2,235 research outputs found
A remark on asymptotic completeness for the critical nonlinear Klein-Gordon equation
We give a short proof of asymptotic completeness and global existence for the
cubic Nonlinear Klein-Gordon equation in one dimension. Our approach to dealing
with the long range behavior of the asymptotic solution is by reducing it, in
hyperbolic coordinates to the study of an ODE. Similar arguments extend to
higher dimensions and other long range type nonlinear problems.Comment: To appear in Lett. Math. Phy
Classical Correlations and Entanglement in Quantum Measurements
We analyze a quantum measurement where the apparatus is initially in a mixed
state. We show that the amount of information gained in a measurement is not
equal to the amount of entanglement between the system and the apparatus, but
is instead equal to the degree of classical correlations between the two. As a
consequence, we derive an uncertainty-like expression relating the information
gain in the measurement and the initial mixedness of the apparatus. Final
entanglement between the environment and the apparatus is also shown to be
relevant for the efficiency of the measurement.Comment: to appear in Physical Review Letter
Correlations in optically-controlled quantum emitters
We address the problem of optically controlling and quantifying the
dissipative dynamics of quantum and classical correlations in a set-up of
individual quantum emitters under external laser excitation. We show that both
types of correlations, the former measured by the quantum discord, are present
in the system's evolution even though the emitters may exhibit an early stage
disentanglement. In the absence of external laser pumping,we demonstrate
analytically, for a set of suitable initial states, that there is an entropy
bound for which quantum discord and entanglement of the emitters are always
greater than classical correlations, thus disproving an early conjecture that
classical correlations are greater than quantum correlations. Furthermore, we
show that quantum correlations can also be greater than classical correlations
when the system is driven by a laser field. For scenarios where the emitters'
quantum correlations are below their classical counterparts, an optimization of
the evolution of the quantum correlations can be carried out by appropriately
tailoring the amplitude of the laser field and the emitters' dipole-dipole
interaction. We stress the importance of using the entanglement of formation,
rather than the concurrence, as the entanglement measure, since the latter can
grow beyond the total correlations and thus give incorrect results on the
actual system's degree of entanglement.Comment: 11 pages, 10 figures, this version contains minor modifications; to
appear in Phys. Rev.
Steady state entanglement in open and noisy quantum systems at high temperature
We show that quantum mechanical entanglement can prevail even in noisy open
quantum systems at high temperature and far from thermodynamical equilibrium,
despite the deteriorating effect of decoherence. The system consists of a
number N of interacting quantum particles, and it can interact and exchange
particles with some environment. The effect of decoherence is counteracted by a
simple mechanism, where system particles are randomly reset to some standard
initial state, e.g. by replacing them with particles from the environment. We
present a master equation that describes this process, which we can solve
analytically for small N. If we vary the interaction strength and the reset
against decoherence rate, we find a threshold below which the equilibrium state
is classically correlated, and above which there is a parameter region with
genuine entanglement.Comment: 5 pages, 3 figure
Transient dynamics of linear quantum amplifiers
The transient dynamics of a quantum linear amplifier during the transition
from damping to amplification regime is studied. The master equation for the
quantized mode of the field is solved, and the solution is used to describe the
statistics of the output field. The conditions under which a nonclassical input
field may retain nonclassical features at the output of the amplifier are
analyzed and compared to the results of earlier theories. As an application we
give a dynamical description of the departure of the system from thermal
equilibrium.Comment: 10 pages, 6 figures. V2: extended discussion on application
Kinematic approach to the mixed state geometric phase in nonunitary evolution
A kinematic approach to the geometric phase for mixed quantal states in
nonunitary evolution is proposed. This phase is manifestly gauge invariant and
can be experimentally tested in interferometry. It leads to well-known results
when the evolution is unitary.Comment: Minor changes; journal reference adde
Geometric phase distributions for open quantum systems
In an open system, the geometric phase should be described by a distribution.
We show that a geometric phase distribution for open system dynamics is in
general ambiguous, but the imposition of reasonable physical constraints on the
environment and its coupling with the system yields a unique geometric phase
distribution that applies even for mixed states, non-unitary dynamics, and
non-cyclic evolutions.Comment: Some minor revisions, references update
Universal bounds for the Holevo quantity, coherent information \\ and the Jensen-Shannon divergence
The Holevo quantity provides an upper bound for the mutual information
between the sender of a classical message encoded in quantum carriers and the
receiver. Applying the strong sub-additivity of entropy we prove that the
Holevo quantity associated with an initial state and a given quantum operation
represented in its Kraus form is not larger than the exchange entropy. This
implies upper bounds for the coherent information and for the quantum
Jensen--Shannon divergence. Restricting our attention to classical information
we bound the transmission distance between any two probability distributions by
the entropic distance, which is a concave function of the Hellinger distance.Comment: 5 pages, 2 figure
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