. We compare the coherent rotation and quasiparticle descriptions of rf spectroscopy which are analogous to NMR experiments in superfluid He3 and tunneling in solids, respectively. We suggest that rf spectroscopy in ultracold gases provides an interesting crossover between these descriptions that could be used for studying decoherence in quantum measurement, in the context of many-body quantum states.Peer reviewe