(1)Proposal for a chaotic ratchet using cold atoms in optical lattices , Monteiro T S.,Dando P A, Hutchings N , Isherwood M, Phys.Rev Lett 89 194102 (2002)
(2)Chaotic filtering of cold atoms in an optical lattice by control of dynamical localization T.Jonckheere, ,Isherwood M. and Monteiro T. S. Phys.Rev Lett 91 253003 (2003).
(3)Directed motion for delta-kicked atoms with broken symmetries: comparison with experiment P H Jones, M Goonasekara, D R Meacher, T Jonckheere, T S Monteiro Phys.Rev Lett 98 073002 (2007)
(4)"Chaotic quantum ratchets with cold atoms in optical lattices: Floquet states" G.Hur, C.Creffield, P.H. Jones and T.S. Monteiro, physics/0407100 Phys.Rev.A 72 013402 (2005).
Ratchets are devices for generating a current even in the absence of a net force. There is an enormous body of work on Brownian ratchets driven by the need to understand biophysical systems such as molecular motors. However the question of whether one can design a generic ratchet where instead of noise one exploits the chaotic dynamics has only been addressed recently. If in addition one also requires that the ratchet has no dissipation, then one has a Hamiltonian ratchet; this has the advantage of fully preserving quantum coherence so is what one requires to design a ratchet suitable for atom optics devices without decoherence.
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We have proposed a fully chaotic atom optics ratchet which exploits the phenomenon
of Dynamical Localization (DL). DL has been described as the
so-called quantum suppression of classical chaotic diffusion.
In experiments,cold atoms in pulsed standing waves of light
make a transition to chaotic classical
dynamics for sufficiently strong laser intensity. The classical energy,
is unbounded and grows diffusively with time.
For the corresponding quantum system,
in contrast,the diffusion is suppressed after an \hbar-dependent timescale, the
`break-time' t*.
Prior to our work it was thought that the only Hamiltonian ratchets which were possible
were mixed phase-space (classical phase space has a mixture of stable
islands and chaotic regions).
Our ratchet is fully chaotic and relies on a hitherto unnoticed effect (Refs 1,2):
for particles in an asymmetric lattice, subjected to
a repeating cycle of unequally spaced kicks, the classical momentum diffusion rates
for positive and negative momenta are (in general) different
up to a finite time, t_r the 'ratchet time' a new time-scale
associated with this process.
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The figure (see also ref.(3)) shows the experimental ratchet current plotted as a function of the initial velocity of the atoms. This represents the first ever experimental demonstration of a Hamiltonian quantum ratchet!
Dr Thibaut Jonckheere (PDRA 2002-2003) left to take a tenured position at the Centre de Physique Theorique, campus de Luminy, Marseille.
Dr Matt Isherwood (PhD 2001-2004) is now Associate Vice-president of Financial Risk Management Ltd, London. His PhD thesis on the theory of Chaotic Hamiltonian Ratchets is here (PDF)
