167 research outputs found
A comparative study of relative entropy of entanglement, concurrence and negativity
The problem of ordering of two-qubit states imposed by relative entropy of
entanglement (E) in comparison to concurrence (C) and negativity (N) is
studied. Analytical examples of states consistently and inconsistently ordered
by the entanglement measures are given. In particular, the states for which any
of the three measures imposes order opposite to that given by the other two
measures are described. Moreover, examples are given of pairs of the states,
for which (i) N'=N'' and C'=C'' but E' is different from E'', (ii) N'=N'' and
E'=E'' but C' differs from C'', (iii) E'=E'', N'C'', or (iv) states
having the same E, C, and N but still violating the
Bell-Clauser-Horne-Shimony-Holt inequality to different degrees.Comment: 8 pages, 7 figures, final versio
Kraus representation of damped harmonic oscillator and its application
By definition, the Kraus representation of a harmonic oscillator suffering
from the environment effect, modeled as the amplitude damping or the phase
damping, is directly given by a simple operator algebra solution. As examples
and applications, we first give a Kraus representation of a single qubit whose
computational basis states are defined as bosonic vacuum and single particle
number states. We further discuss the environment effect on qubits whose
computational basis states are defined as the bosonic odd and even coherent
states. The environment effects on entangled qubits defined by two different
kinds of computational basis are compared with the use of fidelity.Comment: 9 pages, 3 figure
Sudden vanishing and reappearance of nonclassical effects: General occurrence of finite-time decays and periodic vanishings of nonclassicality and entanglement witnesses
Analyses of phenomena exhibiting finite-time decay of quantum entanglement
have recently attracted considerable attention. Such decay is often referred to
as sudden vanishing (or sudden death) of entanglement, which can be followed by
its sudden reappearance (or sudden rebirth). We analyze various finite-time
decays (for dissipative systems) and analogous periodic vanishings (for unitary
systems) of nonclassical correlations as described by violations of classical
inequalities and the corresponding nonclassicality witnesses (or quantumness
witnesses), which are not necessarily entanglement witnesses. We show that
these sudden vanishings are universal phenomena and can be observed: (i) not
only for two- or multi-mode but also for single-mode nonclassical fields, (ii)
not solely for dissipative systems, and (iii) at evolution times which are
usually different from those of sudden vanishings and reappearances of quantum
entanglement.Comment: 10 pages, 3 figure
Wehrl information entropy and phase distributions of Schrodinger cat and cat-like states
The Wehrl information entropy and its phase density, the so-called Wehrl
phase distribution, are applied to describe Schr\"odinger cat and cat-like
(kitten) states. The advantages of the Wehrl phase distribution over the Wehrl
entropy in a description of the superposition principle are presented. The
entropic measures are compared with a conventional phase distribution from the
Husimi Q-function. Compact-form formulae for the entropic measures are found
for superpositions of well-separated states. Examples of Schr\"odinger cats
(including even, odd and Yurke-Stoler coherent states), as well as the cat-like
states generated in Kerr medium are analyzed in detail. It is shown that, in
contrast to the Wehrl entropy, the Wehrl phase distribution properly
distinguishes between different superpositions of unequally-weighted states in
respect to their number and phase-space configuration.Comment: 10 pages, 4 figure
Qubit-induced phonon blockade as a signature of quantum behavior in nanomechanical resonators
The observation of quantized nanomechanical oscillations by detecting
femtometer-scale displacements is a significant challenge for experimentalists.
We propose that phonon blockade can serve as a signature of quantum behavior in
nanomechanical resonators. In analogy to photon blockade and Coulomb blockade
for electrons, the main idea for phonon blockade is that the second phonon
cannot be excited when there is one phonon in the nonlinear oscillator. To
realize phonon blockade, a superconducting quantum two-level system is coupled
to the nanomechanical resonator and is used to induce the phonon
self-interaction. Using Monte Carlo simulations, the dynamics of the induced
nonlinear oscillator is studied via the Cahill-Glauber -parametrized
quasiprobability distributions. We show how the oscillation of the resonator
can occur in the quantum regime and demonstrate how the phonon blockade can be
observed with currently accessible experimental parameters
Generation of Kerr non-Gaussian motional states of trapped ions
Non-Gaussian states represent a powerful resource for quantum information
protocols in the continuous variables regime. Cat states, in particular, have
been produced in the motional degree of freedom of trapped ions by controlled
displacements dependent on the ionic internal state. An alternative method
harnesses the Kerr nonlinearity naturally existent in this kind of system. We
present detailed calculations confirming its feasibility for typical
experimental conditions. Additionally, this method permits the generation of
complex non-Gaussian states with negative Wigner functions. Especially,
superpositions of many coherent states are achieved at a fraction of the time
necessary to produce the cat state.Comment: 6 pages, 5 figure
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