1,108 research outputs found
Probing quantum-mechanical level repulsion in disordered systems by means of time-resolved selectively-excited resonance fluorescence
We argue that the time-resolved spectrum of selectively-excited resonance
fluorescence at low temperature provides a tool for probing the
quantum-mechanical level repulsion in the Lifshits tail of the electronic
density of states in a wide variety of disordered materials. The technique,
based on detecting the fast growth of a fluorescence peak that is red-shifted
relative to the excitation frequency, is demonstrated explicitly by simulations
on linear Frenkel exciton chains.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Box traps on an atom chip for one-dimensional quantum gases
We present the implementation of tailored trapping potentials for ultracold
gases on an atom chip. We realize highly elongated traps with box-like
confinement along the long, axial direction combined with conventional harmonic
confinement along the two radial directions. The design, fabrication and
characterization of the atom chip and the box traps is described. We load
ultracold (K) clouds of Rb in a box trap, and demonstrate
Bose-gas focusing as a means to characterize these atomic clouds in arbitrarily
shaped potentials. Our results show that box-like axial potentials on atom
chips are very promising for studies of one-dimensional quantum gases.Comment: 9 pages 4 figure
Short to long-range charge-transfer excitations in the zincbacteriochlorin-bacteriochlorin complex: a Bethe-Salpeter study
We study using the Bethe-Salpeter formalism the excitation energies of the
zincbacteriochlorinbacteriochlorin dyad, a paradigmatic photosynthetic complex.
In great contrast with standard timedependent density functional theory
calculations with (semi)local kernels, charge transfer excitations are
correctly located above the intramolecular Q-bands transitions found to be in
excellent agreement with experiment. Further, the asymptotic Coulomb behavior
towards the true quasiparticle gap for charge transfer excitations at long
distance is correctly reproduced, showing that the present scheme allows to
study with the same accuracy intramolecular and charge transfer excitations at
various spatial range and screening environment without any adjustable
parameter.Comment: 5 pages, 2 figures, 1 tabl
Three-dimensional character of atom-chip-based rf-dressed potentials
We experimentally investigate the properties of radio-frequency-dressed
potentials for Bose-Einstein condensates on atom chips. The three-dimensional
potential forms a connected pair of parallel waveguides. We show that
rf-dressed potentials are robust against the effect of small magnetic-field
variations on the trap potential. Long-lived dipole oscillations of condensates
induced in the rf-dressed potentials can be tuned to a remarkably low damping
rate. We study a beam-splitter for Bose-Einstein condensates and show that a
propagating condensate can be dynamically split in two vertically separated
parts and guided along two paths. The effect of gravity on the potential can be
tuned and compensated for using a rf-field gradient.Comment: 9 pages, 7 figure
Linear motor motion control using a learning feedforward controller
The design and realization of an online learning motion controller for a linear motor is presented, and its usefulness is evaluated. The controller consists of two components: (1) a model-based feedback component, and (2) a learning feedforward component. The feedback component is designed on the basis of a simple second-order linear model, which is known to have structural errors. In the design, an emphasis is placed on robustness. The learning feedforward component is a neural-network-based controller, comprised of a one-hidden-layer structure with second-order B-spline basis functions. Simulations and experimental evaluations show that, with little effort, a high-performance motion system can be obtained with this approach
Yang-Yang thermodynamics on an atom chip
We investigate the behavior of a weakly interacting nearly one-dimensional
(1D) trapped Bose gas at finite temperature. We perform in situ measurements of
spatial density profiles and show that they are very well described by a model
based on exact solutions obtained using the Yang-Yang thermodynamic formalism,
in a regime where other, approximate theoretical approaches fail. We use
Bose-gas focusing [Shvarchuck etal., Phys. Rev. Lett. 89, 270404 (2002)] to
probe the axial momentum distribution of the gas, and find good agreement with
the in situ results.Comment: extended introduction and conclusions, and minor changes throughout;
accepted for publication in Phys. Rev. Let
Yang-Yang thermometry and momentum distribution of a trapped one-dimensional Bose gas
We describe the use of the exact Yang-Yang solutions for the one-dimensional
Bose gas to enable accurate kinetic-energy thermometry based on the
root-mean-square width of an experimentally measured momentum distribution.
Furthermore, we use the stochastic projected Gross-Pitaevskii theory to provide
the first quantitative description of the full momentum distribution
measurements of Van Amerongen et al., Phys. Rev. Lett. 100, 090402 (2008). We
find the fitted temperatures from the stochastic projected Gross-Pitaevskii
approach are in excellent agreement with those determined by Yang-Yang
kinetic-energy thermometry.Comment: 5 pages, 3 figures. v2: Updated to published versio
Tunable exciton interactions in optical lattices with polar molecules
Rotational excitation of polar molecules trapped in an optical lattice gives
rise to rotational excitons. Here we show that non-linear interactions of such
excitons can be controlled by an electric field. The exciton--exciton
interactions can be tuned to induce exciton pairing, leading to the formation
of biexcitons. Tunable non-linear interactions between excitons can be used for
many applications ranging from the controlled preparation of entangled
quasiparticles to the study of polaron interactions and the effects of
non-linear interactions on quantum energy transport in molecular aggregates.Comment: Some typos have been corrected in this versio
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