4,340 research outputs found
Correlated projection operator approach to non-Markovian dynamics in spin baths
The dynamics of an open quantum system is usually studied by performing a
weak-coupling and weak-correlation expansion in the system-bath interaction.
For systems exhibiting strong couplings and highly non-Markovian behavior this
approach is not justified. We apply a recently proposed correlated projection
superoperator technique to the model of a central spin coupled to a spin bath
via full Heisenberg interaction. Analytical solutions to both the
Nakajima-Zwanzig and the time-convolutionless master equation are determined
and compared with the results of the exact solution. The correlated projection
operator technique significantly improves the standard methods and can be
applied to many physical problems such as the hyperfine interaction in a
quantum dot
Dissipative Entanglement of Quantum Spin Fluctuations
We consider two non-interacting infinite quantum spin chains immersed in a
common thermal environment and undergoing a local dissipative dynamics of
Lindblad type. We study the time evolution of collective mesoscopic quantum
spin fluctuations that, unlike macroscopic mean-field observables, retain a
quantum character in the thermodynamical limit. We show that the microscopic
dissipative dynamics is able to entangle these mesoscopic degrees of freedom,
through a purely mixing mechanism. Further, the behaviour of the dissipatively
generated quantum correlations between the two chains is studied as a function
of temperature and dissipation strength.Comment: 54 pages, 8 figure
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
Correlated errors can lead to better performance of quantum codes
A formulation for evaluating the performance of quantum error correcting
codes for a general error model is presented. In this formulation, the
correlation between errors is quantified by a Hamiltonian description of the
noise process. We classify correlated errors using the system-bath interaction:
local versus nonlocal and two-body versus many-body interactions. In
particular, we consider Calderbank-Shor-Steane codes and observe a better
performance in the presence of correlated errors depending on the timing of the
error recovery. We also find this timing to be an important factor in the
design of a coding system for achieving higher fidelities.Comment: 5 pages, 3 figures. Replaced by the published version. Title change
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.
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
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
Stochastic wave function approach to the calculation of multitime correlation functions of open quantum systems
Within the framework of probability distributions on projective Hilbert space
a scheme for the calculation of multitime correlation functions is developed.
The starting point is the Markovian stochastic wave function description of an
open quantum system coupled to an environment consisting of an ensemble of
harmonic oscillators in arbitrary pure or mixed states. It is shown that matrix
elements of reduced Heisenberg picture operators and general time-ordered
correlation functions can be expressed by time-symmetric expectation values of
extended operators in a doubled Hilbert space. This representation allows the
construction of a stochastic process in the doubled Hilbert space which enables
the determination of arbitrary matrix elements and correlation functions. The
numerical efficiency of the resulting stochastic simulation algorithm is
investigated and compared with an alternative Monte Carlo wave function method
proposed first by Dalibard et al. [Phys. Rev. Lett. {\bf 68}, 580 (1992)]. By
means of a standard example the suggested algorithm is shown to be more
efficient numerically and to converge faster. Finally, some specific examples
from quantum optics are presented in order to illustrate the proposed method,
such as the coupling of a system to a vacuum, a squeezed vacuum within a finite
solid angle, and a thermal mixture of coherent states.Comment: RevTex, 19 pages, 3 figures, uses multico
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
Hybrid method for simulating front propagation in reaction-diffusion systems
We study the propagation of pulled fronts in the
microscopic reaction-diffusion process using Monte Carlo (MC) simulations. In
the mean field approximation the process is described by the deterministic
Fisher-Kolmogorov-Petrovsky-Piscounov (FKPP) equation. In particular we
concentrate on the corrections to the deterministic behavior due to the number
of particles per site . By means of a new hybrid simulation scheme, we
manage to reach large macroscopic values of which allows us to show
the importance in the dynamics of microscopic pulled fronts of the interplay of
microscopic fluctuations and their macroscopic relaxation.Comment: 5 pages, 4 figure
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