463 research outputs found
Violation of Bell inequality for thermal states of interaction qubits via a multi-qubit Heisenberg model
We study the violations of Bell inequality for thermal states of qubits in a
multi-qubit Heisenberg model as a function of temperature and external magnetic
fields. Unlike the behaviors of the entanglement the violation can not be
obtained by increasing the temperature or the magnetic field. The threshold
temperatures of the violation are found be less than that of the entanglement.
We also consider a realistic cavity-QED model which is a special case of the
mutli-qubit Heisenberg model.Comment: 5 pages, 5 figures, few changed, accepted by New J. Phy
Quantum information with continuous variables
Quantum information is a rapidly advancing area of interdisciplinary
research. It may lead to real-world applications for communication and
computation unavailable without the exploitation of quantum properties such as
nonorthogonality or entanglement. We review the progress in quantum information
based on continuous quantum variables, with emphasis on quantum optical
implementations in terms of the quadrature amplitudes of the electromagnetic
field.Comment: accepted for publication in Reviews of Modern Physic
Quantum metrology with nonclassical states of atomic ensembles
Quantum technologies exploit entanglement to revolutionize computing,
measurements, and communications. This has stimulated the research in different
areas of physics to engineer and manipulate fragile many-particle entangled
states. Progress has been particularly rapid for atoms. Thanks to the large and
tunable nonlinearities and the well developed techniques for trapping,
controlling and counting, many groundbreaking experiments have demonstrated the
generation of entangled states of trapped ions, cold and ultracold gases of
neutral atoms. Moreover, atoms can couple strongly to external forces and light
fields, which makes them ideal for ultra-precise sensing and time keeping. All
these factors call for generating non-classical atomic states designed for
phase estimation in atomic clocks and atom interferometers, exploiting
many-body entanglement to increase the sensitivity of precision measurements.
The goal of this article is to review and illustrate the theory and the
experiments with atomic ensembles that have demonstrated many-particle
entanglement and quantum-enhanced metrology.Comment: 76 pages, 40 figures, 1 table, 603 references. Some figures bitmapped
at 300 dpi to reduce file siz
Cloning Entangled Qubits to Scales One Can See
By amplifying photonic qubits it is possible to produce states that contain
enough photons to be seen with a human eye, potentially bringing quantum
effects to macroscopic scales [1]. In this paper we theoretically study quantum
states obtained by amplifying one side of an entangled photon pair with
different types of optical cloning machines for photonic qubits. We propose a
detection scheme that involves lossy threshold detectors (such as human eye) on
the amplified side and conventional photon detectors on the other side. We show
that correlations obtained with such coarse-grained measurements prove the
entanglement of the initial photon pair and do not prove the entanglement of
the amplified state. We emphasize the importance of the detection loophole in
Bell violation experiments by giving a simple preparation technique for
separable states that violate a Bell inequality without closing this loophole.
Finally we analyze the genuine entanglement of the amplified states and its
robustness to losses before, during and after amplification.Comment: 15 pages, 9 figure
Entanglement detection
How can one prove that a given state is entangled? In this paper we review
different methods that have been proposed for entanglement detection. We first
explain the basic elements of entanglement theory for two or more particles and
then entanglement verification procedures such as Bell inequalities,
entanglement witnesses, the determination of nonlinear properties of a quantum
state via measurements on several copies, and spin squeezing inequalities. An
emphasis is given on the theory and application of entanglement witnesses. We
also discuss several experiments, where some of the presented methods have been
implemented.Comment: review article, 90 pages, 22 figures. v2: more content, v3: small
change
Genuine quantum correlations in quantum many-body systems: a review of recent progress
Quantum information theory has considerably helped in the understanding of
quantum many-body systems. The role of quantum correlations and in particular,
bipartite entanglement, has become crucial to characterise, classify and
simulate quantum many body systems. Furthermore, the scaling of entanglement
has inspired modifications to numerical techniques for the simulation of
many-body systems leading to the, now established, area of tensor networks.
However, the notions and methods brought by quantum information do not end with
bipartite entanglement. There are other forms of correlations embedded in the
ground, excited and thermal states of quantum many-body systems that also need
to be explored and might be utilised as potential resources for quantum
technologies. The aim of this work is to review the most recent developments
regarding correlations in quantum many-body systems focussing on multipartite
entanglement, quantum nonlocality, quantum discord, mutual information but also
other non classical measures of correlations based on quantum coherence.
Moreover, we also discuss applications of quantum metrology in quantum
many-body systems.Comment: Review. Close to published version. Comments are welcome! Please
write an email to g.dechiara[(at)]qub.ac.u
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