Generic Regularities in Quasienergy Spectra and Free Quantum Dynamics Independently of Chaos Strength

Generic and significant regularities are shown to occur in the quasienergy spectra of the generalized quantum kicked particle for arbitrary quasimomentum, a quantity most relevant in atom-optics experimental realizations of this paradigmatic nonintegrable system. The spectral regularities are basically different from the usual ones associated with ordered regions in a mixed phase space, since they are \emph{completely independent} of the chaos strength. Their origin are free-motion features unaffected by the nonintegrability. The generic regularities are \emph{dense} subspectra, each being either a regular level sequence with approximately a Poisson level-spacing distribution or a ladder of equally-spaced levels. The quantum dynamics associated with each regular subspectrum is essentially \emph{free} for \emph{arbitrarily strong} chaos.