384 research outputs found
Resonant control of spin dynamics in ultracold quantum gases by microwave dressing
We study experimentally interaction-driven spin oscillations in optical
lattices in the presence of an off-resonant microwave field. We show that the
energy shift induced by this microwave field can be used to control the spin
oscillations by tuning the system either into resonance to achieve near-unity
contrast or far away from resonance to suppress the oscillations. Finally, we
propose a scheme based on this technique to create a flat sample with either
singly- or doubly-occupied sites, starting from an inhomogeneous Mott
insulator, where singly- and doubly-occupied sites coexist.Comment: 4 pages, 5 figure
Upscaling of bottom-generated turbulence in large-scale 3D models for sediment transport in estuaries and coastal zones
Currently used 3D numerical sediment transport models still fail to make good quantitative predictions. To a great extent, this can be attributed to the inadequate description of physical processes which occur at the subgrid scale level. From flume experiments it is known that particle-turbulence interactions near the bed significantly change the effective roughness experienced by the overlying water column. This results in different transport rates if not accounted for.From a theoretical perspective, bed load transport, sheet flow and fluid mud flow are all occurrences of supersaturated suspension flow in the inner near-bed layer comprising the viscous sublayer and the transient layer. Its thickness increases with sediment load, since particle-particle interactions (four-way coupling effects) consume considerable amounts of the available stream power. In order to know how much energy is left over to compute the transport capacity of the outer, fully-developed layer, it is necessary to quantify the energy budget in the inner layer.This is a difficult task. Every modelling approach has its draw-backs and limitations. Lagrangean particle tracking is hopeless, since the required number of particles to approach field conditions is much too high, and the volumes occupied by the particles cannot be neglected. Grain sizes are non-uniform in nature and concentrations near the bed very high, making it very difficult to give an accurate description of the momentum exchange between fluid and solid phase, which accounts for particle collisions. Therefore, in view of large-scale applications, a one-fluid approach is adopted. This implies that the momentum equation is solved for the suspension, together with a turbulence closure model and the sediment mass balance.Since the thickness of the supersaturated inner layer mostly is very small relative to the water depth and the vertical discretization in large scale applications, it is not possible to resolve this layer with a traditional low-Reynolds model approach, which requires a very fine grid. A new approach is proposed, where a modified Prandtl-mixing length (PML) model is used for the bed layer, and a new low-Reynolds model is applied in the outer layers. In this way it is possible to obtain a correct behaviour for tidal oscillating flow in estuaries, where low-Re effects enter high in the water column during slack water.The correction factor for the PML eddy viscosity and the damping functions for the low-Re k-epsilon turbulence model are constructed based on theoretical constraints, DNS and LES generated data, as well as experimental flume data. In parallel, LES and improved two-layer low-Re models are developed to simulate flow over rough bottoms without and with sediment, in order to generate data very close to the bed surface, where no measurements can be made. These additional data are used to help interpret experimental flume data, which always show relatively high experimental errors, and to extend the new damping functions for the cases with bottom roughness and suspended sediment.Preliminary results of the new coarse grid RANS model for open-channel flow with various roughness conditions without and with suspended sediment will be shown, compared to LES results for flow over a wavy bottom, low-Reynolds RANS results over rough bottom and experimental flume data
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
Coherent collisional spin dynamics in optical lattices
We report on the observation of coherent, purely collisionally driven spin
dynamics of neutral atoms in an optical lattice. For high lattice depths, atom
pairs confined to the same lattice site show weakly damped Rabi-type
oscillations between two-particle Zeeman states of equal magnetization, induced
by spin changing collisions. This paves the way towards the efficient creation
of robust entangled atom pairs in an optical lattice. Moreover, measurement of
the oscillation frequency allows for precise determination of the spin-changing
collisional coupling strengths, which are directly related to fundamental
scattering lengths describing interatomic collisions at ultracold temperatures.Comment: revised version; 4 pages, 5 figure
What's in the
Detectors for accents and phrase boundaries have been developed which derive prosodic features from the speech signal and its fundamental frequency to support other modules of a speech understanding system in an early analysis stage, or in cases where no word hypotheses are available. The detectors' underlying Gaussian distribution classifiers were trained with 50 minutes and tested with 30 minutes of spontaneous speech, yielding recognition rates of 74% for accents and 86% for phrase boundaries. Since this material was prosodically hand labelled, the question was: which labels for phrase boundaries and accentuation were only guided by syntactic or semantic knowledge, and which ones are really prosodically marked? Therefore a small test subset has been resynthesized in such a way that comprehensibility was lost, but the prosodic characteristics were kept. This subset has been relabelled by 11 listeners with nearly the same accuracy as the detector
Bayesian feedback control of a two-atom spin-state in an atom-cavity system
We experimentally demonstrate real-time feedback control of the joint
spin-state of two neutral Caesium atoms inside a high finesse optical cavity.
The quantum states are discriminated by their different cavity transmission
levels. A Bayesian update formalism is used to estimate state occupation
probabilities as well as transition rates. We stabilize the balanced two-atom
mixed state, which is deterministically inaccessible, via feedback control and
find very good agreement with Monte-Carlo simulations. On average, the feedback
loops achieves near optimal conditions by steering the system to the target
state marginally exceeding the time to retrieve information about its state.Comment: 4 pages, 4 figure
Correlated hopping of bosonic atoms induced by optical lattices
In this work we analyze a particular setup with ultracold atoms trapped in
state-dependent lattices. We show that any asymmetry in the contact interaction
translates into one of two classes of correlated hopping. After deriving the
effective lattice Hamiltonian for the atoms, we obtain analytically and
numerically the different phases and quantum phase transitions. We find for
weak correlated hopping both Mott insulators and charge density waves, while
for stronger correlated hopping the system transitions into a pair superfluid.
We demonstrate that this phase exists for a wide range of interaction
asymmetries and has interesting correlation properties that differentiate it
from an ordinary atomic Bose-Einstein condensate.Comment: 24 pages with 9 figures, to appear in New Journal of Physic
Three-Dimensional Dirac Electrons at the Fermi Energy in Cubic Inverse Perovskites: Ca_3PbO and its Family
The band structure of cubic inverse perovskites, Ca_3PbO and its family, are
investigated with the first-principles method. A close observation of the band
structure reveals that six equivalent Dirac electrons with a very small mass
exist on the line connecting the Gamma- and X-points, and at the symmetrically
equivalent points in the Brillouin zone. The discovered Dirac electrons are
three-dimensional and remarkably located exactly at the Fermi energy. A
tight-binding model describing the low-energy band structure is also
constructed and used to discuss the origin of the Dirac electrons in this
material. Materials related to Ca_3PbO are also studied, and some design
principles for the Dirac electrons in this series of materials are proposed.Comment: 4.2 pages, refined versio
Fictitious Magnetic Resonance by Quasi-Electrostatic Field
We propose a new kind of spin manipulation method using a {\it fictitious}
magnetic field generated by a quasi-electrostatic field. The method can be
applicable to every atom with electron spins and has distinct advantages of
small photon scattering rate and local addressability. By using a
laser as a quasi-electrostatic field, we have experimentally demonstrated the
proposed method by observing the Rabi-oscillation of the ground state hyperfine
spin F=1 of the cold atoms and the Bose-Einstein condensate.Comment: 5 pages, 5 figure
- …