168 research outputs found
Collisional properties of ultracold K-Rb mixtures
We determine the inter-species s-wave triplet scattering length a3 for all
K-Rb isotopic mixtures by measuring the cross-section for collisions between
41K and 87Rb in different temperature regimes. The positive value
a3=+163(+57,-12)a0 ensures the stability of binary 41K-87Rb Bose-Einstein
condensates. For the fermion-boson mixture 40K-87Rb we obtain a large and
negative scattering length which implies an efficient sympathetic cooling of
the fermionic species down to the degenerate regime.Comment: 4 pages, 4 figures; revised version (references added and small
changes
Optics with an Atom Laser Beam
We report on the atom optical manipulation of an atom laser beam. Reflection,
focusing and its storage in a resonator are demonstrated. Precise and versatile
mechanical control over an atom laser beam propagating in an inhomogeneous
magnetic field is achieved by optically inducing spin-flips between atomic
ground states with different magnetic moment. The magnetic force acting on the
atoms can thereby be effectively switched on and off. The surface of the atom
optical element is determined by the resonance condition for the spin-flip in
the inhomogeneous magnetic field. A mirror reflectivity of more than 98% is
measured
Spectral Properties of Coupled Bose-Einstein Condensates
We investigate the energy spectrum structure of a system of two (identical)
interacting bosonic wells occupied by N bosons within the Schwinger realization
of the angular momentum. This picture enables us to recognize the symmetry
properties of the system Hamiltonian H and to use them for characterizing the
energy eigenstates. Also, it allows for the derivation of the single-boson
picture which is shown to be the background picture naturally involved by the
secular equation for H. After deriving the corresponding eigenvalue equation,
we recast it in a recursive N-dependent form which suggests a way to generate
the level doublets (characterizing the H spectrum) via suitable inner
parameters. Finally, we show how the presence of doublets in the spectrum
allows to recover, in the classical limit, the symmetry breaking effect that
characterizes the system classically.Comment: 8 pages, 3 figures; submitted to Phys. Rev. A. The present extended
form replaces the first version in the letter forma
An Atom Laser with a cw Output Coupler
We demonstrate a continuous output coupler for magnetically trapped atoms.
Over a period of up to 100 ms a collimated and monoenergetic beam of atoms is
continuously extracted from a Bose- Einstein condensate. The intensity and
kinetic energy of the output beam of this atom laser are controlled by a weak
rf-field that induces spin flips between trapped and untrapped states.
Furthermore, the output coupler is used to perform a spectroscopic measurement
of the condensate, which reveals the spatial distribution of the magnetically
trapped condensate and allows manipulation of the condensate on a micrometer
scale.Comment: 4 pages, 4 figure
Superfluid to Mott insulator transition in one, two, and three dimensions
We have created one-, two-, and three-dimensional quantum gases and study the
superfluid to Mott insulator transition. Measurements of the transition using
Bragg spectroscopy show that the excitation spectra of the low-dimensional
superfluids differ significantly from the three-dimensional case
Quantum statistics of atoms in microstructures
This paper proposes groove-like potential structures for the observation of
quantum information processing by trapped particles. As an illustration the
effect of quantum statistics at a 50-50 beam splitter is investigated. For
non-interacting particles we regain the results known from photon experiments,
but we have found that particle interactions destroy the perfect bosonic
correlations. Fermions avoid each other due to the exclusion principle and
hence they are far less sensitive to particle interactions. For bosons, the
behavior can be explained with simple analytic considerations which predict a
certain amount of universality. This is verified by detailed numerical
calculations.Comment: 18 pages incl. 13 figure
Surface acoustic wave attenuation by a two-dimensional electron gas in a strong magnetic field
The propagation of a surface acoustic wave (SAW) on GaAs/AlGaAs
heterostructures is studied in the case where the two-dimensional electron gas
(2DEG) is subject to a strong magnetic field and a smooth random potential with
correlation length Lambda and amplitude Delta. The electron wave functions are
described in a quasiclassical picture using results of percolation theory for
two-dimensional systems. In accordance with the experimental situation, Lambda
is assumed to be much smaller than the sound wavelength 2*pi/q. This restricts
the absorption of surface phonons at a filling factor \bar{\nu} approx 1/2 to
