624 research outputs found
Stability of networks of delay-coupled delay oscillators
Dynamical networks with time delays can pose a considerable challenge for
mathematical analysis. Here, we extend the approach of generalized modeling to
investigate the stability of large networks of delay-coupled delay oscillators.
When the local dynamical stability of the network is plotted as a function of
the two delays then a pattern of tongues is revealed. Exploiting a link between
structure and dynamics, we identify conditions under which perturbations of the
topology have a strong impact on the stability. If these critical regions are
avoided the local stability of large random networks can be well approximated
analytically
Microscopy of a scalable superatom
Strong interactions can amplify quantum effects such that they become
important on macroscopic scales. Controlling these coherently on a single
particle level is essential for the tailored preparation of strongly correlated
quantum systems and opens up new prospects for quantum technologies. Rydberg
atoms offer such strong interactions which lead to extreme nonlinearities in
laser coupled atomic ensembles. As a result, multiple excitation of a
Micrometer sized cloud can be blocked while the light-matter coupling becomes
collectively enhanced. The resulting two-level system, often called
"superatom", is a valuable resource for quantum information, providing a
collective Qubit. Here we report on the preparation of two orders of magnitude
scalable superatoms utilizing the large interaction strength provided by
Rydberg atoms combined with precise control of an ensemble of ultracold atoms
in an optical lattice. The latter is achieved with sub shot noise precision by
local manipulation of a two-dimensional Mott insulator. We microscopically
confirm the superatom picture by in-situ detection of the Rydberg excitations
and observe the characteristic square root scaling of the optical coupling with
the number of atoms. Furthermore, we verify the presence of entanglement in the
prepared states and demonstrate the coherent manipulation of the superatom.
Finally, we investigate the breakdown of the superatom picture when two Rydberg
excitations are present in the system, which leads to dephasing and a loss of
coherence.Comment: 7 pages, 5 figure
Quantum Fiel Theoretic Treatment of the Non-Forward Compton Amplitude in the Generalized Bjorken Region
A quantum field theoretic treatment of the leading light-cone part of the
virtual Compton amplitude is presented. The twist-decomposition of the
operators is performed by a group-theoretic procedure respecting the Lorentz
group O(3,1). The twist-2 contributions to the Compton amplitude are calculated
and it is shown that the electromagnetic current is conserved for these terms.
Relations between the amplitude functions associated to the symmetric and
asymmetric part of the Compton amplitude are derived. These relations
generalize the Callan-Gross and Wandzura-Wilczek relations of forward
scattering for the non-forward Compton amplitude.Comment: 7 pages LATEX, 1 style file, DESY 00-045, Contribution to the
Proceedings of `Loops and Legs in Quantum Field Theory', Bastei, Germany,
April 2000, Nucl. Phys. B (Proc. Suppl.) (2000) to appea
Spatially Resolved Detection of a Spin-Entanglement Wave in a Bose-Hubbard Chain
Entanglement is an essential property of quantum many-body systems. However,
its local detection is challenging and was so far limited to spin degrees of
freedom in ion chains. Here we measure entanglement between the spins of atoms
located on two lattice sites in a one-dimensional Bose-Hubbard chain which
features both local spin- and particle-number fluctuations. Starting with an
initially localized spin impurity, we observe an outwards propagating
entanglement wave and show quantitatively how entanglement in the spin sector
rapidly decreases with increasing particle-number fluctuations in the chain.Comment: 6 pages, 4 figure
Coherent many-body spin dynamics in a long-range interacting Ising chain
Coherent many-body quantum dynamics lies at the heart of quantum simulation
and quantum computation. Both require coherent evolution in the exponentially
large Hilbert space of an interacting many-body system. To date, trapped ions
have defined the state of the art in terms of achievable coherence times in
interacting spin chains. Here, we establish an alternative platform by
reporting on the observation of coherent, fully interaction-driven quantum
revivals of the magnetization in Rydberg-dressed Ising spin chains of atoms
trapped in an optical lattice. We identify partial many-body revivals at up to
about ten times the characteristic time scale set by the interactions. At the
same time, single-site-resolved correlation measurements link the magnetization
dynamics with inter-spin correlations appearing at different distances during
the evolution. These results mark an enabling step towards the implementation
of Rydberg atom based quantum annealers, quantum simulations of higher
dimensional complex magnetic Hamiltonians, and itinerant long-range interacting
quantum matter.Comment: 11 pages, 9 figure
HOW DO UNIVERSITY STUDENTS SELECT AND USE THEIR LEARNING TOOLS? A MIXED-METHOD STUDY ON PERSONALISED LEARNING (21)
Universities often blend traditional learning and e-learning by providing software licenses, electronic learning materials, and access to Learning Management Systems. Following the idea of personalised learning in higher education, students are free to choose between a wide range of learning tools constructing their Personalised Learning Environment. However, the characteristics of the chosen tools need to match the characteristics of the learning tasks to support students adequately. In the present paper, a mixed-method approach is used to analyse which types of tools are used in practice and which types of learning tasks are performed using these learning tools. Furthermore, important factors influencing the decision to select learning tools are identified. This study shows that a wide array of learning tools is used in practice. Although students consider individual factors (such as perceived ease of use and task-technology fit) to be most important when selecting their tools, several exogenous factors such as the lecturers’ targeted pedagogy, social norm and the occurrence of higher order thinking skills limit the range of adequate learning tools
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