596 research outputs found
Global-to-local incompatibility, monogamy of entanglement, and ground-state dimerization: Theory and observability of quantum frustration in systems with competing interactions
Frustration in quantum many body systems is quantified by the degree of
incompatibility between the local and global orders associated, respectively,
to the ground states of the local interaction terms and the global ground state
of the total many-body Hamiltonian. This universal measure is bounded from
below by the ground-state bipartite block entanglement. For many-body
Hamiltonians that are sums of two-body interaction terms, a further inequality
relates quantum frustration to the pairwise entanglement between the
constituents of the local interaction terms. This additional bound is a
consequence of the limits imposed by monogamy on entanglement shareability. We
investigate the behavior of local pair frustration in quantum spin models with
competing interactions on different length scales and show that valence bond
solids associated to exact ground-state dimerization correspond to a transition
from generic frustration, i.e. geometric, common to classical and quantum
systems alike, to genuine quantum frustration, i.e. solely due to the
non-commutativity of the different local interaction terms. We discuss how such
frustration transitions separating genuinely quantum orders from classical-like
ones are detected by observable quantities such as the static structure factor
and the interferometric visibility.Comment: 11 pages, 7 figures. Matches published versio
Dependence of the BEC transition temperature on interaction strength: a perturbative analysis
We compute the critical temperature T_c of a weakly interacting uniform Bose
gas in the canonical ensemble, extending the criterion of condensation provided
by the counting statistics for the uniform ideal gas. Using ordinary
perturbation theory, we find in first order , where T_c^0 is the transition temperature of the corresponding
ideal Bose gas, a is the scattering length, and is the particle number
density.Comment: 14 pages (RevTeX
Energy Spectrum of Anyons in a Magnetic Field
For the many-anyon system in external magnetic field, we derive the energy
spectrum as an exact solution of the quantum eigenvalue problem with particular
topological constraints. Our results agree with the numerical spectra recently
obtained for the 3- and the 4-anyon systems.Comment: 11 pages in Plain LaTeX (plus 4 figures available on request), DFPD
92/TH/4
Decoherence of number states in phase-sensitive reservoirs
The non-unitary evolution of initial number states in general Gaussian
environments is solved analytically. Decoherence in the channels is quantified
by determining explicitly the purity of the state at any time. The influence of
the squeezing of the bath on decoherence is discussed. The behavior of coherent
superpositions of number states is addressed as well.Comment: 5 pages, 2 figures, minor changes, references adde
Exact Multiplicities in the Three-Anyon Spectrum
Using the symmetry properties of the three-anyon spectrum, we obtain exactly
the multiplicities of states with given energy and angular momentum. The
results are shown to be in agreement with the proper quantum mechanical and
semiclassical considerations, and the unexplained points are indicated.Comment: 16 pages plus 3 postscript figures, Kiev Institute for Theoretical
Physics preprint ITP-93-32
Optimal estimation of losses at the ultimate quantum limit with non-Gaussian states
We address the estimation of the loss parameter of a bosonic channel probed
by arbitrary signals. Unlike the optimal Gaussian probes, which can attain the
ultimate bound on precision asymptotically either for very small or very large
losses, we prove that Fock states at any fixed photon number saturate the bound
unconditionally for any value of the loss. In the relevant regime of low-energy
probes, we demonstrate that superpositions of the first low-lying Fock states
yield an absolute improvement over any Gaussian probe. Such few-photon states
can be recast quite generally as truncations of de-Gaussified photon-subtracted
states.Comment: 4 pages, 3 figure
Discriminating quantum gravity models by gravitational decoherence
Several phenomenological approaches to quantum gravity assume the existence
of a minimal measurable length and/or a maximum measurable momentum near the
Planck scale. When embedded into the framework of quantum mechanics, such
constraints induce a modification of the canonical commutation relations and
thus a generalization of the Heisenberg uncertainty relations, commonly
referred to as generalized uncertainty principle (GUP). Different models of
quantum gravity imply different forms of the GUP. For instance, in the
framework of string theory the GUP is quadratic in the momentum operator, while
in the context of doubly special relativity it includes an additional linear
dependence. Among the possible physical consequences, it was recently shown
that the quadratic GUP induces a universal decoherence mechanism, provided one
assumes a foamy structure of quantum spacetime close to the Planck length.
