73 research outputs found
Parametrization of projector-based witnesses for bipartite systems
Entanglement witnesses are nonpositive Hermitian operators which can detect
the presence of entanglement. In this paper, we provide a general
parametrization for orthonormal basis of and use it to
construct projector-based witness operators for entanglement detection in the
vicinity of pure bipartite states. Our method to parameterize entanglement
witnesses is operationally simple and could be used for doing symbolic and
numerical calculations. As an example we use the method for detecting
entanglement between an atom and the single mode of quantized field, described
by the Jaynes-Cummings model. We also compare the detection of witnesses with
the negativity of the state, and show that in the vicinity of pure stats such
constructed witnesses able to detect entanglement of the state.Comment: 12 pages, four figure
Phonon Dispersion Relations in PrBa2Cu3O6+x (x ~ 0.2)
We report measurements of the phonon dispersion relations in
non-superconducting, oxygen-deficient PrBa2Cu3O6+x (x ~ 0.2) by inelastic
neutron scattering. The data are compared with a model of the lattice dynamics
based on a common interaction potential. Good agreement is achieved for all but
two phonon branches, which are significantly softer than predicted. These modes
are found to arise predominantly from motion of the oxygen ions in the CuO2
planes. Analogous modes in YBa2Cu3O6 are well described by the common
interaction potential model.Comment: 4 pages, 3 figures. Minor changes following referees' comment
Hamiltonicity of 3-arc graphs
An arc of a graph is an oriented edge and a 3-arc is a 4-tuple of
vertices such that both and are paths of length two. The
3-arc graph of a graph is defined to have vertices the arcs of such
that two arcs are adjacent if and only if is a 3-arc of
. In this paper we prove that any connected 3-arc graph is Hamiltonian, and
all iterative 3-arc graphs of any connected graph of minimum degree at least
three are Hamiltonian. As a consequence we obtain that if a vertex-transitive
graph is isomorphic to the 3-arc graph of a connected arc-transitive graph of
degree at least three, then it is Hamiltonian. This confirms the well known
conjecture, that all vertex-transitive graphs with finitely many exceptions are
Hamiltonian, for a large family of vertex-transitive graphs. We also prove that
if a graph with at least four vertices is Hamilton-connected, then so are its
iterative 3-arc graphs.Comment: in press Graphs and Combinatorics, 201
Exact analytical solutions to the master equation of quantum Brownian motion for a general environment
We revisit the model of a quantum Brownian oscillator linearly coupled to an
environment of quantum oscillators at finite temperature. By introducing a
compact and particularly well-suited formulation, we give a rather quick and
direct derivation of the master equation and its solutions for general spectral
functions and arbitrary temperatures. The flexibility of our approach allows
for an immediate generalization to cases with an external force and with an
arbitrary number of Brownian oscillators. More importantly, we point out an
important mathematical subtlety concerning boundary-value problems for
integro-differential equations which led to incorrect master equation
coefficients and impacts on the description of nonlocal dissipation effects in
all earlier derivations. Furthermore, we provide explicit, exact analytical
results for the master equation coefficients and its solutions in a wide
variety of cases, including ohmic, sub-ohmic and supra-ohmic environments with
a finite cut-off.Comment: 37 pages (26 + appendices), 14 figures; this paper is an evolution of
arXiv:0705.2766v1, but contains far more general and significant results; v2
minor changes, double column, improved Appendix
Entanglement transfer from dissociated molecules to photons
We introduce and study the concept of a reversible transfer of the quantum
state of two internally-translationally entangled fragments, formed by
molecular dissociation, to a photon pair. The transfer is based on intracavity
stimulated Raman adiabatic passage and it requires a combination of processes
whose principles are well established.Comment: 5 pages, 3 figure
Ponderomotive entangling of atomic motions
We propose the use of ponderomotive forces to entangle the motions of
different atoms. Two situations are analyzed: one where the atoms belong to the
same optical cavity and interact with the same radiation field mode; the other
where each atom is placed in own optical cavity and the output field of one
cavity enters the other.Comment: Revtex file, five pages, two eps figure
Information and entropy in quantum Brownian motion: Thermodynamic entropy versus von Neumann entropy
We compare the thermodynamic entropy of a quantum Brownian oscillator derived
from the partition function of the subsystem with the von Neumann entropy of
its reduced density matrix. At low temperatures we find deviations between
these two entropies which are due to the fact that the Brownian particle and
its environment are entangled. We give an explanation for these findings and
point out that these deviations become important in cases where statements
about the information capacity of the subsystem are associated with
thermodynamic properties, as it is the case for the Landauer principle.Comment: 8 pages, 7 figure
Number--conserving model for boson pairing
An independent pair ansatz is developed for the many body wavefunction of
dilute Bose systems. The pair correlation is optimized by minimizing the
expectation value of the full hamiltonian (rather than the truncated Bogoliubov
one) providing a rigorous energy upper bound. In contrast with the Jastrow
model, hypernetted chain theory provides closed-form exactly solvable equations
for the optimized pair correlation. The model involves both condensate and
coherent pairing with number conservation and kinetic energy sum rules
satisfied exactly and the compressibility sum rule obeyed at low density. We
compute, for bulk boson matter at a given density and zero temperature, (i) the
two--body distribution function, (ii) the energy per particle, (iii) the sound
velocity, (iv) the chemical potential, (v) the momentum distribution and its
condensate fraction and (vi) the pairing function, which quantifies the ODLRO
resulting from the structural properties of the two--particle density matrix.
The connections with the low--density expansion and Bogoliubov theory are
analyzed at different density values, including the density and scattering
length regime of interest of trapped-atoms Bose--Einstein condensates.
Comparison with the available Diffusion Monte Carlo results is also made.Comment: 21 pages, 12 figure
Chaos in a double driven dissipative nonlinear oscillator
We propose an anharmonic oscillator driven by two periodic forces of
different frequencies as a new time-dependent model for investigating quantum
dissipative chaos. Our analysis is done in the frame of statistical ensemble of
quantum trajectories in quantum state diffusion approach. Quantum dynamical
manifestation of chaotic behavior, including the emergence of chaos, properties
of strange attractors, and quantum entanglement are studied by numerical
simulation of ensemble averaged Wigner function and von Neumann entropy.Comment: 9 pages, 18 figure
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