Experimental Realization of Quantum-Resonance Ratchets

Quantum-resonance ratchets associated with the periodically kicked particle are experimentally realized for the first time. This is achieved by using a Bose-Einstein condensate exposed to a pulsed standing light wave and prepared in an initial state differing from the usual plane wave. Both the standing-wave potential and the initial state have a point symmetry around some center and the ratchet arises from the non-coincidence of the two centers. The dependence of the directed quantum transport on the quasimomentum is studied. A detailed theoretical analysis is used to explain the experimental results.Comment: Accepted for publication in Physical Review Letters (November 2007

Controlling the Momentum Current of an Off-resonant Ratchet

We experimentally investigate the phenomenon of a quantum ratchet created by exposing a Bose-Einstein Condensate to short pulses of a potential which is periodic in both space and time. Such a ratchet is manifested by a directed current of particles, even though there is an absence of a net bias force. We confirm a recent theoretical prediction [M. Sadgrove and S. Wimberger, New J. Phys. \textbf{11}, 083027 (2009)] that the current direction can be controlled by experimental parameters which leave the underlying symmetries of the system unchanged. We demonstrate that this behavior can be understood using a single variable containing many of the experimental parameters and thus the ratchet current is describable using a single universal scaling law.Comment: arXiv admin note: substantial text overlap with arXiv:1210.565

Sub-Fourier characteristics of a δ\delta-kicked rotor resonance

We experimentally investigate the sub-Fourier behavior of a $\delta$-kicked rotor resonance by performing a measurement of the fidelity or overlap of a Bose-Einstein condensate (BEC) exposed to a periodically pulsed standing wave. The temporal width of the fidelity resonance peak centered at the Talbot time and zero initial momentum exhibits an inverse cube pulse number ($1/N^{3}$) dependent scaling compared to a $1/N^{2}$ dependence for the mean energy width at the same resonance. A theoretical analysis shows that for an accelerating potential the width of the resonance in acceleration space depends on $1/N^{3}$, a property which we also verify experimentally. Such a sub-Fourier effect could be useful for high precision gravity measurements.Comment: 4 pages, 5 figure

Creation mechanism of quantum accelerator modes

We investigate the creation mechanism of quantum accelerator modes which are attributed to the existence of the stability islands in an underlying pseudoclassical phase space of the quantum delta-kicked accelerator. Quantum accelerator modes can be created by exposing a Bose-Einstein condensate to a pulsed standing light wave. We show that constructive interference between momentum states populated by the pulsed light determines the stability island’s existence in the underlying pseudoclassical phase space. We generalize this interference model to incorporate higher-order accelerator modes, showing that they are generated if the rephasing occurs after multiple pulses. The model is extended to predict the momentum structure of the quantum accelerator modes close to higher-order quantum resonances. These predictions are in good agreement with our experimental observations

Experimental observation of high-order quantum accelerator modes.

Using a freely falling cloud of cold cesium atoms periodically kicked by pulses from a vertical standing wave of laser light, we present the first experimental observation of high-order quantum accelerator modes. This confirms the recent prediction by Fishman, Guarneri, and Rebuzzini [Phys. Rev. Lett.10.1103/PhysRevLett.89.084101 89, 084101 (2002)]. We also show how these accelerator modes can be identified with the stable regions of phase space in a classical-like chaotic system, despite their intrinsically quantum origin

Dynamical tunneling of a Bose-Einstein condensate in periodically driven systems

We report measurements of dynamical tunneling rates of a Bose-Einstein condensate across a barrier in classical phase space. The atoms are initially prepared in quantum states that extend over a classically regular island region. We focus on the specific system of quantum accelerator modes of the kicked rotor in the presence of gravity. Our experimental data is supported by numerical simulations taking into account imperfections mainly from spontaneous emission. Furthermore, we predict experimentally accessible parameter ranges over which direct tunneling could be readily observed if spontaneous emission was further suppressed. Altogether, we provide a proof-of-principle for the experimental accessibility of dynamical tunneling rates in periodically driven systems.Comment: Improved versio

Experimental investigation of optical atom traps with a frequency jump

We study the evolution of a trapped atomic cloud subject to a trapping frequency jump for two cases: stationary and moving center of mass. In the first case, the frequency jump initiates oscillations in the cloud's momentum and size. At certain times we find the temperature is significantly reduced. When the oscillation amplitude becomes large enough, local density increases induced by the anharmonicity of the trapping potential are observed. In the second case, the oscillations are coupled to the center of mass motion through the anharmonicity of the potential. This induces oscillations with even larger amplitudes, enhancing the temperature reduction effects and leading to nonisotropic expansion rates while expanding freely.Comment: 8 figures, Journal of Physics B: At. Mol. Op. Phy