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arXiv:1407.1150 · Physical Review A

Quantum paddlewheel with ultracold atoms in waveguides

Kunal K. Das, Matthew R. Meehan, Andrew J. Pyle

Published Jun 30, 2014• 3 citations
arXivHTMLPDF

Abstract

We propose and study a quantum pump which emulates a traditional paddlewheel, that can be implemented with ultracold atoms in waveguides. We use wave-packet propagation to study its single-mode dynamics, which also determines its multimode current for mesoscopic setups. Energy flow with or without particle transport is possible. The spectrum reveals unusual features such as nonuniform Floquet sidebands and counterintuitive scattering. Explanations are found by examining the scattering dynamics comparatively using quantum, classical, and semiclassical pictures, indicating a rich system and experimentally accessible method to explore quantum versus classical dynamics.

DOI
10.1103/physreva.89.063626

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We propose and study a quantum pump which emulates a traditional paddlewheel, that can be implemented with ultracold atoms in waveguides. We use wave-packet propagation to study its single-mode dynamics, which also determines its multimode current for mesoscopic setups. Energy flow with or without particle transport is possible. The spectrum reveals unusual features such as nonuniform Floquet sidebands and counterintuitive scattering. Explanations are found by examining the scattering dynamics comparatively using quantum, classical, and semiclassical pictures, indicating a rich system and experimentally accessible method to explore quantum versus classical dynamics.
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We propose a way to simulate mesoscopic transport processes with counterpropagating wave packets of ultracold atoms in quasi-one-dimensional (1D) waveguides and show quantitative agreement with analytical results. The method allows the study of a broad range of transport processes at the level of individual modes, not possible in electronic systems. Typically suppressed effects of quantum coherence become manifest, along with the effects of tunable interactions, which can be used to develop a simpler type of sensitive atom interferometer.
Quantum pumping is a method of transporting particles through a circuit without an external applied voltage or chemical potential, but instead with localized time-varying potentials.Ballistic quantum pumping has been theorized in mesoscopic solid state sys tems, but experimental verifications have been challenging due to the capacitive coupling and rectification inherent to electronic systems.Using ultracold neutral atoms to test and verify the theoretical predictions of quantum pumping reduces these challenges while providing the advantages of easy confinement and manipulation, a high degree of coher ence, and bosons as well as fermions.In addition, ultracold atoms allow smooth tunability between classical thermal gases and quantum gases.Here, we design, build, characterize, and optimize a dual-species experimental apparatus capable of quantum pumping exper iments with Bose-Einstein condensates on an atom chip.We produce quasi-pure 87Rb Bose-Einstein condensates of ~ 104 atoms and successfully laser-cool 39K.We lay the experimental groundwork for quantum pumping experiments with ultracold atoms and characterize the classical dynamics of the system.These classical dynamics are supported with theoretical modeling that we have developed to compare the dramatic differences between the classical and quantum results of a double barrier turnstile pump as well as its single barrier building blocks.
A wave-packet propagation method is developed and applied to investigate the quantum dynamics of scattering processes of identical and distinguishable atoms in harmonic waveguides. The quantum dynamics of the confinement-induced resonances (CIRs) for ultracold collisions of identical particles, sss-wave CIRs for bosons and ppp-wave CIRs for fermions, is explored in detail. Our multigrid approach allows us to fully take into account the coupling between the center-of-mass (c.m.) and relative motions in the case of distinguishable atoms. The latter includes, in particular, sss- and ppp-partial-wave mixing, caused by the confining trap, which acts differently on the different atomic species. Specifically, we explore in detail the recently discovered [J. I. Kim, V. S. Melezhik, and P. Schmelcher, Phys. Rev. Lett. 97, 193203 (2006)] dual CIR, which is based on a destructive interference mechanism leading to complete transmission in the waveguide, although the corresponding scattering in free space exhibits strong sss- and ppp-wave scattering.
We report on the transport properties of a single-mode quantum pump that operates by the simultaneous translation and oscillation of a potential well. We examine the dynamics comparatively using quantum, classical, and semiclassical simulations. The use of an attractive or well potential is found to present several striking features absent if a barrier potential is used instead, as usually favored. The trapping of particles by the well for variable durations and subsequent release leads to a fractal-like structure in the distribution of the classical scattering trajectories. Interference among them leads to a rich dynamical structure in the quantum current, conspicuously missing in the classical current. Specifically, we observe sharp steps, spikes, and dips in the current as a function of the incident energy of the carriers and determine that a dynamical version of Fano resonance has a role that depends on the direction of incidence and on multiple scattering by the potential.
The properties of low-density, low-energy matter wave packets propagating through waveguide bends are investigated. Time-dependent quantum-mechanical calculations using simple harmonic oscillator confining potentials are performed for a range of parameters close to those accessible by recent ``atom chip''-based experiments. We compare classical calculations based on Ehrenfest's theorem to these results to determine whether classical mechanics can predict the amount of transverse excitation as measured by the transverse heating. The present results thus elucidate some of the limits for which matter wave propagation through microstructures can be reliably considered using classical particle motion.