68 research outputs found
Experimental Evidence for the Incorporation of Two Metals at Equivalent Lattice Positions in Mixed-Metal MetalâOrganic Frameworks
Metalâorganic frameworks containing multiple metals distributed over crystallographically equivalent framework positions (mixedâmetal MOFs) represent an interesting class of materials, since the close vicinity of isolated metal centers often gives rise to synergistic effects. However, appropriate characterization techniques for detailed investigations of these mixedâmetal metalâorganic framework materials, particularly addressing the distribution of metals within the lattice, are rarely available. The synthesis of mixedâmetal FeCuBTC materials in direct syntheses proved to be difficult and only a thorough characterization using various techniques, like powder Xâray diffraction, Xâray absorption spectroscopy and electron paramagnetic resonance spectroscopy, unambiguously evidenced the formation of a mixedâmetal FeCuBTC material with HKUSTâ1 structure, which contained bimetallic FeâCu paddlewheels as well as monometallic CuâCu and FeâFe units under optimized synthesis conditions. The inâdepth characterization showed that other synthetic procedures led to impurities, which contained the majority of the applied iron and were impossible or difficult to identify using solely standard characterization techniques. Therefore, this study shows the necessity to characterize mixedâmetal MOFs extensively to unambiguously prove the incorporation of both metals at the desired positions. The controlled positioning of metal centers in mixedâmetal metalâorganic framework materials and the thorough characterization thereof is particularly important to derive structureâproperty or structureâactivity correlations
Rapid Prototyping of a Frequency Hopping Ad Hoc Network System
Wireless networks in-the-loop is a method to develop
software radio systems which operate in networks. It eliminates the gap between simulations and live testing by allowing the developer to use the same code for both types of testing. This allows the parallel development of signal processing and protocols in a network context without having to use intermediary tools. We demonstrate this concept using the example of frequency hopping ad hoc networks, which are both challenging on the signal processing side as well as on the network side
Robust optical delay lines via topological protection
Phenomena associated with topological properties of physical systems are
naturally robust against perturbations. This robustness is exemplified by
quantized conductance and edge state transport in the quantum Hall and quantum
spin Hall effects. Here we show how exploiting topological properties of
optical systems can be used to implement robust photonic devices. We
demonstrate how quantum spin Hall Hamiltonians can be created with linear
optical elements using a network of coupled resonator optical waveguides (CROW)
in two dimensions. We find that key features of quantum Hall systems, including
the characteristic Hofstadter butterfly and robust edge state transport, can be
obtained in such systems. As a specific application, we show that the
topological protection can be used to dramatically improve the performance of
optical delay lines and to overcome limitations related to disorder in photonic
technologies.Comment: 9 pages, 5 figures + 12 pages of supplementary informatio
Demonstration of Universal Parametric Entangling Gates on a Multi-Qubit Lattice
We show that parametric coupling techniques can be used to generate selective
entangling interactions for multi-qubit processors. By inducing coherent
population exchange between adjacent qubits under frequency modulation, we
implement a universal gateset for a linear array of four superconducting
qubits. An average process fidelity of is estimated for
three two-qubit gates via quantum process tomography. We establish the
suitability of these techniques for computation by preparing a four-qubit
maximally entangled state and comparing the estimated state fidelity against
the expected performance of the individual entangling gates. In addition, we
prepare an eight-qubit register in all possible bitstring permutations and
monitor the fidelity of a two-qubit gate across one pair of these qubits.
