123 research outputs found
Realistic continuous-variable quantum teleportation with non-Gaussian resources
We present a comprehensive investigation of nonideal continuous-variable
quantum teleportation implemented with entangled non-Gaussian resources. We
discuss in a unified framework the main decoherence mechanisms, including
imperfect Bell measurements and propagation of optical fields in lossy fibers,
applying the formalism of the characteristic function. By exploiting
appropriate displacement strategies, we compute analytically the success
probability of teleportation for input coherent states, and two classes of
non-Gaussian entangled resources: Two-mode squeezed Bell-like states (that
include as particular cases photon-added and photon-subtracted de-Gaussified
states), and two-mode squeezed cat-like states. We discuss the optimization
procedure on the free parameters of the non-Gaussian resources at fixed values
of the squeezing and of the experimental quantities determining the
inefficiencies of the non-ideal protocol. It is found that non-Gaussian
resources enhance significantly the efficiency of teleportation and are more
robust against decoherence than the corresponding Gaussian ones. Partial
information on the alphabet of input states allows further significant
improvement in the performance of the non-ideal teleportation protocol.Comment: 14 pages, 6 figure
Continuous variable quantum teleportation with non-Gaussian resources
We investigate continuous variable quantum teleportation using non-Gaussian
states of the radiation field as entangled resources. We compare the
performance of different classes of degaussified resources, including two-mode
photon-added and two-mode photon-subtracted squeezed states. We then introduce
a class of two-mode squeezed Bell-like states with one-parameter dependence for
optimization. These states interpolate between and include as subcases
different classes of degaussified resources. We show that optimized squeezed
Bell-like resources yield a remarkable improvement in the fidelity of
teleportation both for coherent and nonclassical input states. The
investigation reveals that the optimal non-Gaussian resources for continuous
variable teleportation are those that most closely realize the simultaneous
maximization of the content of entanglement, the degree of affinity with the
two-mode squeezed vacuum and the, suitably measured, amount of non-Gaussianity.Comment: 12 pages, 12 figure
Structure of multiphoton quantum optics. II. Bipartite systems, physical processes, and heterodyne squeezed states
Extending the scheme developed for a single mode of the electromagnetic field
in the preceding paper ``Structure of multiphoton quantum optics. I. Canonical
formalism and homodyne squeezed states'', we introduce two-mode nonlinear
canonical transformations depending on two heterodyne mixing angles. They are
defined in terms of hermitian nonlinear functions that realize heterodyne
superpositions of conjugate quadratures of bipartite systems. The canonical
transformations diagonalize a class of Hamiltonians describing non degenerate
and degenerate multiphoton processes. We determine the coherent states
associated to the canonical transformations, which generalize the non
degenerate two--photon squeezed states. Such heterodyne multiphoton squeezed
are defined as the simultaneous eigenstates of the transformed, coupled
annihilation operators. They are generated by nonlinear unitary evolutions
acting on two-mode squeezed states. They are non Gaussian, highly non
classical, entangled states. For a quadratic nonlinearity the heterodyne
multiphoton squeezed states define two--mode cubic phase states. The
statistical properties of these states can be widely adjusted by tuning the
heterodyne mixing angles, the phases of the nonlinear couplings, as well as the
strength of the nonlinearity. For quadratic nonlinearity, we study the
higher-order contributions to the susceptibility in nonlinear media and we
suggest possible experimental realizations of multiphoton conversion processes
generating the cubic-phase heterodyne squeezed states.Comment: 16 pages, 23 figure
Optimization of the transmission of observable expectation values and observable statistics in Continuous Variable Teleportation
We analyze the statistics of observables in continuous variable quantum
teleportation in the formalism of the characteristic function. We derive
expressions for average values of output state observables in particular
cumulants which are additive in terms of the input state and the resource of
teleportation. Working with Squeezed Bell-like states, which may be optimized
in a free parameter for better teleportation performance we discuss the
relation between resources optimal for fidelity and for different observable
averages. We obtain the values of the free parameter which optimize the central
momenta and cumulants up to fourth order. For the cumulants the distortion
between in and out states due to teleportation depends only on the resource. We
obtain optimal parameters for the second and fourth order cumulants which do
not depend on the squeezing of the resource. The second order central momenta
which is equal to the second order cumulants and the photon number average are
optimized by the same resource. We show that the optimal fidelity resource,
found in reference (Phys. Rev. A {\bf 76}, 022301 (2007)) to depend also on the
characteristics of input, tends for high squeezing to the resource which
optimizes the second order momenta. A similar behavior is obtained for the
resource which optimizes the photon statistics which is treated here using the
