25 research outputs found
Discrete squeezed states for finite-dimensional spaces
We show how discrete squeezed states in an -dimensional phase space
can be properly constructed out of the finite-dimensional context. Such
discrete extensions are then applied to the framework of quantum tomography and
quantum information theory with the aim of establishing an initial study on the
interference effects between discrete variables in a finite phase-space.
Moreover, the interpretation of the squeezing effects is seen to be direct in
the present approach, and has some potential applications in different branches
of physics.Comment: 16 pages; 3 figure
Entanglement universality of two-qubit X-states
We demonstrate that for every two-qubit state there is a X-counterpart, i.e.,
a corresponding two-qubit X-state of same spectrum and entanglement, as
measured by concurrence, negativity or relative entropy of entanglement. By
parametrizing the set of two-qubit X-states and a family of unitary
transformations that preserve the sparse structure of a two-qubit X-state
density matrix, we obtain the parametric form of a unitary transformation that
converts arbitrary two-qubit states into their X-counterparts. Moreover, we
provide a semi-analytic prescription on how to set the parameters of this
unitary transformation in order to preserve concurrence or negativity. We also
explicitly construct a set of X-state density matrices, parametrized by their
purity and concurrence, whose elements are in one-to-one correspondence with
the points of the concurrence versus purity (CP) diagram for generic two-qubit
states.Comment: 24 pages, 6 figures. v2 includes new references and minor changes
(accepted version
Heuristic for estimation of multiqubit genuine multipartite entanglement
For every N-qubit density matrix written in the computational basis, an
associated "X-density matrix" can be obtained by vanishing all entries out of
the main- and anti-diagonals. It is very simple to compute the genuine
multipartite (GM) concurrence of this associated N-qubit X-state, which,
moreover, lower bounds the GM-concurrence of the original (non-X) state. In
this paper, we rely on these facts to introduce and benchmark a heuristic for
estimating the GM-concurrence of an arbitrary multiqubit mixed state. By
explicitly considering two classes of mixed states, we illustrate that our
estimates are usually very close to the standard lower bound on the
GM-concurrence, being significantly easier to compute. In addition, while
evaluating the performance of our proposed heuristic, we provide the first
characterization of GM-entanglement in the steady states of the driven Dicke
model at zero temperature.Comment: 19 pages, 5 figure
Generalized squeezing operators, bipartite Wigner functions and entanglement via Wehrl's entropy functionals
We introduce a new class of unitary transformations based on the su(1,1) Lie
algebra that generalizes, for certain particular representations of its
generators, well-known squeezing transformations in quantum optics. To
illustrate our results, we focus on the two-mode bosonic representation and
show how the parametric amplifier model can be modified in order to generate
such a generalized squeezing operator. Furthermore, we obtain a general
expression for the bipartite Wigner function which allows us to identify two
distinct sources of entanglement, here labelled by dynamical and kinematical
entanglement. We also establish a quantitative estimate of entanglement for
bipartite systems through some basic definitions of entropy functionals in
continuous phase-space representations.Comment: 16 page