3,242 research outputs found
Non-Markovian dissipative dynamics of two coupled qubits in independent reservoirs: a comparison between exact solutions and master equation approaches
The reduced dynamics of two interacting qubits coupled to two independent
bosonic baths is investigated. The one-excitation dynamics is derived and
compared with that based on the resolution of appropriate non-Markovian master
equations. The Nakajima-Zwanzig and the time-convolutionless projection
operator techniques are exploited to provide a description of the non-Markovian
features of the dynamics of the two-qubits system. The validity of such
approximate methods and their range of validity in correspondence to different
choices of the parameters describing the system are brought to light.Comment: 6 pages, 3 figures. Submitted to PR
Transition from diffusive to ballistic dynamics for a class of finite quantum models
The transport of excitation probabilities amongst weakly coupled subunits is
investigated for a class of finite quantum systems. It is demonstrated that the
dynamical behavior of the transported quantity depends on the considered length
scale, e. g., the introduced distinction between diffusive and ballistic
transport appears to be a scale-dependent concept, especially since a
transition from diffusive to ballistic behavior is found in the limit of small
as well as in the limit of large length scales. All these results are derived
by an application of the time-convolutionless projection operator technique and
are verified by the numerical solution of the full time-dependent Schroedinger
equation which is obtained by exact diagonalization for a range of model
parameters.Comment: 4 pages, 5 figures, approved for publication in Physical Review
Letter
Reduced density matrix hybrid approach: Application to electronic energy transfer
Electronic energy transfer in the condensed phase, such as that occurring in
photosynthetic complexes, frequently occurs in regimes where the energy scales
of the system and environment are similar. This situation provides a challenge
to theoretical investigation since most approaches are accurate only when a
certain energetic parameter is small compared to others in the problem. Here we
show that in these difficult regimes, the Ehrenfest approach provides a good
starting point for a dynamical description of the energy transfer process due
to its ability to accurately treat coupling to slow environmental modes. To
further improve on the accuracy of the Ehrenfest approach, we use our reduced
density matrix hybrid framework to treat the faster environmental modes quantum
mechanically, at the level of a perturbative master equation. This combined
approach is shown to provide an efficient and quantitative description of
electronic energy transfer in a model dimer and the Fenna-Matthews-Olson
complex and is used to investigate the effect of environmental preparation on
the resulting dynamics.Comment: 11 pages, 8 figure
Intersexual conflict influences female reproductive success in a female-dispersing primate
In group-living mammals, individual efforts to maximize reproductive success result in conflicts and compromises between the sexes. Females utilize counterstrategies to minimize the costs of sexual coercion by males, but few studies have examined the effect of such behaviors on female reproductive success. Secondary dispersal by females is rare among group-living mammals, but in western gorillas, it is believed to be a mate choice strategy to minimize infanticide risk and infant mortality. Previous research suggested that females choose males that are good protectors. However, how much female reproductive success varies depending on male competitive ability and whether female secondary dispersal leads to reproductive costs or benefits has not been examined. We used data on 100 females and 229 infants in 36 breeding groups from a 20-year long-term study of wild western lowland gorillas to investigate whether male tenure duration and female transfer rate had an effect on interbirth interval, female birth rates, and offspring mortality. We found that offspring mortality was higher near the end of males’ tenures, even after excluding potential infanticide when those males died, suggesting that females suffer a reproductive cost by being with males nearing the end of their tenures. Females experience a delay in breeding when they dispersed, having a notable effect on birth rates of surviving offspring per female if females transfer multiple times in their lives. This study exemplifies that female counterstrategies to mitigate the effects of male-male competition and sexual coercion may not be sufficient to overcome the negative consequences of male behavior
Long range order in non-equilibrium interacting quantum spin chains
We conjecture that non-equilibrium boundary conditions generically trigger
long range order in non-equilibrium steady states of locally interacting
quantum chains. Our result is based on large scale density matrix
renormalization group simulations of several models of quantum spin 1/2 chains
which are driven far from equilibrium by coupling to a pair of unequal Lindblad
reservoirs attached locally to the ends of the chain. In particular, we find a
phase transition from exponentially decaying to long range spin-spin
correlations in integrable Heisenberg XXZ chain by changing the anisotropy
parameter. Long range order also typically emerges after breaking the
integrability of the model.Comment: 4 pages, 4 figure
The equilibrium states of open quantum systems in the strong coupling regime
In this work we investigate the late-time stationary states of open quantum
