164 research outputs found
Quantum repeaters with individual rare-earth ions at telecommunication wavelengths
We present a quantum repeater scheme that is based on individual erbium and
europium ions. Erbium ions are attractive because they emit photons at
telecommunication wavelength, while europium ions offer exceptional spin
coherence for long-term storage. Entanglement between distant erbium ions is
created by photon detection. The photon emission rate of each erbium ion is
enhanced by a microcavity with high Purcell factor, as has recently been
demonstrated. Entanglement is then transferred to nearby europium ions for
storage. Gate operations between nearby ions are performed using dynamically
controlled electric-dipole coupling. These gate operations allow entanglement
swapping to be employed in order to extend the distance over which entanglement
is distributed. The deterministic character of the gate operations allows
improved entanglement distribution rates in comparison to atomic ensemble-based
protocols. We also propose an approach that utilizes multiplexing in order to
enhance the entanglement distribution rate.Comment: 13 pages, 4 figure
Applications of Spatio-Temporal Graph Neural Network Models for Brain Connectivity Analysis
Comprehending the interplay between spatial and temporal characteristics of neural dynamics can improve our understanding of information processing in the human brain. Graph neural networks provide a novel possibility to interpret graph-structured signals as typically observed in complex brain networks. This thesis presents an application of spatio-temporal graph neural networks to model functional dynamics observed in magnetic resoance imaging data. It is shown that graph neural network models are able to scale to large brain networks, and can help us to derive directed functional dependecies based on the structural brain network
Quantum repeaters based on individual electron spins and nuclear-spin-ensemble memories in quantum dots
Inspired by recent developments in the control and manipulation of quantum
dot nuclear spins, which allow for the transfer of an electron spin state to
the surrounding nuclear-spin ensemble for storage, we propose a quantum
repeater scheme that combines individual quantum dot electron spins and
nuclear-spin ensembles, which serve as spin-photon interfaces and quantum
memories respectively. We consider the use of low-strain quantum dots embedded
in high-cooperativity optical microcavities. Quantum dot nuclear-spin ensembles
allow for the long-term storage of entangled states, and heralded entanglement
swapping is performed using cavity-assisted gates. We highlight the advances in
quantum dot technologies required to realize our quantum repeater scheme which
promises the establishment of high-fidelity entanglement over long distances
with a distribution rate exceeding that of the direct transmission of photons.Comment: 21 pages, 5 figure
Light-cone distribution amplitudes of octet baryons from lattice QCD
We present lattice QCD results for the wave function normalization constants
and the first moments of the distribution amplitudes for the lowest-lying
baryon octet. The analysis is based on a large number of ensembles
comprising multiple trajectories in the quark mass plane including physical
pion (and kaon) masses, large volumes, and, most importantly, five different
lattice spacings down to . This allows us to perform a
controlled extrapolation to the continuum and infinite volume limits by a
simultaneous fit to all available data. We demonstrate that the formerly
observed violation of flavor symmetry breaking constraints can, indeed, be
attributed to discretization effects that vanish in the continuum limit
A Constrained ICA-EMD Model for Group Level fMRI Analysis
Independent component analysis (ICA), being a data-driven method, has been shown to be a powerful tool for functional magnetic resonance imaging (fMRI) data analysis. One drawback of this multivariate approach is that it is not, in general, compatible with the analysis of group data. Various techniques have been proposed to overcome this limitation of ICA. In this paper, a novel ICA-based workflow for extracting resting-state networks from fMRI group studies is proposed. An empirical mode decomposition (EMD) is used, in a data-driven manner, to generate reference signals that can be incorporated into a constrained version of ICA (cICA), thereby eliminating the inherent ambiguities of ICA. The results of the proposed workflow are then compared to those obtained by a widely used group ICA approach for fMRI analysis. In this study, we demonstrate that intrinsic modes, extracted by EMD, are suitable to serve as references for cICA. This approach yields typical resting-state patterns that are consistent over subjects. By introducing these reference signals into the ICA, our processing pipeline yields comparable activity patterns across subjects in a mathematically transparent manner. Our approach provides a user-friendly tool to adjust the trade-off between a high similarity across subjects and preserving individual subject features of the independent components
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