Across all such permutations, an average fidelity of
is observed. These results thus offer a path to a scalable architecture with
high selectivity and low crosstalk
A Green's function approach to transmission of massless Dirac fermions in graphene through an array of random scatterers
We consider the transmission of massless Dirac fermions through an array of
short range scatterers which are modeled as randomly positioned -
function like potentials along the x-axis. We particularly discuss the
interplay between disorder-induced localization that is the hallmark of a
non-relativistic system and two important properties of such massless Dirac
fermions, namely, complete transmission at normal incidence and periodic
dependence of transmission coefficient on the strength of the barrier that
leads to a periodic resonant transmission. This leads to two different types of
conductance behavior as a function of the system size at the resonant and the
off-resonance strengths of the delta function potential. We explain this
behavior of the conductance in terms of the transmission through a pair of such
barriers using a Green's function based approach. The method helps to
understand such disordered transport in terms of well known optical phenomena
such as Fabry Perot resonances.Comment: 22 double spaced single column pages. 15 .eps figure
Relativistic quantum effects of Dirac particles simulated by ultracold atoms
Quantum simulation is a powerful tool to study a variety of problems in
physics, ranging from high-energy physics to condensed-matter physics. In this
article, we review the recent theoretical and experimental progress in quantum
simulation of Dirac equation with tunable parameters by using ultracold neutral
atoms trapped in optical lattices or subject to light-induced synthetic gauge
fields. The effective theories for the quasiparticles become relativistic under
certain conditions in these systems, making them ideal platforms for studying
the exotic relativistic effects. We focus on the realization of one, two, and
three dimensional Dirac equations as well as the detection of some relativistic
effects, including particularly the well-known Zitterbewegung effect and Klein
tunneling. The realization of quantum anomalous Hall effects is also briefly
discussed.Comment: 22 pages, review article in Frontiers of Physics: Proceedings on
Quantum Dynamics of Ultracold Atom
Synaptic Vesicle Docking: Sphingosine Regulates Syntaxin1 Interaction with Munc18
Consensus exists that lipids must play key functions in synaptic activity but precise mechanistic information is limited. Acid sphingomyelinase knockout mice (ASMko) are a suitable model to address the role of sphingolipids in synaptic regulation as they recapitulate a mental retardation syndrome, Niemann Pick disease type A (NPA), and their neurons have altered levels of sphingomyelin (SM) and its derivatives. Electrophysiological recordings showed that ASMko hippocampi have increased paired-pulse facilitation and post-tetanic potentiation. Consistently, electron microscopy revealed reduced number of docked vesicles. Biochemical analysis of ASMko synaptic membranes unveiled higher amounts of SM and sphingosine (Se) and enhanced interaction of the docking molecules Munc18 and syntaxin1. In vitro reconstitution assays demonstrated that Se changes syntaxin1 conformation enhancing its interaction with Munc18. Moreover, Se reduces vesicle docking in primary neurons and increases paired-pulse facilitation when added to wt hippocampal slices. These data provide with a novel mechanism for synaptic vesicle control by sphingolipids and could explain cognitive deficits of NPA patients
Enhancement of Rydberg-mediated single-photon nonlinearities by electrically tuned Förster resonances
We demonstrate experimentally that Stark-tuned Förster resonances can be used to substantially increase the interaction between individual photons mediated by Rydberg interaction inside an optical medium. This technique is employed to boost the gain of a Rydberg-mediated single-photon transistor and to enhance the non-destructive detection of single Rydberg atoms. Furthermore, our all-optical detection scheme enables high-resolution spectroscopy of two-state Förster resonances, revealing the fine structure splitting of high-n Rydberg states and the non-degeneracy of Rydberg Zeeman substates in finite fields. We show that the âŁ50S1/2,48S1/2â©ââŁ49P1/2,48P1/2â© pair state resonance in 87Rb enables simultaneously a transistor gain G>100 and all-optical detection fidelity of single Rydberg atoms F>0.8. We demonstrate for the first time the coherent operation of the Rydberg transistor with G>2 by reading out the gate photon after scattering source photons. Comparison of the observed readout efficiency to a theoretical model for the projection of the stored spin wave yields excellent agreement and thus successfully identifies the main decoherence mechanism of the Rydberg transistor
Basal cytokeratins and their relationship to the cellular origin and functional classification of breast cancer
Recent publications have classified breast cancers on the basis of expression of cytokeratin-5 and -17 at the RNA and protein levels, and demonstrated the importance of these markers in defining sporadic tumours with bad prognosis and an association with BRCA1-related breast cancers. These important observations using different technology platforms produce a new functional classification of breast carcinoma. However, it is important in developing hypotheses about the pathogenesis of this tumour type to review the nomenclature that is being used to emphasize potential confusion between terminology that defines clinical subgroups and markers of cell lineage. This article reviews the lineages in the normal breast in relation to what have become known as the 'basal-like' carcinomas
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