367 research outputs found
Inelastic Neutron and X-ray Scattering from Incommensurate Magnetic Systems
Neutrons and X-rays are powerful probes for studying magnetic and lattice
excitations in strongly correlated materials over very wide ranges of momentum
and energy transfers. In the focus of the present work are the incommensurate
magnetic systems MnSi and Cr. Under application of a magnetic field, helically
ordered MnSi transforms into a weak itinerant ferromagnet. Using polarized
neutrons we demonstrate that the Stoner excitations are spin flip excitations.
The amplitude (longitudinal) fluctuations associated with the magnon modes are
already strong far away from T_C. Interestingly, even the non spin flip
excitations associated with the Stoner modes are observable. In Cr, we have
observed Kohn anomalies in the phonon spectrum at those incommensurate
positions in reciprocal space, where the spin density wave is observed. The
corresponding phonon and magnon modes are not coupled. In addition, an
anomalous softening of a transverse phonon branch along the N-H zone boundary
line is observed that is caused by strong electron phonon coupling. High
resolution neutron scattering indicate that the low energy Fincher-Burke
excitations may rather correspond to localized modes in momentum and energy and
not to propagating collective modes. Finally, we demonstrate that in the near
future it may become feasible to investigate excitations in very small samples
thus allowing to measure the dynamics of strongly correlated materials under
extreme conditions and in the vicinity of quantum phase transitions
Iterative algorithm for reconstruction of entangled states
An iterative algorithm for the reconstruction of an unknown quantum state
from the results of incompatible measurements is proposed. It consists of
Expectation-Maximization step followed by a unitary transformation of the
eigenbasis of the density matrix. The procedure has been applied to the
reconstruction of the entangled pair of photons.Comment: 4 pages, no figures, some formulations changed, a minor mistake
correcte
Phonon lineshapes in the vortex state of the phonon-mediated superconductor YNiBC
We present an inelastic neutron scattering study of phonon lineshapes in the
vortex state of the type-II superconductor YNiBC. In a previous study
[Phys. Rev. Lett. \textbf{101}, 237002 (2008)] it was shown that certain
phonons exhibit a clear signature of the superconducting gap on
entering the superconducting state. Our interest was to find out whether or not
the lineshape of such phonons reflects the inhomogeneous nature of the vortex
state induced by a magnetic field smaller than the upper critical field
.We found that this is indeed the case because the observed phonon
lineshapes can be well described by a model considering the phonon as a local
probe of the spatial variation of the superconducting gap. We found that even
at , where the inter-vortex distance is less than \AA, the
phonon lineshape still shows evidence for a variation of the gap
Diluted maximum-likelihood algorithm for quantum tomography
We propose a refined iterative likelihood-maximization algorithm for
reconstructing a quantum state from a set of tomographic measurements. The
algorithm is characterized by a very high convergence rate and features a
simple adaptive procedure that ensures likelihood increase in every iteration
and convergence to the maximum-likelihood state.
We apply the algorithm to homodyne tomography of optical states and quantum
tomography of entangled spin states of trapped ions and investigate its
convergence properties.Comment: v2: Convergence proof adde
Informational completeness of continuous-variable measurements
We justify that homodyne tomography turns out to be informationally complete
when the number of independent quadrature measurements is equal to the
dimension of the density matrix in the Fock representation. Using this as our
thread, we examine the completeness of other schemes, when continuous-variable
observations are truncated to discrete finite-dimensional subspaces.Comment: To appear in Phys. Rev.
Quantum theory of incompatible observations
Maximum likelihood principle is shown to be the best measure for relating the
experimental data with the predictions of quantum theory.Comment: 3 page
Development of the Magnetic Excitations of Charge-Stripe Ordered La(2-x)Sr(x)NiO(4) on Doping Towards Checkerboard Charge Order
The magnetic excitation spectrums of charge stripe ordered La(2-x)Sr(x)NiO(4)
x = 0.45 and x = 0.4 were studied by inelastic neutron scattering. We found the
magnetic excitation spectrum of x = 0.45 from the ordered Ni^2+ S = 1 spins to
match that of checkerboard charge ordered La(1.5)Sr(0.5)NiO(4). The distinctive
asymmetry in the magnetic excitations above 40 meV was observed for both doping
levels, but an additional ferromagnetic mode was observed in x = 0.45 and not
in the x = 0.4. We discuss the origin of crossover in the excitation spectrum
between x = 0.45 and x = 0.4 with respect to discommensurations in the charge
stripe structure.Comment: 4 Figures. To be appear in the J. Kor. Phys. Soc. as a proceedings
paper from the ICM 2012 conferenc
Optimal measurements for quantum spatial superresolution
We construct optimal measurements, achieving the ultimate precision predicted
by quantum theory, for the simultaneous estimation of centroid, separation, and
relative intensities of two incoherent point sources using a linear optical
system. We discuss the physical feasibility of the scheme, which could pave the
way for future practical implementations of quantum inspired imaging.Comment: 7 pages. 3 color figures. Title change
Testing of quantum phase in matter wave optics
Various phase concepts may be treated as special cases of the maximum
likelihood estimation. For example the discrete Fourier estimation that
actually coincides with the operational phase of Noh, Fouge`res and Mandel is
obtained for continuous Gaussian signals with phase modulated mean.Since
signals in quantum theory are discrete, a prediction different from that given
by the Gaussian hypothesis should be obtained as the best fit assuming a
discrete Poissonian statistics of the signal. Although the Gaussian estimation
gives a satisfactory approximation for fitting the phase distribution of almost
any state the optimal phase estimation offers in certain cases a measurable
better performance. This has been demonstrated in neutron--optical experiment.Comment: 8 pages, 4 figure
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