78 research outputs found
Dynamic generation of spin-squeezed states in bosonic Josephson junctions
We analyze the formation of squeezed states in a condensate of ultracold
bosonic atoms confined by a double-well potential. The emphasis is set on the
dynamical formation of such states from initially coherent many-body quantum
states. Two cases are described: the squeezing formation in the evolution of
the system around the stable point, and in the short time evolution in the
vicinity of an unstable point. The latter is shown to produce highly squeezed
states on very short times. On the basis of a semiclassical approximation to
the Bose-Hubbard Hamiltonian, we are able to predict the amount of squeezing,
its scaling with and the speed of coherent spin formation with simple
analytical formulas which successfully describe the numerical Bose-Hubbard
results. This new method of producing highly squeezed spin states in systems of
ultracold atoms is compared to other standard methods in the literature.Comment: 12 pages, revised discussion + added reference
Spin-Waves in the Mid-Infrared Spectrum of Antiferromagnetic YBaCuO
The mid-infrared spin-wave spectrum of antiferromagnetic
YBaCuO\ was determined by infrared transmission and reflection
measurements (\bbox{k} \!\! \parallel c) at ~K.\@ Excitation of
single magnons of the optical branch was observed at
~meV.\@ Two further peaks at ~meV
() and ~meV
() both belong to the two-magnon spectrum. Linear
spin wave theory is in good agreement with the measured two-magnon spectrum,
and allows to determine the exchange constant to be about ~meV,
whereas the intrabilayer coupling is approximately .Comment: 3 figures in uuencoded for
Theory of semiconductor quantum-wire based single- and two-qubit gates
A GaAs/AlGaAs based two-qubit quantum device that allows the controlled
generation and straightforward detection of entanglement by measuring a
stationary current-voltage characteristic is proposed. We have developed a
two-particle Green's function method of open systems and calculate the
properties of three-dimensional interacting entangled systems
non-perturbatively. We present concrete device designs and detailed, charge
self-consistent predictions. One of the qubits is an all-electric Mach-Zehnder
interferometer that consists of two electrostatically defined quantum wires
with coupling windows, whereas the second qubit is an electrostatically defined
double quantum dot located in a second two-dimensional electron gas beneath the
quantum wires. We find that the entanglement of the device can be controlled
externally by tuning the tunneling coupling between the two quantum dots.Comment: 16 pages, 13 figures, RevTex4 two-column format, to appear in Phys.
Rev.
Systematics of c-axis Phonons in the Thallium and Bismuth Based Cuprate Superconductors
We present grazing incidence reflectivity measurements in the far infrared
region at temperatures above and below Tc for a series of thallium (Tl2Ba2CuO6,
Tl2Ba2CaCu2O8) and bismuth (Bi2Sr2CuO6, Bi2Sr2CaCu2O8, and
Bi(2-x)Pb(x)Sr2CaCu2O8) based cuprate superconductors. From the spectra, which
are dominated by the c-axis phonons, longitudinal frequencies (LO) are directly
obtained. The reflectivity curves are well fitted by a series of Lorentz
oscillators. In this way the transverse (TO) phonon frequencies were accurately
determined. On the basis of the comparative study of the Bi and Tl based
cuprates with different number of CuO2 layers per unit cell, we suggest
modifications of the assignment of the main oxygen modes. We compare the LO
frequencies in Bi2Sr2CaCu2O8 and Tl2Ba2Ca2Cu3O10 obtained from intrinsic
Josephson junction characteristics with our measurements, and explain the
discrepancy in LO frequencies obtained by the two different methods.Comment: 8 pages Revtex, 6 eps figures, 3 tables, to appear in Phys. Rev.
Nonlinear atom interferometer surpasses classical precision limit
Interference is fundamental to wave dynamics and quantum mechanics. The
quantum wave properties of particles are exploited in metrology using atom
interferometers, allowing for high-precision inertia measurements [1, 2].