systems coupled to a thermal reservoir in the strong coupling regime. In
general such systems do not necessarily relax to a Boltzmann distribution if
the coupling to the thermal reservoir is non-vanishing or equivalently if the
relaxation timescales are finite. Using a variety of non-equilibrium formalisms
valid for non-Markovian processes, we show that starting from a product state
of the closed system = system + environment, with the environment in its
thermal state, the open system which results from coarse graining the
environment will evolve towards an equilibrium state at late-times. This state
can be expressed as the reduced state of the closed system thermal state at the
temperature of the environment. For a linear (harmonic) system and environment,
which is exactly solvable, we are able to show in a rigorous way that all
multi-time correlations of the open system evolve towards those of the closed
system thermal state. Multi-time correlations are especially relevant in the
non-Markovian regime, since they cannot be generated by the dynamics of the
single-time correlations. For more general systems, which cannot be exactly
solved, we are able to provide a general proof that all single-time
correlations of the open system evolve to those of the closed system thermal
state, to first order in the relaxation rates. For the special case of a
zero-temperature reservoir, we are able to explicitly construct the reduced
closed system thermal state in terms of the environmental correlations.Comment: 20 pages, 2 figure
New method to simulate quantum interference using deterministic processes and application to event-based simulation of quantum computation
We demonstrate that networks of locally connected processing units with a
primitive learning capability exhibit behavior that is usually only attributed
to quantum systems. We describe networks that simulate single-photon
beam-splitter and Mach-Zehnder interferometer experiments on a causal,
event-by-event basis and demonstrate that the simulation results are in
excellent agreement with quantum theory. We also show that this approach can be
generalized to simulate universal quantum computers.Comment: J. Phys. Soc. Jpn. (in press) http://www.compphys.net/dl
Collective multipole-like signatures of entanglement in symmetric N-qubit systems
A cogent theory of collective multipole-like quantum correlations in
symmetric multiqubit states is presented by employing SO(3) irreducible
spherical tensor representation. An arbitrary bipartite division of this system
leads to a family of inequalities to detect entanglement involving averages of
these tensors expressed in terms of the total system angular momentum operator.
Implications of this theory to the quantum nature of multipole-like
correlations of all orders in the Dicke states are deduced. A selected set of
examples illustrate these collective tests. Such tests detect entanglement in
macroscopic atomic ensembles, where individual atoms are not accessible.Comment: REVTEX, 4 pages with 1 figure; To appear in Phys. Rev.
Entanglement in SO(3)-invariant bipartite quantum systems
The structure of the state spaces of bipartite (N tensor N) quantum systems
which are invariant under product representations of the group SO(3) of
three-dimensional proper rotations is analyzed. The subsystems represent
particles of arbitrary spin j which transform according to an irreducible
representation of the rotation group. A positive map theta is introduced which
describes the time reversal symmetry of the local states and which is unitarily
equivalent to the transposition of matrices. It is shown that the partial time
reversal transformation theta_2 = (I tensor theta) acting on the composite
system can be expressed in terms of the invariant 6-j symbols introduced by
Wigner into the quantum theory of angular momentum. This fact enables a
complete geometrical construction of the manifold of states with positive
partial transposition and of the sets of separable and entangled states of (4
tensor 4) systems. The separable states are shown to form a three-dimensional
prism and a three-dimensional manifold of bound entangled states is identified.
A positive maps is obtained which yields, together with the time reversal, a
necessary and sufficient condition for the separability of states of (4 tensor
4) systems. The relations to the reduction criterion and to the recently
proposed cross norm criterion for separability are discussed.Comment: 15 pages, 3 figure
Dynamical invariants and nonadiabatic geometric phases in open quantum systems
We introduce an operational framework to analyze non-adiabatic Abelian and
non-Abelian, cyclic and non-cyclic, geometric phases in open quantum systems.
In order to remove the adiabaticity condition, we generalize the theory of
dynamical invariants to the context of open systems evolving under arbitrary
convolutionless master equations. Geometric phases are then defined through the
Jordan canonical form of the dynamical invariant associated with the
super-operator that governs the master equation. As a by-product, we provide a
sufficient condition for the robustness of the phase against a given decohering
process. We illustrate our results by considering a two-level system in a
Markovian interaction with the environment, where we show that the
non-adiabatic geometric phase acquired by the system can be constructed in such
a way that it is robust against both dephasing and spontaneous emission.Comment: 9 pages, 3 figures. v2: minor corrections and subsection IV.D added.
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