1,625 research outputs found
The quantum Heisenberg antiferromagnet on the square lattice
The pure-quantum self-consistent harmonic approximation, a semiclassical
method based on the path-integral formulation of quantum statistical mechanics,
is applied to the study of the thermodynamic behaviour of the quantum
Heisenberg antiferromagnet on the square lattice (QHAF). Results for various
properties are obtained for different values of the spin and successfully
compared with experimental data.Comment: Proceedings of the Conference "Path Integrals from peV to TeV - 50
Years from Feynman's paper" (Florence, August 1998) -- 2 pages, ReVTeX, 2
figure
Optimal dynamics for quantum-state and entanglement transfer through homogeneous quantum wires
It is shown that effective quantum-state and entanglement transfer can be
obtained by inducing a coherent dynamics in quantum wires with homogeneous
intrawire interactions. This goal is accomplished by tuning the coupling
between the wire endpoints and the two qubits there attached, to an optimal
value. A general procedure to determine such value is devised, and scaling laws
between the optimal coupling and the length of the wire are found. The
procedure is implemented in the case of a wire consisting of a spin-1/2 XY
chain: results for the time dependence of the quantities which characterize
quantum-state and entanglement transfer are found of extremely good quality and
almost independent of the wire length. The present approach does not require
`ad hoc' engineering of the intrawire interactions nor a specific initial pulse
shaping, and can be applied to a vast class of quantum channels.Comment: 5 pages, 5 figure
Spectral shapes of solid neon
We present a Path Integral Monte Carlo calculation of the first three moments
of the displacement-displacement correlation functions of solid neon at
different temperatures for longitudinal and transverse phonon modes. The
Lennard-Jones potential is considered. The relevance of the quantum effects on
the frequency position of the peak and principally on the line-width of the
spectral shape is clearly pointed out. The spectrum is reconstructed via a
continued fraction expansion; the approximations introduced using the effective
potential quantum molecular dynamics are discussed.Comment: 3 pages, 2 figures, 3 table
Using the J1-J2 Quantum Spin Chain as an Adiabatic Quantum Data Bus
This paper investigates numerically a phenomenon which can be used to
transport a single q-bit down a J1-J2 Heisenberg spin chain using a quantum
adiabatic process. The motivation for investigating such processes comes from
the idea that this method of transport could potentially be used as a means of
sending data to various parts of a quantum computer made of artificial spins,
and that this method could take advantage of the easily prepared ground state
at the so called Majumdar-Ghosh point. We examine several annealing protocols
for this process and find similar result for all of them. The annealing process
works well up to a critical frustration threshold.Comment: 14 pages, 13 figures (2 added), revisions made to add citations and
additional discussion at request of referee
Long quantum channels for high-quality entanglement transfer
High-quality quantum-state and entanglement transfer can be achieved in an
unmodulated spin bus operating in the ballistic regime, which occurs when the
endpoint qubits A and B are coupled to the chain by an exchange interaction
comparable with the intrachain exchange. Indeed, the transition amplitude
characterizing the transfer quality exhibits a maximum for a finite optimal
value , where is the channel length. We show that
scales as for large and that it ensures a
high-quality entanglement transfer even in the limit of arbitrarily long
channels, almost independently of the channel initialization. For instance, the
average quantum-state transmission fidelity exceeds 90% for any chain length.
We emphasize that, taking the reverse point of view, should be
experimentally constrained, high-quality transfer can still be obtained by
adjusting the channel length to its optimal value.Comment: 12 pages, 9 figure
Quantum fluctuations in one-dimensional arrays of condensates
The effects of quantum and thermal fluctuations upon the fringe structure
predicted to be observable in the momentum distribution of coupled
Bose-Einstein condensates are studied by the effective-potential method. For a
double-well trap, the coherence factor recently introduced by Pitaevskii and
Stringari [Phys. Rev. Lett. 87, 180402 (2001)] is calculated using the
effective potential approach and is found in good agreement with their result.
The calculations are extended to the case of a one-dimensional array of
condensates, showing that quantum effects are essentially described through a
simple renormalization of the energy scale in the classical analytical
expression for the fringe structure. The consequences for the experimental
observability are discussed.Comment: RevTeX, 4 pages, 5 eps figures (published version with updated
references
Quantum thermodynamics of systems with anomalous dissipative coupling
The standard {\em system-plus-reservoir} approach used in the study of
dissipative systems can be meaningfully generalized to a dissipative coupling
involving the momentum, instead of the coordinate: the corresponding equation
of motion differs from the Langevin equation, so this is called {\em anomalous}
dissipation. It occurs for systems where such coupling can indeed be derived
from the physical analysis of the degrees of freedom which can be treated as a
dissipation bath. Starting from the influence functional corresponding to
anomalous dissipation, it is shown how to derive the effective classical
potential that gives the quantum thermal averages for the dissipative system in
terms of classical-like calculations; the generalization to many degrees of
freedom is given. The formalism is applied to a single particle in a
double-well and to the discrete model. At variance with the standard
case, the fluctuations of the coordinate are enhanced by anomalous dissipative
coupling.Comment: 12 pages, 5 figures, to be published in Phys. Rev.
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