6,031 research outputs found
Dipolar Bose gases: Many-body versus mean-field description
We characterize zero-temperature dipolar Bose gases under external spherical
confinement as a function of the dipole strength using the essentially exact
many-body diffusion Monte Carlo (DMC) technique. We show that the DMC energies
are reproduced accurately within a mean-field framework if the variation of the
s-wave scattering length with the dipole strength is accounted for properly.
Our calculations suggest stability diagrams and collapse mechanisms of dipolar
Bose gases that differ significantly from those previously proposed in the
literature
Quasi-one-dimensional Bose gases with large scattering length
Bose gases confined in highly-elongated harmonic traps are investigated over
a wide range of interaction strengths using quantum Monte Carlo techniques. We
find that the properties of a Bose gas under tight transverse confinement are
well reproduced by a 1d model Hamiltonian with contact interactions. We point
out the existence of a unitary regime, where the properties of the quasi-1d
Bose gas become independent of the actual value of the 3d scattering length. In
this unitary regime, the energy of the system is well described by a hard rod
equation of state. We investigate the stability of quasi-1d Bose gases with
positive and negative 3d scattering length.Comment: 5 pages, 3 figure
System Size Dependence of Particle Production at the SPS
Recent results on the system size dependence of net-baryon and hyperon
production as measured at the CERN SPS are discussed. The observed Npart
dependences of yields, but also of dynamical properties, such as average
transverse momenta, can be described in the context of the core corona
approach. Other observables, such as antiproton yields and net-protons at
forward rapidities, do not follow the predictions of this model. Possible
implications for a search for a critical point in the QCD phase diagram are
discussed. Event-by-event fluctuations of the relative core to corona source
contributions might influence fluctuation observables (e.g. multiplicity
fluctuations). The magnitude of this effect is investigated.Comment: 10 pages, 4 figurs. Proceedings of the 6th International Workshop on
Critical Point and Onset of Deconfinement in Dubna, Aug. 201
Disentangling multipole resonances through a full x-ray polarization analysis
Complete polarization analysis applied to resonant x-ray scattering at the Cr
K-edge in K2CrO4 shows that incident linearly polarized x-rays can be converted
into circularly polarized x-rays by diffraction at the Cr pre-edge (E = 5994
eV). The physical mechanism behind this phenomenon is a subtle interference
effect between purely dipole (E1-E1) and purely quadrupole (E2-E2) transitions,
leading to a phase shift between the respective scattering amplitudes. This
effect may be exploited to disentangle two close-lying resonances that appear
as a single peak in a conventional energy scan, in this way allowing to single
out and identify the different multipole order parameters involved.Comment: 6 pages, 6 figure
Optimal, reliable estimation of quantum states
Accurately inferring the state of a quantum device from the results of
measurements is a crucial task in building quantum information processing
hardware. The predominant state estimation procedure, maximum likelihood
estimation (MLE), generally reports an estimate with zero eigenvalues. These
cannot be justified. Furthermore, the MLE estimate is incompatible with error
bars, so conclusions drawn from it are suspect. I propose an alternative
procedure, Bayesian mean estimation (BME). BME never yields zero eigenvalues,
its eigenvalues provide a bound on their own uncertainties, and it is the most
accurate procedure possible. I show how to implement BME numerically, and how
to obtain natural error bars that are compatible with the estimate. Finally, I
briefly discuss the differences between Bayesian and frequentist estimation
techniques.Comment: RevTeX; 14 pages, 2 embedded figures. Comments enthusiastically
welcomed
Quantum Monte Carlo study of quasi-one-dimensional Bose gases
We study the behavior of quasi-one-dimensional (quasi-1d) Bose gases by Monte
Carlo techniques, i.e., by the variational Monte Carlo, the diffusion Monte
Carlo, and the fixed-node diffusion Monte Carlo technique. Our calculations
confirm and extend our results of an earlier study [Astrakharchik et al.,
cond-mat/0308585]. We find that a quasi-1d Bose gas i) is well described by a
1d model Hamiltonian with contact interactions and renormalized coupling
constant; ii) reaches the Tonks-Girardeau regime for a critical value of the 3d
scattering length a_3d; iii) enters a unitary regime for |a_3d| -> infinity,
where the properties of the gas are independent of a_3d and are similar to
those of a 1d gas of hard-rods; and iv) becomes unstable against cluster
formation for a critical value of the 1d gas parameter. The accuracy and
implications of our results are discussed in detail.Comment: 15 pages, 9 figure
Valence-Bond Crystal, and Lattice Distortions in a Pyrochlore Antiferromagnet with Orbital Degeneracy
We discuss the ground state properties of a spin 1/2 magnetic ion with
threefold orbital degeneracy on a highly frustrated pyrochlore
lattice, like Ti ion in B-spinel MgTiO. We formulate an
effective spin-orbital Hamiltonian and study its low energy sector by
constructing several exact-eigenstates in the limit of vanishing Hund's
coupling. We find that orbital degrees of freedom modulate the spin-exchange
energies, release the infinite spin-degeneracy of pyrochlore structure, and
drive the system to a non-magnetic spin-singlet manifold. The latter is a
collection of spin-singlet dimers and is, however, highly degenerate with
respect of dimer orientations. This ``orientational'' degeneracy is then lifted
by a magneto-elastic interaction that optimizes the previous energy gain by
distorting the bonds in suitable directions and leading to a tetragonal phase.
In this way a valence bond crystal state is formed, through the condensation of
dimers along helical chains running around the tetragonal c-axis, as actually
observed in MgTiO. The orbitally ordered pattern in the dimerized phase
is predicted to be of ferro-type along the helices and of antiferro-type
between them. Finally, through analytical considerations as well as numerical
ab-initio simulations, we predict a possible experimental tool for the
observation of such an orbital ordering, through resonant x-ray scattering.Comment: 15 pages, 8 figure
Tricritical behaviour of Ising spin glasses with charge fluctuations
We show that tricritical points displaying unusal behaviour exist in phase
diagrams of fermionic Ising spin glasses as the chemical potential or the
filling assumes characteristic values. Exact results for infinite range
interaction and a one loop renormalization group analysis of thermal
tricritical fluctuations for finite range models are presented. Surprising
similarities with zero temperature transitions and a new tricritical
point of metallic quantum spin glasses are derived.Comment: 4 pages, 1 Postscript figure, minor change
Verifying multi-partite mode entanglement of W states
We construct a method for verifying mode entanglement of N-mode W states. The
ideal W state contains exactly one excitation symmetrically shared between N
modes, but our method takes the existence of higher numbers of excitations into
account, as well as the vacuum state and other deviations from the ideal state.
Moreover, our method distinguishes between full N-party entanglement and states
with M-party entanglement with M<N, including mixtures of the latter. We
specialize to the case N=4 for illustrative purposes. In the optical case,
where excitations are photons, our method can be implemented using linear
optics.Comment: 11 pages, 12 figure
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