sum of the squared differences in photon probabilities of input and output
states as the distortion measure. This is interpreted to mean that the
distortions associated to second order momenta dominates the behavior of the
output state for large squeezing of the resource. Optimal fidelity and optimal
photon statistics resources are compared and is shown that for mixtures of Fock
states they are equivalent.Comment: 25 pages, 11 figure
Optimal estimation of losses at the ultimate quantum limit with non-Gaussian states
We address the estimation of the loss parameter of a bosonic channel probed
by arbitrary signals. Unlike the optimal Gaussian probes, which can attain the
ultimate bound on precision asymptotically either for very small or very large
losses, we prove that Fock states at any fixed photon number saturate the bound
unconditionally for any value of the loss. In the relevant regime of low-energy
probes, we demonstrate that superpositions of the first low-lying Fock states
yield an absolute improvement over any Gaussian probe. Such few-photon states
can be recast quite generally as truncations of de-Gaussified photon-subtracted
states.Comment: 4 pages, 3 figure
Informality, Inequality and ICT in Transition economies
In this paper, we examine the role of the quality of institutional infrastructure and information and communication technology (ICT) in the relationship between the size of the informal sector (IS) and income inequality. Following our results, the sign of the relationship between IS and income inequality depends on the quality of institutions. When institutions are weak, agents invest less human capital and ICT in the formal sector (FS), thereby reducing income inequality. Utilizing panel data for sixteen transition countries we show that the relationship between the size of the IS and the level of income inequality is ambiguous. Our findings highlight the problem of measuring the relative size of the IS which is a hidden entity. We control for robustness of our findings using alternative proxies of ICT, human capital, and institutional quality and some interaction terms among these variables
Hierarchies of Geometric Entanglement
We introduce a class of generalized geometric measures of entanglement. For
pure quantum states of elementary subsystems, they are defined as the
distances from the sets of -separable states (). The entire set
of generalized geometric measures provides a quantification and hierarchical
ordering of the different bipartite and multipartite components of the global
geometric entanglement, and allows to discriminate among the different
contributions. The extended measures are applied to the study of entanglement
in different classes of -qubit pure states. These classes include and
states, and their symmetric superpositions; symmetric multi-magnon
states; cluster states; and, finally, asymmetric generalized -like
superposition states. We discuss in detail a general method for the explicit
evaluation of the multipartite components of geometric entanglement, and we
show that the entire set of geometric measures establishes an ordering among
the different types of bipartite and multipartite entanglement. In particular,
it determines a consistent hierarchy between and states, clarifying
the original result of Wei and Goldbart that states possess a larger global
entanglement than states. Furthermore, we show that all multipartite
components of geometric entanglement in symmetric states obey a property of
self-similarity and scale invariance with the total number of qubits and the
number of qubits per party.Comment: 16 pages, 7 figures. Final version, to appear in Phys. Rev.
Multipartite entangled states in particle mixing
In the physics of flavor mixing, the flavor states are given by
superpositions of mass eigenstates. By using the occupation number to define a
multiqubit space, the flavor states can be interpreted as multipartite
mode-entangled states. By exploiting a suitable global measure of entanglement,
based on the entropies related to all possible bipartitions of the system, we
analyze the correlation properties of such states in the instances of three-
and four-flavor mixing. Depending on the mixing parameters, and, in particular,
on the values taken by the free phases, responsible for the CP-violation,
entanglement concentrates in preferred bipartitions. We quantify in detail the
amount and the distribution of entanglement in the physically relevant cases of
flavor mixing in quark and neutrino systems. By using the wave packet
description for localized particles, we use the global measure of entanglement,
suitably adapted for the instance of multipartite mixed states, to analyze the
decoherence induced by the free evolution dynamics on the quantum correlations
of stationary neutrino beams. We define a decoherence length as the distance
associated with the vanishing of the coherent interference effects among
massive neutrino states. We investigate the role of the CP-violating phase in
the decoherence process.Comment: 18 pages, 7 figure
A study of the efficiency of the class of -states as a quantum channel
Recently, a new class of -states has been defined by Agarwal and Pati
\cite{agarwal} and it has been shown that they can be used as a quantum channel
for teleportation and superdense coding. In this work, we identify those
three-qubit states from the set of the new class of -states which are most
efficient or suitable for quantum teleportation. We show that with some
probability is best suited for
teleportation channel in the sense that it does not depend on the input state.Comment: 7 pages, Late
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