94 research outputs found
Phase dependence of localization in the driven two-level model
A two-level system subjected to a high-frequency driving field can exhibit an
effect termed ``coherent destruction of tunneling'', in which the tunneling of
the system is suppressed at certain values of the frequency and strength of the
field. This suppression becomes less effective as the frequency of the driving
field is reduced, and we show here how the detailed form of its fall-off
depends on the phase of the driving, which for certain values can produce small
local maxima (or revivals) in the overall decay. By considering a squarewave
driving field, which has the advantage of being analytically tractable, we show
how this surprising behavior can be interpreted geometrically in terms of
orbits on the Bloch sphere. These results are of general applicability to more
commonly used fields, such as sinusoidal driving, which display a similar
phenomenology.Comment: 4 pages,4 eps figures. V2: minor changes, this version to be
published in Europhysics Letter
Coherent control of a self-trapped Bose-Einstein condensate
We study the behavior of a Bose-Einstein condensate held in an optical
lattice. We first show how a self-trapping transition can be induced in the
system by either increasing the number of atoms occupying a lattice site, or by
raising the interaction strength above a critical value. We then investigate
how applying a periodic driving potential to the self-trapped state can be used
to coherently control the emission of a precise number of correlated bosons
from the trapping-site. This allows the formation and transport of entangled
bosonic states, which are of great relevance to novel technologies such as
quantum information processing.Comment: 4 pages, 5 EPS figure
Relativistic motion of an Airy wavepacket in a lattice potential
We study the dynamics of an Airy wavepacket moving in a one-dimensional
lattice potential. In contrast to the usual case of propagation in a continuum,
for which such a wavepacket experiences a uniform acceleration, the lattice
bounds its velocity, and so the acceleration cannot continue indefinitely.
Instead, we show that the wavepacket's motion is described by relativistic
equations of motion, which surprisingly, arise naturally from evolution under
the standard non-relativistic Schr\"odinger equation. The presence of the
lattice potential allows the wavepacket's motion to be controlled by means of
Floquet engineering. In particular, in the deep relativistic limit when the
wavepacket's motion is photon-like, this form of control allows it to mimic
both standard and negative refraction. Airy wavepackets held in lattice
potentials can thus be used as powerful and flexible simulators of relativistic
quantum systems.Comment: 9 pages, 8 figures. Higher resolution versions of Figs. 7a, 7b, 7c
can be supplied on reques
Comment on "Creating artificial magnetic fields for cold atoms by photon-assisted tunneling" by Kolovsky A.R
We comment briefly on the scheme proposed in EPL 93, 20003 (2011) to produce
synthetic gauge fields by means of photon-assisted tunneling.Comment: 2 pages, EPL forma
Generation of uniform synthetic magnetic fields by split driving of an optical lattice
We describe a method to generate a synthetic gauge potential for ultracold
atoms held in an optical lattice. Our approach uses a time-periodic driving
potential based on two quickly alternating signals to engineer the appropriate
Aharonov-Bohm phases, and permits the simulation of a uniform tunable magnetic
field. We explicitly demonstrate that our split driving scheme reproduces the
behavior of a charged quantum particle in a magnetic field over the complete
range of field strengths, and obtain the Hofstadter butterfly band-structure
for the Floquet quasienergies at high fluxes.Comment: 5 pages, 3 eps figure
Instability and control of a periodically-driven Bose-Einstein condensate
We investigate the dynamics of a Bose-Einstein condensate held in an optical
lattice under the influence of a strong periodic driving potential. Studying
the mean-field version of the Bose-Hubbard model reveals that the condensate
becomes highly unstable when the effective intersite tunneling becomes
negative. We further show how controlling the sign of the tunneling can be used
as a powerful tool to manage the dispersion of an atomic wavepacket, and thus
to create a pulsed atomic soliton laser.Comment: 4 pages, 3 eps figure
Coherent ratchets in driven Bose-Einstein condensates
We study the response of a Bose-Einstein condensate to an unbiased periodic
driving potential. By controlling the space and time symmetries of the driving
we show how a directed current can be induced, producing a coherent quantum
ratchet. Weak driving induces a regular behavior that is strongly governed by
the interparticle interaction. Breaking both space and time symmetries is
required to produce current flow. For strong driving the behavior becomes
chaotic. The resulting effective irreversibility renders the space asymmetry
sufficient to produce the ratchet effect, although the system is completely
coherent.Comment: 5 pages, 4 eps figures. Minor changes, this version to be published
in PR
Controlled generation of coherent matter-currents using a periodic driving field
We study the effect of a strong, oscillating driving field on the dynamics of
ultracold bosons held in an optical lattice. Modeling the system as a
Bose-Hubbard model, we show how the driving field can be used to produce and
maintain a coherent atomic current by controlling the phase of the intersite
tunneling processes. We investigate both the stroboscopic and time-averaged
behavior using Floquet theory, and demonstrate that this procedure provides a
stable and precise method of controlling coherent quantum systems.Comment: 4.1 pages, 4 eps figure
Finding zeros of the Riemann zeta function by periodic driving of cold atoms
The Riemann hypothesis, which states that the non-trivial zeros of the
Riemann zeta function all lie on a certain line in the complex plane, is one of
the most important unresolved problems in mathematics. Inspired by the
P\'olya-Hilbert conjecture, we propose a new approach to finding a physical
system to study the Riemann zeros, which in contrast to previous examples, is
based on applying a time-periodic driving field. This driving allows us to
mould the quasienergies of the system (the analogue of the eigenenergies in the
absence of driving), so that they are directly governed by the zeta function.
We further show by numerical simulations that this allows the Riemann zeros to
be measured in currently accessible cold atom experiments.Comment: 6 pages, accepted for publication in Phys. Rev.
Optimum pinning of the vortex lattice in extremely type-II layered superconductors
The two-dimensional (2D) vortex lattice in the extreme type-II limit is
studied by Monte Carlo simulation of the corresponding 2D Coulomb gas, with
identical pins placed at sites coinciding with the zero-temperature triangular
vortex lattice. At weak pinning we find evidence for 2D melting into an
intermediate hexatic phase. The strong pinning regime shows a
Kosterlitz-Thouless transition, driven by interstitial vortex/anti-vortex
excitations. A stack of such identical layers with a weak Josephson coupling
models a layered superconductor with a triangular arrangement of columnar pins
at the matching field. A partial duality analysis finds that layer decoupling
of the flux-line lattice does not occur at weak pinning for temperatures below
2D melting.Comment: 5 pgs., 4 figs. To appear in PRB. Added size study of hexatic phas
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