827 research outputs found
Household-Level Livestock Marketing Behavior Among Northern Kenyan and Southern Ethiopian Pastoralists
Pastoralists in East Africa's arid and semi-arid lands (ASAL) regularly confront climatic shocks triggering massive herd die-offs and loss of scarce wealth. On the surface, it appears puzzling that pastoralists do not make extensive use of livestock markets to offload animals when climatic shocks temporarily reduce the carrying capacity of local rangelands, and then use markets to restock their herds when local conditions recover. In recent years, donors and policy makers have begun to hypothesize that investments in livestock marketing systems might quickly pay for themselves through reduced demand for relief aid,by increasing pastoralist marketing responsiveness to temporal variation in range conditions.Marketing,
Excitation and Entanglement Transfer Near Quantum Critical Points
Recently, there has been growing interest in employing condensed matter
systems such as quantum spin or harmonic chains as quantum channels for short
distance communication. Many properties of such chains are determined by the
spectral gap between their ground and excited states. In particular this gap
vanishes at critical points of quantum phase transitions. In this article we
study the relation between the transfer speed and quality of such a system and
the size of its spectral gap. We find that the transfer is almost perfect but
slow for large spectral gaps and fast but rather inefficient for small gaps.Comment: submitted to Optics and Spectroscopy special issue for ICQO'200
Quantum State Transfer in Spin-1 Chains
We study the transfer of quantum information through a Heisenberg spin-1
chain prepared in its ground state. We measure the efficiency of such a quantum
channel {\em via} the fidelity of retrieving an arbitrarily prepared state and
{\em via} the transfer of quantum entanglement. The Heisenberg spin-1 chain has
a very rich quantum phase diagram. We show that the phase boundaries are
reflected in sharp variations of the transfer efficiency. In the vicinity of
the border between the dimer and the ferromagnetic phase (in the conjectured
spin-nematic region), we find strong indications for a qualitative change of
the excitation spectrum. Moreover, we identify two regions of the phase diagram
which give rise to particularly high transfer efficiency; the channel might be
non-classical even for chains of arbitrary length, in contrast to spin-1/2
chains.Comment: 4 pages, 4 figures, published versio
One-Dimensional Impenetrable Anyons in Thermal Equilibrium. II. Determinant Representation for the Dynamic Correlation Functions
We have obtained a determinant representation for the time- and
temperature-dependent field-field correlation function of the impenetrable
Lieb-Liniger gas of anyons through direct summation of the form factors. In the
static case, the obtained results are shown to be equivalent to those that
follow from the anyonic generalization of Lenard's formula.Comment: 16 pages, RevTeX
Bethe Ansatz solution of a new class of Hubbard-type models
We define one-dimensional particles with generalized exchange statistics. The
exact solution of a Hubbard-type Hamiltonian constructed with such particles is
achieved using the Coordinate Bethe Ansatz. The chosen deformation of the
statistics is equivalent to the presence of a magnetic field produced by the
particles themselves, which is present also in a ``free gas'' of these
particles.Comment: 4 pages, revtex. Essentially modified versio
Three-tangle for mixtures of generalized GHZ and generalized W states
We give a complete solution for the three-tangle of mixed three-qubit states
composed of a generalized GHZ state, a|000>+b|111>, and a generalized W state,
c|001>+d|010>+f|100>. Using the methods introduced by Lohmayer et al. we
provide explicit expressions for the mixed-state three-tangle and the
corresponding optimal decompositions for this more general case. Moreover, as a
special case we obtain a general solution for a family of states consisting of
a generalized GHZ state and an orthogonal product state
Robustness of adiabatic passage trough a quantum phase transition
We analyze the crossing of a quantum critical point based on exact results
for the transverse XY model. In dependence of the change rate of the driving
field, the evolution of the ground state is studied while the transverse
magnetic field is tuned through the critical point with a linear ramping. The
excitation probability is obtained exactly and is compared to previous studies
and to the Landau-Zener formula, a long time solution for non-adiabatic
transitions in two-level systems. The exact time dependence of the excitations
density in the system allows to identify the adiabatic and diabatic regions
during the sweep and to study the mesoscopic fluctuations of the excitations.
The effect of white noise is investigated, where the critical point transmutes
into a non-hermitian ``degenerate region''. Besides an overall increase of the
excitations during and at the end of the sweep, the most destructive effect of
the noise is the decay of the state purity that is enhanced by the passage
through the degenerate region.Comment: 16 pages, 15 figure
Pastoralist Livestock Marketing Behavior in Northern Kenya and Southern Ethiopia: An Analysis of Constraints Limiting Off-take Rates
Pastoralists in East Africa\u27s arid and semi-arid lands (ASAL) regularly confront climatic shocks that plunge them into massive herd die-offs and loss of scarce wealth. One of the most puzzling features of pastoralist behavior in times of stress has been their relatively low and non-responsive rate of marketed off-take of animals when faced with likely losses to herd mortality. As Figure 1, from Desta (1999), finds in 17-year herd history data from Borana pastoralists in southern Ethiopia, mortality always exceeds net sales as a share of beginning period herd size, with the latter never exceeding three percent and moving hardly at all in response to shocks to rangeland carrying capacity that cause regular spikes in mortality rates. This case might be more pronounced than others, but the basic pattern is widely believed representative of herd dynamics and marketing patterns among east African ASAL pastoralists
The invariant-comb approach and its relation to the balancedness of multipartite entangled states
The invariant-comb approach is a method to construct entanglement measures
for multipartite systems of qubits. The essential step is the construction of
an antilinear operator that we call {\em comb} in reference to the {\em
hairy-ball theorem}. An appealing feature of this approach is that for qubits
(or spins 1/2) the combs are automatically invariant under SL(2,\CC), which
implies that the obtained invariants are entanglement monotones by
construction. By asking which property of a state determines whether or not it
is detected by a polynomial SL(2,\CC) invariant we find that it is the
presence of a {\em balanced part} that persists under local unitary
transformations. We present a detailed analysis for the maximally entangled
states detected by such polynomial invariants, which leads to the concept of
{\em irreducibly balanced} states. The latter indicates a tight connection with
SLOCC classifications of qubit entanglement. \\ Combs may also help to define
measures for multipartite entanglement of higher-dimensional subsystems.
However, for higher spins there are many independent combs such that it is
non-trivial to find an invariant one. By restricting the allowed local
operations to rotations of the coordinate system (i.e. again to the
SL(2,\CC)) we manage to define a unique extension of the concurrence to
general half-integer spin with an analytic convex-roof expression for mixed
states.Comment: 17 pages, revtex4. Substantially extended manuscript (e.g. proofs
have been added); title and abstract modified
Algebraic invariants of five qubits
The Hilbert series of the algebra of polynomial invariants of pure states of
five qubits is obtained, and the simplest invariants are computed.Comment: 4 pages, revtex. Short discussion of quant-ph/0506073 include
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