We investigate a new type of quantum ratchet which may be realized by cold atoms in a double-well optical lattice, pulsed with unequal periods. The classical dynamics is chaotic and we find the classical diffusion rate D is asymmetric in momentum up to a finite time t r . The quantum behavior produces a corresponding asymmetry in the momentum distribution which is ''frozen-in'' by dynamical localization provided the break time t t r . We conclude that the cold atom ratchets require Db= h 1, where b is a small deviation from period-one pulses. DOI: 10.1103/PhysRevLett.89.194102 PACS numbers: 05.45.Mt, 05.40.Jc, 05.60.-k, 32.80.Pj Cold atoms in optical lattices provide an excellent experimental demonstration of the phenomenon of dynamical localization [1,2]. Dynamical localization (DL) has been described as the so-called ''quantum suppression of classical chaos.'' In the usual realizations, a periodically driven or kicked system makes a transition to chaotic classical dynamics for sufficiently strong perturbation. The classical energy is unbounded and grows diffusively with time. For the corresponding quantum system, in contrast, the diffusion is suppressed after an h-dependent time scale, the ''break time'' t . The final quantum momentum distribution is localized with a characteristic exponential profile. The formal analogy established with Anderson localization [2] forms a key analysis of this phenomenon. A series of recent experiments on cesium atoms in pulsed optical lattices [3] gave a classic demonstration of this effect.The possibility of experiments with asymmetric lattices, in particular, with asymmetric double wells [4,5], leads us to investigate the possibility of constructing a ''clean'' atomic ratchet, where the transport results purely from the chaotic Hamiltonian dynamics, with no Brownian or dissipative ingredients. Ratchets are spatially periodic systems which, by means of a suitable spatialtemporal asymmetry, can generate a current even in the absence of a net force. There is already an extensive body of work on Brownian and deterministic ratchets with dissipation [6,7], driven by the need to understand biophysical systems such as molecular motors and certain mesoscopic systems. Some of this work encompasses the quantum dynamics [8]. For a full review see [9]. However, to date there has been very little work on Hamiltonian ratchets. One notable exception is the work by Flach et al.[10] where the general form of the spatial and temporal desymmetrization required to generate transport was investigated. The only substantial study of quantum Hamiltonian ratchets, however, is the work of Dittrich et al. [11] which showed how transport can occur in mixed phase spaces. They demonstrated that transport is zero if starting conditions cover all regions of phase space uniformly. A key result was a sum rule showing transport in the chaotic manifold is balanced by transport in the adjoining regular manifolds (stable islands/tori). Very recently [12], it was shown that a kicked map with a ''rocking'' linea...

We present and compare three generically applicable signal processing methods for periodic orbit quantization via harmonic inversion of semiclassical recurrence functions. In a first step of each method, a band-limited decimated periodic orbit signal is obtained by analytical frequency windowing of the periodic orbit sum. In a second step, the frequencies and amplitudes of the decimated signal are determined by either Decimated Linear Predictor, Decimated Padé Approximant, or Decimated Signal Diagonalization. These techniques, which would have been numerically unstable without the windowing, provide numerically more accurate semiclassical spectra than does the filter-diagonalization method.

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