728 research outputs found
Decay of fidelity in terms of correlation functions
We consider, within the algebraic formalism, the time dependence of fidelity
for qubits encoded into an open physical system. We relate the decay of
fidelity to the evolution of correlation functions and, in the particular case
of a Markovian dynamics, to the spectral gap of the generator of the semigroup.
The results are applicable to the analysis of models of quantum memories.Comment: 9 pages, no figure
Short Time Cycles of Purely Quantum Refrigerators
Four stroke Otto refrigerator cycles with no classical analogue are studied.
Extremely short cycle times with respect to the internal time scale of the
working medium characterize these refrigerators. Therefore these cycles are
termed sudden. The sudden cycles are characterized by the stable limit cycle
which is the invariant of the global cycle propagator. During their operation
the state of the working medium possesses significant coherence which is not
erased in the equilibration segments due to the very short time allocated. This
characteristic is reflected in a difference between the energy entropy and the
Von Neumann entropy of the working medium. A classification scheme for sudden
refrigerators is developed allowing simple approximations for the cooling power
and coefficient of performance.Comment: 20 pages, 12 figures. Among the figures there are 6 figures which are
double, namely with two parts, Top and Botto
Information-theoretical meaning of quantum dynamical entropy
The theory of noncommutative dynamical entropy and quantum symbolic dynamics
for quantum dynamical systems is analised from the point of view of quantum
information theory. Using a general quantum dynamical system as a communication
channel one can define different classical capacities depending on the
character of resources applied for encoding and decoding procedures and on the
type of information sources. It is shown that for Bernoulli sources the
entanglement-assisted classical capacity, which is the largest one, is bounded
from above by the quantum dynamical entropy defined in terms of operational
partitions of unity. Stronger results are proved for the particular class of
quantum dynamical systems -- quantum Bernoulli shifts. Different classical
capacities are exactly computed and the entanglement-assisted one is equal to
the dynamical entropy in this case.Comment: 6 page
Linear dynamical entropy and free-independence for quantized maps on the torus
We study the relations between the averaged linear entropy production in
periodically measured quantum systems and ergodic properties of their classical
counterparts. Quantized linear automorphisms of the torus, both classically
chaotic and regular ones, are used as examples. Numerical calculations show
different entropy production regimes depending on the relation between the
Kolmogorov-Sinai entropy and the measurement entropy. The hypothesis of free
independence relations between the dynamics and measurement proposed to explain
the initial constant and maximal entropy production is tested numerically for
those models.Comment: 7 pages, 5 figure
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