419 research outputs found
Long-distance entanglement and quantum teleportation in XX spin chains
Isotropic XX models of one-dimensional spin-1/2 chains are investigated with
the aim to elucidate the formal structure and the physical properties that
allow these systems to act as channels for long-distance, high-fidelity quantum
teleportation. We introduce two types of models: I) open, dimerized XX chains,
and II) open XX chains with small end bonds. For both models we obtain the
exact expressions for the end-to-end correlations and the scaling of the energy
gap with the length of the chain. We determine the end-to-end concurrence and
show that model I) supports true long-distance entanglement at zero
temperature, while model II) supports {\it ``quasi long-distance''}
entanglement that slowly falls off with the size of the chain. Due to the
different scalings of the gaps, respectively exponential for model I) and
algebraic in model II), we demonstrate that the latter allows for efficient
qubit teleportation with high fidelity in sufficiently long chains even at
moderately low temperatures.Comment: 9 pages, 6 figure
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
Quantifying nonclassicality: global impact of local unitary evolutions
We show that only those composite quantum systems possessing nonvanishing
quantum correlations have the property that any nontrivial local unitary
evolution changes their global state. We derive the exact relation between the
global state change induced by local unitary evolutions and the amount of
quantum correlations. We prove that the minimal change coincides with the
geometric measure of discord (defined via the Hilbert- Schmidt norm), thus
providing the latter with an operational interpretation in terms of the
capability of a local unitary dynamics to modify a global state. We establish
that two-qubit Werner states are maximally quantum correlated, and are thus the
ones that maximize this type of global quantum effect. Finally, we show that
similar results hold when replacing the Hilbert-Schmidt norm with the trace
norm.Comment: 5 pages, 1 figure. To appear in Physical Review
Continuous variable tangle, monogamy inequality, and entanglement sharing in Gaussian states of continuous variable systems
For continuous-variable systems, we introduce a measure of entanglement, the
continuous variable tangle ({\em contangle}), with the purpose of quantifying
the distributed (shared) entanglement in multimode, multipartite Gaussian
states. This is achieved by a proper convex roof extension of the squared
logarithmic negativity. We prove that the contangle satisfies the
Coffman-Kundu-Wootters monogamy inequality in all three--mode Gaussian states,
and in all fully symmetric --mode Gaussian states, for arbitrary . For
three--mode pure states we prove that the residual entanglement is a genuine
tripartite entanglement monotone under Gaussian local operations and classical
communication. We show that pure, symmetric three--mode Gaussian states allow a
promiscuous entanglement sharing, having both maximum tripartite residual
entanglement and maximum couplewise entanglement between any pair of modes.
These states are thus simultaneous continuous-variable analogs of both the GHZ
and the states of three qubits: in continuous-variable systems monogamy
does not prevent promiscuity, and the inequivalence between different classes
of maximally entangled states, holding for systems of three or more qubits, is
removed.Comment: 13 pages, 1 figure. Replaced with published versio
On reduced density matrices for disjoint subsystems
We show that spin and fermion representations for solvable quantum chains
lead in general to different reduced density matrices if the subsystem is not
singly connected. We study the effect for two sites in XX and XY chains as well
as for sublattices in XX and transverse Ising chains.Comment: 10 pages, 4 figure
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.
Multipartite entanglement in three-mode Gaussian states of continuous variable systems: Quantification, sharing structure and decoherence
We present a complete analysis of multipartite entanglement of three-mode
Gaussian states of continuous variable systems. We derive standard forms which
characterize the covariance matrix of pure and mixed three-mode Gaussian states
up to local unitary operations, showing that the local entropies of pure
Gaussian states are bound to fulfill a relationship which is stricter than the
general Araki-Lieb inequality. Quantum correlations will be quantified by a
proper convex roof extension of the squared logarithmic negativity (the
contangle), satisfying a monogamy relation for multimode Gaussian states, whose
proof will be reviewed and elucidated. The residual contangle, emerging from
the monogamy inequality, is an entanglement monotone under Gaussian local
operations and classical communication and defines a measure of genuine
tripartite entanglement. We analytically determine the residual contangle for
arbitrary pure three-mode Gaussian states and study the distribution of quantum
correlations for such states. This will lead us to show that pure, symmetric
states allow for a promiscuous entanglement sharing, having both maximum
tripartite residual entanglement and maximum couplewise entanglement between
any pair of modes. We thus name these states GHZ/ states of continuous
variable systems because they are simultaneous continuous-variable counterparts
of both the GHZ and the states of three qubits. We finally consider the
action of decoherence on tripartite entangled Gaussian states, studying the
decay of the residual contangle. The GHZ/ states are shown to be maximally
robust under both losses and thermal noise.Comment: 20 pages, 5 figures. (v2) References updated, published versio
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
Characterization of separability and entanglement in - and -dimensional systems by single-qubit and single-qutrit unitary transformations
We investigate the geometric characterization of pure state bipartite
entanglement of - and -dimensional composite
quantum systems. To this aim, we analyze the relationship between states and
their images under the action of particular classes of local unitary
operations. We find that invariance of states under the action of single-qubit
and single-qutrit transformations is a necessary and sufficient condition for
separability. We demonstrate that in the -dimensional case the
von Neumann entropy of entanglement is a monotonic function of the minimum
squared Euclidean distance between states and their images over the set of
single qubit unitary transformations. Moreover, both in the - and
in the -dimensional cases the minimum squared Euclidean distance
exactly coincides with the linear entropy (and thus as well with the tangle
measure of entanglement in the -dimensional case). These results
provide a geometric characterization of entanglement measures originally
established in informational frameworks. Consequences and applications of the
formalism to quantum critical phenomena in spin systems are discussed.Comment: 8 pages, 1 figur
Multipartite Entanglement and Frustration
Some features of the global entanglement of a composed quantum system can be
quantified in terms of the purity of a balanced bipartition, made up of half of
its subsystems. For the given bipartition, purity can always be minimized by
taking a suitable (pure) state. When many bipartitions are considered, the
requirement that purity be minimal for all bipartitions can engender conflicts
and frustration arises. This unearths an interesting link between frustration
and multipartite entanglement, defined as the average purity over all
(balanced) bipartitions.Comment: 15 pages, 7 figure
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