Along this line, in the present work we investigate the gravitational
decoherence associated to the linear-quadratic GUP and we compare it with the
one associated to the quadratic GUP. We find that, despite their similarities,
the two generalizations of the Heisenberg uncertainty principle yield
decoherence times that are completely uncorrelated and significantly distinct.
Motivated by this result, we introduce a theoretical and experimental scheme
based on cavity optomechanics to measure the different time evolution of
nonlocal quantum correlations corresponding to the two aforementioned
decoherence mechanisms. We find that the deviation between the two predictions
occurs on time scales that are macroscopic and thus potentially amenable to
experimental verification. This scenario provides a possible setting to
discriminate between different forms of the GUP and therefore different models
of quantum gravity.Comment: v1: 11 pages, 2 figures; v2: 11 pages, 2 figures, title, abstract and
references slightly edited and updated for better clarity; no change in the
physics; v3: 14 pages, 3 figures, title changed, new section and
clarifications adde
Hierarchies of Geometric Entanglement
We introduce a class of generalized geometric measures of entanglement. For
pure quantum states of elementary subsystems, they are defined as the
distances from the sets of -separable states (). The entire set
of generalized geometric measures provides a quantification and hierarchical
ordering of the different bipartite and multipartite components of the global
geometric entanglement, and allows to discriminate among the different
contributions. The extended measures are applied to the study of entanglement
in different classes of -qubit pure states. These classes include and
states, and their symmetric superpositions; symmetric multi-magnon
states; cluster states; and, finally, asymmetric generalized -like
superposition states. We discuss in detail a general method for the explicit
evaluation of the multipartite components of geometric entanglement, and we
show that the entire set of geometric measures establishes an ordering among
the different types of bipartite and multipartite entanglement. In particular,
it determines a consistent hierarchy between and states, clarifying
the original result of Wei and Goldbart that states possess a larger global
entanglement than states. Furthermore, we show that all multipartite
components of geometric entanglement in symmetric states obey a property of
self-similarity and scale invariance with the total number of qubits and the
number of qubits per party.Comment: 16 pages, 7 figures. Final version, to appear in Phys. Rev.
Broadband detection of squeezed vacuum: A spectrum of quantum states
We demonstrate the simultaneous quantum state reconstruction of the spectral
modes of the light field emitted by a continuous wave degenerate optical
parametric amplifier. The scheme is based on broadband measurement of the
quantum fluctuations of the electric field quadratures and subsequent Fourier
decomposition into spectral intervals. Applying the standard reconstruction
algorithms to each bandwidth-limited quantum trajectory, a "spectrum" of
density matrices and Wigner functions is obtained. The recorded states show a
smooth transition from the squeezed vacuum to a vacuum state. In the time
domain we evaluated the first order correlation function of the squeezed output
field, showing good agreement with the theory.Comment: 11 pages, 5 figure
A multidisciplinary approach to study the reproductive biology of wild prawns
This work aims to provide deeper knowledge on reproductive biology of P. kerathurus in a multidisciplinary way. Upon 789 examined females, 285 were found inseminated. The logistic equation enabled to estimate the size at first maturity at 30.7 mm CL for female. The Gono-Somatic Index (GSI) showed a pronounced seasonality, ranged from 0.80 ± 0.34 to 11.24 ± 5.72. Histological analysis highlighted five stages of ovarian development. Gonadal fatty acids analysis performed with gas chromatograph evidenced a pronounced seasonal variation; total lipids varied from 1.7% dry weight (dw) in Winter, to 7.2% dw in Summer. For the first time, a chemometric approach (Principal Component Analysis) was applied to relate GSI with total lipid content and fatty acid composition of gonads. The first two components (PC1 and PC2) showed that seasonality explained about 84% of the variability of all data set. In particular, in the period February-May, lipids were characterized by high PUFAs content, that were probably utilized during embryogenesis as energy source and as constituent of the cell membranes. During the summer season, gonads accumulated saturated FAs, that will be used during embryogenesis and early larval stages, while in the cold season total lipids decreased drastically and the gonad reached a quiescent state
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