1,405 research outputs found
Bound entanglement in the XY model
We study the multi-spin entanglement for the 1D anisotropic XY model
concentrating on the simplest case of three-spin entanglement. As compared to
the pairwise entanglement, three-party quantum correlations have a longer range
and they are more robust on increasing the temperature.
We find regions of the phase diagram of the system where bound entanglement
occurs, both at zero and finite temperature. Bound entanglement in the ground
state can be obtained by tuning the magnetic field. Thermal bound entanglement
emerges naturally due to the effect of temperature on the free ground state
entanglement.Comment: 7 pages, 3 figures; some typos corrected, references adde
Electrostatic analogy for integrable pairing force Hamiltonians
For the exactly solved reduced BCS model an electrostatic analogy exists; in
particular it served to obtain the exact thermodynamic limit of the model from
the Richardson Bethe ansatz equations. We present an electrostatic analogy for
a wider class of integrable Hamiltonians with pairing force interactions. We
apply it to obtain the exact thermodynamic limit of this class of models. To
verify the analytical results, we compare them with numerical solutions of the
Bethe ansatz equations for finite systems at half-filling for the ground state.Comment: 14 pages, 6 figures, revtex4. Minor change
Algebraic Bethe Ansatz for a discrete-state BCS pairing model
We show in detail how Richardson's exact solution of a discrete-state BCS
(DBCS) model can be recovered as a special case of an algebraic Bethe Ansatz
solution of the inhomogeneous XXX vertex model with twisted boundary
conditions: by implementing the twist using Sklyanin's K-matrix construction
and taking the quasiclassical limit, one obtains a complete set of conserved
quantities, H_i, from which the DBCS Hamiltonian can be constructed as a second
order polynomial. The eigenvalues and eigenstates of the H_i (which reduce to
the Gaudin Hamiltonians in the limit of infinitely strong coupling) are exactly
known in terms of a set of parameters determined by a set of on-shell Bethe
Ansatz equations, which reproduce Richardson's equations for these parameters.
We thus clarify that the integrability of the DBCS model is a special case of
the integrability of the twisted inhomogeneous XXX vertex model. Furthermore,
by considering the twisted inhomogeneous XXZ model and/or choosing a generic
polynomial of the H_i as Hamiltonian, more general exactly solvable models can
be constructed. -- To make the paper accessible to readers that are not Bethe
Ansatz experts, the introductory sections include a self-contained review of
those of its feature which are needed here.Comment: 17 pages, 5 figures, submitted to Phys. Rev.
Superfluid qubit systems with ring shaped optical lattices
We study an experimentally feasible qubit system employing neutral atomic
currents. Our system is based on bosonic cold atoms trapped in ring-shaped
optical lattice potentials. The lattice makes the system strictly one
dimensional and it provides the infrastructure to realize a tunable ring-ring
interaction. Our implementation combines the low decoherence rates of of
neutral cold atoms systems, overcoming single site addressing, with the
robustness of topologically protected solid state Josephson flux qubits.
Characteristic fluctuations in the magnetic fields affecting Josephson junction
based flux qubits are expected to be minimized employing neutral atoms as flux
carriers. By breaking the Galilean invariance we demonstrate how atomic
currents through the lattice provide a implementation of a qubit. This is
realized either by artificially creating a phase slip in a single ring, or by
tunnel coupling of two homogeneous ring lattices. The single qubit
infrastructure is experimentally investigated with tailored optical potentials.
Indeed, we have experimentally realized scaled ring-lattice potentials that
could host, in principle, of such ring-qubits, arranged in a stack
configuration, along the laser beam propagation axis.
An experimentally viable scheme of the two-ring-qubit is discussed, as well.
Based on our analysis, we provide protocols to initialize, address, and
read-out the qubit.Comment: 14 revtex4-1 pages, 7 figs; to be published in Scientific Report
Out of equilibrium correlation functions of quantum anisotropic XY models: one-particle excitations
We calculate exactly matrix elements between states that are not eigenstates
of the quantum XY model for general anisotropy. Such quantities therefore
describe non equilibrium properties of the system; the Hamiltonian does not
contain any time dependence. These matrix elements are expressed as a sum of
Pfaffians. For single particle excitations on the ground state the Pfaffians in
the sum simplify to determinants.Comment: 11 pages, no figures; revtex. Minor changes in the text; list of
refs. modifie
Exact relationship between the entanglement entropies of XY and quantum Ising chains
We consider two prototypical quantum models, the spin-1/2 XY chain and the
quantum Ising chain and study their entanglement entropy, S(l,L), of blocks of
l spins in homogeneous or inhomogeneous systems of length L. By using two
different approaches, free-fermion techniques and perturbational expansion, an
exact relationship between the entropies is revealed. Using this relation we
translate known results between the two models and obtain, among others, the
additive constant of the entropy of the critical homogeneous quantum Ising
chain and the effective central charge of the random XY chain.Comment: 6 page
Self-trapping mechanisms in the dynamics of three coupled Bose-Einstein condensates
We formulate the dynamics of three coupled Bose-Einstein condensates within a
semiclassical scenario based on the standard boson coherent states. We compare
such a picture with that of Ref. 1 and show how our approach entails a simple
formulation of the dimeric regime therein studied. This allows to recognize the
parameters that govern the bifurcation mechanism causing self-trapping, and
paves the way to the construction of analytic solutions. We present the results
of a numerical simulation showing how the three-well dynamics has, in general,
a cahotic behavior.Comment: 4 pages, 5 figure
Exactly-Solvable Models Derived from a Generalized Gaudin Algebra
We introduce a generalized Gaudin Lie algebra and a complete set of mutually
commuting quantum invariants allowing the derivation of several families of
exactly solvable Hamiltonians. Different Hamiltonians correspond to different
representations of the generators of the algebra. The derived exactly-solvable
generalized Gaudin models include the Bardeen-Cooper-Schrieffer,
Suhl-Matthias-Walker, the Lipkin-Meshkov-Glick, generalized Dicke, the Nuclear
Interacting Boson Model, a new exactly-solvable Kondo-like impurity model, and
many more that have not been exploited in the physics literature yet
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