458 research outputs found
Entanglement of mixed macroscopic superpositions: an entangling-power study
We investigate entanglement properties of a recently introduced class of
macroscopic quantum superpositions in two-mode mixed states. One of the tools
we use in order to infer the entanglement in this non-Gaussian class of states
is the power to entangle a qubit system. Our study reveals features which are
hidden in a standard approach to entanglement investigation based on the
uncertainty principle of the quadrature variables. We briefly describe the
experimental setup corresponding to our theoretical scenario and a suitable
modification of the protocol which makes our proposal realizable within the
current experimental capabilities.Comment: 9 pages, 7 figures, RevTeX
Towards variance-matrix characterization of complementarity relations in a continuous variable system
We discuss complementarity relations in a bipartite continuous variable
system. Building up from the work done on discrete d-dimensional systems, we
prove that for symmetric two-mode states, quantum complementarity relations can
be put in a simple relation with the elements of the variance matrix. When this
condition is not satisfied, such a connection becomes non-trivial. Our
investigation is the first step towards an operative characterization of the
complementarity in a scenario that has not been investigated so far.Comment: 7 pages, 4 figures, RevTeX
A dissipative scheme to approach the boundary of two-qubit entangled mixed states
We discuss the generation of states close to the boundary-family of maximally
entangled mixed states as defined by the use of concurrence and linear entropy.
The coupling of two qubits to a dissipation-affected bosonic mode is able to
produce a bipartite state having, for all practical purposes, the entanglement
and purity properties of one of such boundary states. We thoroughly study the
effects that thermal and squeezed character of the bosonic mode have in such a
process and we discuss tolerance to qubit phase-damping mechanisms. The
non-demanding nature of the scheme makes it realizable in a matter-light based
physical set-up, which we address in some details.Comment: 9 pages, 7 figures, RevTeX4, Accepted for publication by Physics
Review
Enhanced dynamical entanglement transfer with multiple qubits
We present two strategies to enhance the dynamical entanglement transfer from
continuous variable (CV) to finite dimensional systems by employing multiple
qubits. First, we consider the entanglement transfer to a composite finite
dimensional system of many qubits simultaneously interacting with a bipartite
CV field. We show that, considering realistic conditions in the generation of
CV entanglement, a small number of qubits resonantly coupled to the CV system
is sufficient for an almost complete dynamical transfer of the entanglement.
Our analysis also sheds further light on the transition between microscopic and
macroscopic behaviours of composite finite dimensional systems coupled to
bosonic fields (like atomic clouds interacting with light). Furthermore, we
present a protocol based on sequential interactions of the CV system with some
ancillary qubit systems and on subsequent measurements, allowing to
probabilistically convert CV entanglement into `almost perfect' Bell pairs of
two qubits. Our proposals are suited for realizations in various experimental
settings, ranging from cavity-QED to cavity-integrated superconducting devices.Comment: 10 pages, 8 figures, RevTeX4; terminology revised; accepted for
publicatio
Controllable Gaussian-qubit interface for extremal quantum state engineering
We study state engineering through bilinear interactions between two remote
qubits and two-mode Gaussian light fields. The attainable two-qubit states span
the entire physically allowed region in the entanglement-versus-global-purity
plane. Two-mode Gaussian states with maximal entanglement at fixed global and
marginal entropies produce maximally entangled two-qubit states in the
corresponding entropic diagram. We show that a small set of parameters
characterizing extremally entangled two-mode Gaussian states is sufficient to
control the engineering of extremally entangled two-qubit states, which can be
realized in realistic matter-light scenarios.Comment: 4+3 pages, 6 figures, RevTeX4. Close to published version with
appendi
Bypassing state initialization in Hamiltonian tomography on spin-chains
We provide an extensive discussion on a scheme for Hamiltonian tomography of
a spin-chain model that does not require state initialization [Phys. Rev. Lett.
102, 187203 (2009)]. The method has spurred the attention of the physics
community interested in indirect acquisition of information on the dynamics of
quantum many-body systems and represents a genuine instance of a
control-limited quantum protocol.Comment: 7 pages, 2 figures, RevTeX
Information-flux approach to multiple-spin dynamics
We introduce and formalize the concept of information flux in a many-body
register as the influence that the dynamics of a specific element receive from
any other element of the register. By quantifying the information flux in a
protocol, we can design the most appropriate initial state of the system and,
noticeably, the distribution of coupling strengths among the parts of the
register itself. The intuitive nature of this tool and its flexibility, which
allow for easily manageable numerical approaches when analytic expressions are
not straightforward, are greatly useful in interacting many-body systems such
as quantum spin chains. We illustrate the use of this concept in quantum
cloning and quantum state transfer and we also sketch its extension to
non-unitary dynamics.Comment: 7 pages, 4 figures, RevTeX
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