1,638 research outputs found
Operational Entanglement Families of Symmetric Mixed N-Qubit States
We introduce an operational entanglement classification of symmetric mixed
states for an arbitrary number of qubits based on stochastic local operations
assisted with classical communication (SLOCC operations). We define families of
SLOCC entanglement classes successively embedded into each other, we prove that
they are of non-zero measure, and we construct witness operators to distinguish
them. Moreover, we discuss how arbitrary symmetric mixed states can be realized
in the lab via a one-to-one correspondence between well-defined sets of
controllable parameters and the corresponding entanglement families.Comment: 6 pages, 2 figures, published version, Phys. Rev. A, in pres
Heralded Entanglement of Arbitrary Degree in Remote Qubits
Incoherent scattering of photons off two remote atoms with a Lambda-level
structure is used as a basic Young-type interferometer to herald long-lived
entanglement of an arbitrary degree. The degree of entanglement, as measured by
the concurrence, is found to be tunable by two easily accessible experimental
parameters. Fixing one of them to certain values unveils an analog to the
Malus' law. An estimate of the variation in the degree of entanglement due to
uncertainties in an experimental realization is given.Comment: published version, 4 pages and 2 figure
Operational multipartite entanglement classes for symmetric photonic qubit states
We present experimental schemes that allow to study the entanglement classes
of all symmetric states in multiqubit photonic systems. In addition to
comparing the presented schemes in efficiency, we will highlight the relation
between the entanglement properties of symmetric Dicke states and a recently
proposed entanglement scheme for atoms. In analogy to the latter, we obtain a
one-to-one correspondence between well-defined sets of experimental parameters
and multiqubit entanglement classes inside the symmetric subspace of the
photonic system.Comment: 5 pages, 1 figur
Entanglement Equivalence of -qubit Symmetric States
We study the interconversion of multipartite symmetric -qubit states under
stochastic local operations and classical communication (SLOCC). We demonstrate
that if two symmetric states can be connected with a nonsymmetric invertible
local operation (ILO), then they belong necessarily to the separable, W, or GHZ
entanglement class, establishing a practical method of discriminating subsets
of entanglement classes. Furthermore, we prove that there always exists a
symmetric ILO connecting any pair of symmetric -qubit states equivalent
under SLOCC, simplifying the requirements for experimental implementations of
local interconversion of those states.Comment: Minor correction
Generation of Total Angular Momentum Eigenstates in Remote Qubits
We propose a scheme enabling the universal coupling of angular momentum of
remote noninteracting qubits using linear optical tools only. Our system
consists of single-photon emitters in a -configuration that are
entangled among their long-lived ground-state qubits through suitably designed
measurements of the emitted photons. In this manner, we present an
experimentally feasible algorithm that is able to generate any of the
symmetric and nonsymmetric total angular momentum eigenstates spanning the
Hilbert space of the -qubit compound.Comment: 5 pages, 4 figures, improved presentation. Accepted in Physical
Review
Operational determination of multi-qubit entanglement classes via tuning of local operations
We present a physical setup with which it is possible to produce arbitrary
symmetric long-lived multiqubit entangled states in the internal ground levels
of photon emitters, including the paradigmatic GHZ and W states. In the case of
three emitters, where each tripartite entangled state belongs to one of two
well-defined entanglement classes, we prove a one-to-one correspondence between
well-defined sets of experimental parameters, i.e., locally tunable polarizer
orientations, and multiqubit entanglement classes inside the symmetric
subspace.Comment: Improved version. Accepted in Physical Review Letter
Test-retest reliability of structural brain networks from diffusion MRI
Structural brain networks constructed from diffusion MRI (dMRI) and tractography have been demonstrated in healthy volunteers and more recently in various disorders affecting brain connectivity. However, few studies have addressed the reproducibility of the resulting networks. We measured the test–retest properties of such networks by varying several factors affecting network construction using ten healthy volunteers who underwent a dMRI protocol at 1.5 T on two separate occasions. Each T1-weighted brain was parcellated into 84 regions-of-interest and network connections were identified using dMRI and two alternative tractography algorithms, two alternative seeding strategies, a white matter waypoint constraint and three alternative network weightings. In each case, four common graph-theoretic measures were obtained. Network properties were assessed both node-wise and per network in terms of the intraclass correlation coefficient (ICC) and by comparing within- and between-subject differences. Our findings suggest that test–retest performance was improved when: 1) seeding from white matter, rather than grey; and 2) using probabilistic tractography with a two-fibre model and sufficient streamlines, rather than deterministic tensor tractography. In terms of network weighting, a measure of streamline density produced better test–retest performance than tract-averaged diffusion anisotropy, although it remains unclear which is a more accurate representation of the underlying connectivity. For the best performing configuration, the global within-subject differences were between 3.2% and 11.9% with ICCs between 0.62 and 0.76. The mean nodal within-subject differences were between 5.2% and 24.2% with mean ICCs between 0.46 and 0.62. For 83.3% (70/84) of nodes, the within-subject differences were smaller than between-subject differences. Overall, these findings suggest that whilst current techniques produce networks capable of characterising the genuine between-subject differences in connectivity, future work must be undertaken to improve network reliability
Ion crystals in anharmonic traps
There is currently intensive research into creating a large-scale quantum computer with trapped ions. It is well known that for a linear ion crystal in a harmonic potential, the ions near the center are more closely spaced compared to the ions near the ends. This is problematic as the number of ions increases. Here, we consider a linear ion crystal in an anharmonic potential that is purely quartic in position. We find that the ions are more evenly spaced compared to the harmonic case. We develop a variational approach to calculate the properties of the ground state. We also characterize the zigzag transition in an anharmonic potential
Adaptive thresholding for reliable topological inference in single subject fMRI analysis
Single subject fMRI has proved to be a useful tool for mapping functional areas in clinical procedures such as tumour resection. Using fMRI data, clinicians assess the risk, plan and execute such procedures based on thresholded statistical maps. However, because current thresholding methods were developed mainly in the context of cognitive neuroscience group studies, most single subject fMRI maps are thresholded manually to satisfy specific criteria related to single subject analyses. Here, we propose a new adaptive thresholding method which combines Gamma-Gaussian mixture modelling with topological thresholding to improve cluster delineation. In a series of simulations we show that by adapting to the signal and noise properties, the new method performs well in terms of the trade-off between false negative and positive cluster error rates as well as in terms of over and underestimation of the true activation border. We also show through simulations and a motor test-retest study on ten volunteer subjects that adaptive thresholding improves reliability, mainly by accounting for the global signal variance. This in turn increases the likelihood that the true activation pattern can be determined
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