263 research outputs found
Spontaneously modulated spin textures in a dipolar spinor Bose-Einstein condensate
Helical spin textures in a Rb F=1 spinor Bose-Einstein condensate are
found to decay spontaneously toward a spatially modulated structure of spin
domains. This evolution is ascribed to magnetic dipolar interactions that
energetically favor the short-wavelength domains over the long-wavelength spin
helix. This is confirmed by eliminating the dipolar interactions by a sequence
of rf pulses and observing a suppression of the formation of the short-range
domains. This study confirms the significance of magnetic dipole interactions
in degenerate Rb F=1 spinor gases
Mean field ground state of a spin-1 condensate in a magnetic field
We revisit the topic of the mean field ground state of a spin-1 atomic
condensate inside a uniform magnetic field () under the constraints that
both the total number of atoms () and the magnetization () are
conserved. In the presence of an internal state (spin component) independent
trap, we also investigate the dependence of the so-called single spatial mode
approximation (SMA) on the magnitude of the magnetic field and . Our
result indicate that the quadratic Zeeman effect is an important factor in
balancing the mean field energy from elastic atom-atom collisions that are
known to conserve both and .Comment: 13 pages, 9 figures, to be published in New J. Phys.
(http://www.njp.org/
Topological Excitations in Spinor Bose-Einstein Condensates
We investigate the properties of skyrmion in the ferromagnetic state of
spin-1 Bose-Einstein condensates by means of the mean-field theory and show
that the size of skyrmion is fixed to the order of the healing length. It is
shown that the interaction between two skyrmions with oppositely rotating spin
textures is attractive when their separation is large, following a unique
power-law behavior with a power of -7/2.Comment: 4 pages, 5 figure
Evolution of a spinor condensate: coherent dynamics, dephasing and revivals
We present measurements and a theoretical model for the interplay of spin
dependent interactions and external magnetic fields in atomic spinor
condensates. We highlight general features like quadratic Zeeman dephasing and
its influence on coherent spin mixing processes by focusing on a specific
coherent superposition state in a F=1 Rb Bose-Einstein condensate. In
particular, we observe the transition from coherent spinor oscillations to
thermal equilibration
Generalized spin squeezing inequalities in qubit systems: theory and experiment
We present detailed derivations, various improvements and application to
concrete experimental data of spin squeezing inequalities formulated recently
by some of us [Phys. Rev. Lett. {\bf 95}, 120502 (2005)]. These inequalities
generalize the concept of the spin squeezing parameter, and provide necessary
and sufficient conditions for genuine 2-, or 3- qubit entanglement for
symmetric states, and sufficient entanglement condition for general -qubit
states. We apply our method to theoretical study of Dicke states, and, in
particular, to -states of qubits. Then, we analyze the recently
experimentally generated 7- and 8-ion -states [Nature {\bf 438}, 643
(2005)]. We also present some novel details concerning this experiment.
Finally, we improve criteria for detection of genuine tripartite entanglement
based on entanglement witnesses.Comment: Final versio
Spin squeezing of high-spin, spatially extended quantum fields
Investigations of spin squeezing in ensembles of quantum particles have been
limited primarily to a subspace of spin fluctuations and a single spatial mode
in high-spin and spatially extended ensembles. Here, we show that a wider range
of spin-squeezing is attainable in ensembles of high-spin atoms, characterized
by sub-quantum-limited fluctuations in several independent planes of
spin-fluctuation observables. Further, considering the quantum dynamics of an
ferromagnetic spinor Bose-Einstein condensate, we demonstrate
theoretically that a high degree of spin squeezing is attained in multiple
spatial modes of a spatially extended quantum field, and that such squeezing
can be extracted from spatially resolved measurements of magnetization and
nematicity, i.e.\ the vector and quadrupole magnetic moments, of the quantum
gas. Taking into account several experimental limitations, we predict that the
variance of the atomic magnetization and nematicity may be reduced as far as 20
dB below the standard quantum limits.Comment: 18 pages, 5 figure
Atomic Interactions in Precision Interferometry Using Bose-Einstein Condensates
We present theoretical tools for predicting and reducing the effects of
atomic interactions in Bose-Einstein condensate (BEC) interferometry
experiments. To address mean-field shifts during free propagation, we derive a
robust scaling solution that reduces the three-dimensional Gross-Pitaevskii
equation to a set of three simple differential equations valid for any
interaction strength. To model the other common components of a BEC
interferometer---condensate splitting, manipulation, and recombination---we
generalize the slowly-varying envelope reduction, providing both analytic
handles and dramatically improved simulations. Applying these tools to a BEC
interferometer to measure the fine structure constant (Gupta, et al., 2002), we
find agreement with the results of the original experiment and demonstrate that
atomic interactions do not preclude measurement to better than part-per-billion
accuracy, even for atomic species with relatively large scattering lengths.
These tools help make BEC interferometry a viable choice for a broad class of
precision measurements.Comment: 8 pages, 6 figures, revised based on reviewer comment
Tunable Cavity Optomechanics with Ultracold Atoms
We present an atom-chip-based realization of quantum cavity optomechanics
with cold atoms localized within a Fabry-Perot cavity. Effective sub-wavelength
positioning of the atomic ensemble allows for tuning the linear and quadratic
optomechanical coupling parameters, varying the sensitivity to the displacement
and strain of a compressible gaseous cantilever. We observe effects of such
tuning on cavity optical nonlinearity and optomechanical frequency shifts,
providing their first characterization in the quadratic-coupling regime.Comment: 4 pages, 5 figure
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