17,311 research outputs found
Quantum phase transitions in the sub-ohmic spin-boson model: Failure of the quantum-classical mapping
The effective theories for many quantum phase transitions can be mapped onto
those of classical transitions. Here we show that such a mapping fails for the
sub-ohmic spin-boson model which describes a two-level system coupled to a
bosonic bath with power-law spectral density, J(omega) ~ omega^s. Using an
epsilon expansion we prove that this model has a quantum transition controlled
by an interacting fixed point at small s, and support this by numerical
calculations. In contrast, the corresponding classical long-range Ising model
is known to have an upper-critical dimension at s = 1/2, with mean-field
transition behavior controlled by a non-interacting fixed point for 0 < s <
1/2. The failure of the quantum-classical mapping is argued to arise from the
long-ranged interaction in imaginary time in the quantum model.Comment: 4 pages, 3 figs; (v2) discussion extended; (v3) marginal changes,
final version as published; (v4) added erratum pointing out that main
conclusions were incorrect due to subtle failures of the NR
Blockspin Cluster Algorithms for Quantum Spin Systems
Cluster algorithms are developed for simulating quantum spin systems like the
one- and two-dimensional Heisenberg ferro- and anti-ferromagnets. The
corresponding two- and three-dimensional classical spin models with four-spin
couplings are maped to blockspin models with two-blockspin interactions.
Clusters of blockspins are updated collectively. The efficiency of the method
is investigated in detail for one-dimensional spin chains. Then in most cases
the new algorithms solve the problems of slowing down from which standard
algorithms are suffering.Comment: 11 page
Laser-induced electron localization in H: Mixed quantum-classical dynamics based on the exact time-dependent potential energy surface
We study the exact nuclear time-dependent potential energy surface (TDPES)
for laser-induced electron localization with a view to eventually developing a
mixed quantum-classical dynamics method for strong-field processes. The TDPES
is defined within the framework of the exact factorization [A. Abedi, N. T.
Maitra, and E. K. U. Gross, Phys. Rev. Lett. 105, 123002 (2010)] and contains
the exact effect of the couplings to the electronic subsystem and to any
external fields within a scalar potential. We compare its features with those
of the quasistatic potential energy surfaces (QSPES) often used to analyse
strong-field processes. We show that the gauge-independent component of the
TDPES has a mean-field-like character very close to the density-weighted
average of the QSPESs. Oscillations in this component are smoothened out by the
gauge-dependent component, and both components are needed to yield the correct
force on the nuclei. Once the localization begins to set in, the gradient of
the exact TDPES tracks one QSPES and then switches to the other, similar to the
description provided by surface-hopping between QSPESs. We show that evolving
an ensemble of classical nuclear trajectories on the exact TDPES accurately
reproduces the exact dynamics. This study suggests that the mixed
quantum-classical dynamics scheme based on evolving multiple classical nuclear
trajectories on the exact TDPES will be a novel and useful method to simulate
strong field processes.Comment: 10 pages, 6 figure
Nonequilibrium many-body dynamics along a dissipative Hubbard chain: Symmetries and Quantum Monte Carlo simulations
The nonequilibrium dynamics of correlated charge transfer along a
one-dimensional chain in presence of a phonon environment is investigated
within a dissipative Hubbard model. For this generalization of the ubiquitous
spin-boson model the crucial role of symmetries is analysed in detail and
corresponding invariant subspaces are identified. It is shown that the time
evolution typically occurs in each of the disjunct subspaces independently
leading e.g. asymptotically to a non-Boltzmann equilibrium state. Based on
these findings explicit results are obtained for two interacting electrons by
means of a substantially improved real-time quantum Monte Carlo approach. In
the incoherent regime an appropriate mapping of the many-body dynamics onto an
isomorphic single particle motion allows for an approximate description of the
numerical data in terms of rate equations. These results may lead to new
control schemes of charge transport in tailored quantum systems as e.g.
molecular chains or quantum dot arrays.Comment: 13 pages, 9 figures submitted to Phys. Rev.
Practical purification scheme for decohered coherent-state superpositions via partial homodyne detection
We present a simple protocol to purify a coherent-state superposition that
has undergone a linear lossy channel. The scheme constitutes only a single beam
splitter and a homodyne detector, and thus is experimentally feasible. In
practice, a superposition of coherent states is transformed into a classical
mixture of coherent states by linear loss, which is usually the dominant
decoherence mechanism in optical systems. We also address the possibility of
producing a larger amplitude superposition state from decohered states, and
show that in most cases the decoherence of the states are amplified along with
the amplitude.Comment: 8 pages, 10 figure
- …