electrons occupying extended trajectories of fractal structure. Both
piezoelectric and deformation potential interactions of surface acoustic
phonons with electrons are considered and the corresponding interaction
vertices are derived. These vertices are found to differ from those valid for
three-dimensional bulk phonon systems with respect to the phonon wave vector
dependence. We derive the appropriate dielectric function varepsilon(omega,q)
to describe the effect of screening on the electron-phonon coupling. In the low
temperature, high frequency regime T << Delta (omega_q*Lambda
/v_D)^{alpha/2/nu}, where omega_q is the SAW frequency and v_D is the electron
drift velocity, both the attenuation coefficient Gamma and varepsilon(omega,q)
are independent of temperature. The classical percolation indices give
alpha/2/nu=3/7. The width of the region where a strong absorption of the SAW
occurs is found to be given by the scaling law |Delta \bar{\nu}| approx
(omega_q*Lambda/v_D)^{alpha/2/nu}. The dependence of the electron-phonon
coupling and the screening due to the 2DEG on the filling factor leads to a
double-peak structure for Gamma(\bar{\nu}).Comment: 17 pages, 3 Postscript figures, minor changes mad
Generic model of an atom laser
We present a generic model of an atom laser by including a pump and loss term
in the Gross-Pitaevskii equation. We show that there exists a threshold for the
pump above which the mean matter field assumes a non-vanishing value in
steady-state. We study the transient regime of this atom laser and find
oscillations around the stationary solution even in the presence of a loss
term. These oscillations are damped away when we introduce a position dependent
loss term. For this case we present a modified Thomas-Fermi solution that takes
into account the pump and loss. Our generic model of an atom laser is analogous
to the semi-classical theory of the laser.Comment: 15 pages, including 5 figures, submitted to Phys. Rev. A, revised
manuscript, file also available at
http://www.physik.uni-ulm.de/quan/users/kne
Substrate-based atom waveguide using guided two-color evanescent light fields
We propose a dipole-force linear waveguide which confines neutral atoms up to
lambda/2 above a microfabricated single-mode dielectric optical guide. The
optical guide carries far blue-detuned light in the horizontally-polarized TE
mode and far red-detuned light in the vertically-polarized TM mode, with both
modes close to optical cut-off. A trapping minimum in the transverse plane is
formed above the optical guide due to the differing evanescent decay lengths of
the two modes. This design allows manufacture of mechanically stable
atom-optical elements on a substrate. We calculate the full vector bound modes
for an arbitrary guide shape using two-dimensional non-uniform finite elements
in the frequency-domain, allowing us to optimize atom waveguide properties. We
find that a rectangular optical guide of 0.8um by 0.2um carrying 6mW of total
laser power (detuning +-15nm about the D2 line) gives a trap depth of 200uK for
cesium atoms (m_F = 0), transverse oscillation frequencies of f_x = 40kHz and
f_y = 160kHz, collection area ~ 1um^2 and coherence time of 9ms. We discuss the
effects of non-zero m_F, surface interactions, heating rate, the substrate
refractive index, and the limits on waveguide bending radius.Comment: 12 pages, 4 figures, revtex, submitted to Phys. Rev. A Replaced:
final version accepted by PRA v.61 Feb 2000. (2 paragraphs added
Polarization state of the optical near-field
The polarization state of the optical electromagnetic field lying several
nanometers above complex dielectric structures reveals the intricate
light-matter interaction that occurs in this near-field zone. This information
can only be extracted from an analysis of the polarization state of the
detected light in the near-field. These polarization states can be calculated
by different numerical methods well-suited to near--field optics. In this
paper, we apply two different techniques (Localized Green Function Method and
Differential Theory of Gratings) to separate each polarisation component
associated with both electric and magnetic optical near-fields produced by
nanometer sized objects. The analysis is carried out in two stages: in the
first stage, we use a simple dipolar model to achieve insight into the physical
origin of the near-field polarization state. In the second stage, we calculate
accurate numerical field maps, simulating experimental near-field light
detection, to supplement the data produced by analytical models. We conclude
this study by demonstrating the role played by the near-field polarization in
the formation of the local density of states.Comment: 9 pages, 11 figures, accepted for publication in Phys. Rev.
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