Furthermore, the state-of-the-art time standard is based on an interferometric
technique known as Ramsey spectroscopy. However, the precision of an
interferometer is limited by classical statistics owing to the finite number of
atoms used to deduce the quantity of interest [3]. Here we show experimentally
that the classical precision limit can be surpassed using nonlinear atom
interferometry with a Bose-Einstein condensate. Controlled interactions between
the atoms lead to non-classical entangled states within the interferometer;
this represents an alternative approach to the use of non-classical input
states [4-8]. Extending quantum interferometry [9] to the regime of large atom
number, we find that phase sensitivity is enhanced by 15 per cent relative to
that in an ideal classical measurement. Our nonlinear atomic beam splitter
follows the "one-axis-twisting" scheme [10] and implements interaction control
using a narrow Feshbach resonance. We perform noise tomography of the quantum
state within the interferometer and detect coherent spin squeezing with a
squeezing factor of -8.2dB [11-15]. The results provide information on the
many-particle quantum state, and imply the entanglement of 170 atoms [16]
Resonant Inelastic X-Ray Scattering from Valence Excitations in Insulating Copper-Oxides
We report resonant inelastic x-ray measurements of insulating LaCuO
and SrCuOCl taken with the incident energy tuned near the Cu K
absorption edge. We show that the spectra are well described in a shakeup
picture in 3rd order perturbation theory which exhibits both incoming and
outgoing resonances, and demonstrate how to extract a spectral function from
the raw data. We conclude by showing {\bf q}-dependent measurements of the
charge transfer gap.Comment: minor notational changes, discussion of anderson impurity model
fixed, references added; accepted by PR
Electron interference and entanglement in coupled 1D systems with noise
We estimate the role of noise in the formation of entanglement and in the
appearance of single- and two-electron interference in systems of coupled
one-dimensional channels semiconductors. Two cases are considered: a
single-particle interferometer and a two-particle interferometer exploiting
Coulomb interaction. In both of them, environmental noise yields a
randomization of the carrier phases. Our results assess how that the
complementarity relation linking single-particle behavior to nonlocal
quantities, such as entanglement and environment-induced decoherence, acts in
electron interferometry. We show that, in a experimental implementation of the
setups examined, one- and two-electron detection probability at the output
drains can be used to evaluate the decoherence phenomena and the degree of
entanglement.Comment: 12 pages, 6 figures. v2: added some references and corrected tex
Optical studies of gap, hopping energies and the Anderson-Hubbard parameter in the zigzag-chain compound SrCuO2
We have investigated the electronic structure of the zigzag ladder (chain)
compound SrCuO2 combining polarized optical absorption, reflection,
photoreflectance and pseudo-dielectric function measurements with the model
calculations. These measurements yield an energy gap of 1.42 eV (1.77 eV) at
300 K along (perpendicular) to the Cu-O chains. We have found that the lowest
energy gap, the correlation gap, is temperature independent. The electronic
structure of this oxide is calculated using both the
local-spin-density-approximation with gradient correction method, and the
tight-binding theory for the correlated electrons. The calculated density of
electronic states for non-correlated and correlated electrons shows
quasi-one-dimensional character. The correlation gap values of 1.42 eV
(indirect transition) and 1.88 eV (direct transition) have been calculated with
the electron hopping parameters t = 0.30 eV (along a chain), t_yz = 0.12 eV
(between chains) and the Anderson-Hubbard repulsion on copper sites U= 2.0 eV.
We concluded that SrCuO_2 belongs to the correlated-gap insulators.Comment: 24 pages, 8 figures, to be published in Phys.Rev.
Resonant Two-Magnon Raman Scattering in Cuprate Antiferromagnetic Insulators
We present results of low-temperature two-magnon resonance Raman excitation
profile measurements for single layer Sr_2CuO_2Cl_2 and bilayer YBa_2Cu_3O_{6 +
\delta} antiferromagnets over the excitation region from 1.65 to 3.05 eV. These
data reveal composite structure of the two-magnon line shape and strong
nonmonotic dependence of the scattering intensity on excitation energy. We
analyze these data using the triple resonance theory of Chubukov and Frenkel
(Phys. Rev. Lett., 74, 3057 (1995)) and deduce information about magnetic
interaction and band parameters in these materials.Comment: REVTeX, 4 pages + 2 PostScript (compressed) figure
Rabi flopping induces spatial demixing dynamics
We experimentally investigate the mixing/demixing dynamics of Bose-Einstein
condensates in the presence of a linear coupling between two internal states.
The observed amplitude reduction of the Rabi oscillations can be understood as
a result of demixing dynamics of dressed states as experimentally confirmed by
reconstructing the spatial profile of dressed state amplitudes. The
observations are in quantitative agreement with numerical integration of
coupled Gross-Pitaevskii equations without free parameters, which also reveals
the criticality of the dynamics on the symmetry of the system. Our observations
demonstrate new possibilities for changing effective atomic interactions and
studying critical phenomena.Comment: 4 pages, 4 figure
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