14,853 research outputs found
Genetic correlations among protein yield, productive life, and type traits from the United States and diseases other than mastitis from Denmark and Sweden
Mesoscopic continuous and discrete channels for quantum information transfer
We study the possibility of realizing perfect quantum state transfer in
mesoscopic devices. We discuss the case of the Fano-Anderson model extended to
two impurities. For a channel with an infinite number of degrees of freedom, we
obtain coherent behavior in the case of strong coupling or in weak coupling
off-resonance. For a finite number of degrees of freedom, coherent behavior is
associated to weak coupling and resonance conditions
Vanishing largest Lyapunov exponent and Tsallis entropy
We present a geometric argument that explains why some systems having
vanishing largest Lyapunov exponent have underlying dynamics aspects of which
can be effectively described by the Tsallis entropy. We rely on a comparison of
the generalised additivity of the Tsallis entropy versus the ordinary
additivity of the BGS entropy. We translate this comparison in metric terms by
using an effective hyperbolic metric on the configuration/phase space for the
Tsallis entropy versus the Euclidean one in the case of the BGS entropy.
Solving the Jacobi equation for such hyperbolic metrics effectively sets the
largest Lyapunov exponent computed with respect to the corresponding Euclidean
metric to zero. This conclusion is in agreement with all currently known
results about systems that have a simple asymptotic behaviour and are described
by the Tsallis entropy.Comment: 15 pages, No figures. LaTex2e. Some overlap with arXiv:1104.4869
Additional references and clarifications in this version. To be published in
QScience Connec
Decoherence by Correlated Noise and Quantum Error Correction
We study the decoherence of a quantum computer in an environment which is
inherently correlated in time and space. We first derive the nonunitary time
evolution of the computer and environment in the presence of a stabilizer error
correction code, providing a general way to quantify decoherence for a quantum
computer. The general theory is then applied to the spin-boson model. Our
results demonstrate that effects of long-range correlations can be
systematically reduced by small changes in the error correction codes.Comment: 4 pages, 1 figure, Phys. Rev. Lett. in pres
Decoherence of encoded quantum registers
In order to eliminate disturbing effects of decoherence, encoding of quantum
information in decoherence-free subspaces has been suggested. We analyze the
benefits of this concept for a quantum register that is realized in a spin
chain in contact with a common bosonic bath. Within a dissipation-less model we
provide explicit analytical results for the average fidelity of plain and
encoded quantum registers. For the investigation of dissipative spin-boson
couplings we employ a master equation of Bloch-Redfield type.Comment: 13 pages, 9 figure
Dynamical suppression of telegraph and 1/f noise due to quantum bistable fluctuator
We study dynamical decoupling of a qubit from non gaussian quantum noise due
to discrete sources, as bistable fluctuators and 1/f noise. We obtain analytic
and numerical results for generic operating point. For very large pulse
frequency, where dynamic decoupling compensates decoherence, we found universal
behavior. At intermediate frequencies noise can be compensated or enhanced,
depending on the nature of the fluctuators and on the operating point. Our
technique can be applied to a larger class of non-gaussian environments.Comment: Revtex 4, 5 pages, 3 figures. Title revised and some other minor
changed. Final version as published in PR
Additivity properties of a Gaussian Channel
The Amosov-Holevo-Werner conjecture implies the additivity of the minimum
Re'nyi entropies at the output of a channel. The conjecture is proven true for
all Re'nyi entropies of integer order greater than two in a class of Gaussian
bosonic channel where the input signal is randomly displaced or where it is
coupled linearly to an external environment.Comment: 9 pages, 1 figure (minor error present in the published version
corrected
Effect of a gap on the decoherence of a qubit
We revisit the problem of the decoherence and relaxation of a central spin
coupled to a bath of conduction electrons. We consider both metallic and
semiconducting baths to study the effect of a gap in the bath density of states
(DOS) on the time evolution of the density matrix of the central spin. We use
two weak coupling approximation schemes to study the decoherence. At low
temperatures, though the temperature dependence of the decoherence rate in the
case of a metallic bath is the same irrespective of the details of the bath,
the same is not true for the semiconducting bath. We also calculate the
relaxation and decoherence rates as a function of external magnetic fields
applied both on the central spin and the bath. We find that in the presence of
the gap, there exists a certain regime of fields, for which surprisingly, the
metallic bath has lower rates of relaxation and decoherence than the
semiconducting bath.Comment: 9 pages, 9 figure
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