Coordinate Singularities in Harmonically-sliced Cosmologies

Harmonic slicing has in recent years become a standard way of prescribing the lapse function in numerical simulations of general relativity. However, as was first noticed by Alcubierre (1997), numerical solutions generated using this slicing condition can show pathological behaviour. In this paper, analytic and numerical methods are used to examine harmonic slicings of Kasner and Gowdy cosmological spacetimes. It is shown that in general the slicings are prevented from covering the whole of the spacetimes by the appearance of coordinate singularities. As well as limiting the maximum running times of numerical simulations, the coordinate singularities can lead to features being produced in numerically evolved solutions which must be distinguished from genuine physical effects.Comment: 21 pages, REVTeX, 5 figure

On the efficient Monte Carlo implementation of path integrals

We demonstrate that the Levy-Ciesielski implementation of Lie-Trotter products enjoys several properties that make it extremely suitable for path-integral Monte Carlo simulations: fast computation of paths, fast Monte Carlo sampling, and the ability to use different numbers of time slices for the different degrees of freedom, commensurate with the quantum effects. It is demonstrated that a Monte Carlo simulation for which particles or small groups of variables are updated in a sequential fashion has a statistical efficiency that is always comparable to or better than that of an all-particle or all-variable update sampler. The sequential sampler results in significant computational savings if updating a variable costs only a fraction of the cost for updating all variables simultaneously or if the variables are independent. In the Levy-Ciesielski representation, the path variables are grouped in a small number of layers, with the variables from the same layer being statistically independent. The superior performance of the fast sampling algorithm is shown to be a consequence of these observations. Both mathematical arguments and numerical simulations are employed in order to quantify the computational advantages of the sequential sampler, the Levy-Ciesielski implementation of path integrals, and the fast sampling algorithm.Comment: 14 pages, 3 figures; submitted to Phys. Rev.

A Robot Model of OC-Spectrum Disorders : Design Framework, Implementation and First Experiments

© 2019 Massachusetts Institute of TechnologyComputational psychiatry is increasingly establishing itself as valuable discipline for understanding human mental disorders. However, robot models and their potential for investigating embodied and contextual aspects of mental health have been, to date, largely unexplored. In this paper, we present an initial robot model of obsessive-compulsive (OC) spectrum disorders based on an embodied motivation-based control architecture for decision making in autonomous robots. The OC family of conditions is chiefly characterized by obsessions (recurrent, invasive thoughts) and/or compulsions (an urge to carry out certain repetitive or ritualized behaviors). The design of our robot model follows and illustrates a general design framework that we have proposed to ground research in robot models of mental disorders, and to link it with existing methodologies in psychiatry, and notably in the design of animal models. To test and validate our model, we present and discuss initial experiments, results and quantitative and qualitative analysis regarding the compulsive and obsessive elements of OC-spectrum disorders. While this initial stage of development only models basic elements of such disorders, our results already shed light on aspects of the underlying theoretical model that are not obvious simply from consideration of the model.Peer reviewe

Evaluation der Forschungspreis-Programme der Alexander von Humboldt-Stiftung. Endbericht.

Die Alexander von Humboldt-Stiftung (AvH) verleiht seit 1972 Humboldt-Forschungspreise. Mit dem Preis werden hochrenommierte Wissenschaftler/innen aus dem Ausland für ihr wissenschaftliches Gesamtschaffen ausgezeichnet. Zusätzlich vergibt die AvH seit 2001 Wilhelm Friedrich Bessel-Forschungspreise an jüngere Wissenschaftler/innen, die ihre Promotion vor nicht mehr als 18 Jahren abgeschlossen haben und die in ihren wissenschaftlichen Spezialgebieten bereits als „kommende Stars“ wahrgenommen werden. Die Nominierung der Kandidat/inn/en für die Forschungspreise erfolgt durch Wissenschaftler/innen in Deutschland. Mit der Preisverleihung werden die Preisträger/innen zusätzlich zu einem Aufenthalt nach Deutschland von bis zu einem Jahr einladen, um in Kooperation mit Fachkolleg/inn/en in Deutschland ein selbst gewähltes Forschungsvorhaben durchzuführen. Zweck der Evaluation war es zu überprüfen, inwieweit es der Humboldt-Stiftung gelungen ist, mit der Durchführung die angestrebten Ziele der Forschungspreis-Programme zu erreichen: • Erstens soll mit den Programmen die deutsche Forschungslandschaft gestärkt werden, indem Kooperationen zwischen exzellenten ausländischen und deutschen Wissenschaftler/innen gefördert werden. • Zweitens soll durch den Aufbau und die Vertiefung der persönlichen Netzwerke der Preisträger/innen mit Wissenschaftler/innen in Deutschland (und im Humboldt-Netzwerk) eine nachhaltige persönliche Bindung der wissenschaftlichen Eliten im Ausland an Deutschland erreicht werden. • Schließlich sollen drittens die Preisprogramme dazu beitragen, ein modernes und differenziertes Deutschlandbild zu vermitteln und zu verbreiten

A spin qubit in a fin field-effect transistor

Quantum computing's greatest challenge is scaling up. Several decades ago, classical computers faced the same problem and a single solution emerged: very-large-scale integration using silicon. Today's silicon chips consist of billions of field-effect transistors (FinFETs) in which current flow along the fin-shaped channel is controlled by wrap-around gates. The semiconductor industry currently employs fins of sub-10$\,$nm width, small enough for quantum applications: at low temperature, an electron or hole can be trapped under the gate and serve as a spin qubit. An attractive benefit of silicon's advantageous scaling properties is that quantum hardware and its classical control circuitry can be integrated in the same package. This, however, requires qubit operation at temperatures greater than 1$\,$K where the cooling is sufficient to overcome the heat dissipation. Here, we demonstrate that a silicon FinFET is an excellent host for spin qubits that operate even above 4$\,$K. We achieve fast electrical control of hole spins with driving frequencies up to 150$\,$MHz and single-qubit gate fidelities at the fault-tolerance threshold. The number of spin rotations before coherence is lost at these "hot" temperatures already matches or exceeds values on hole spin qubits at mK temperatures. While our devices feature both industry compatibility and quality, they are fabricated in a flexible and agile way to accelerate their development. This work paves the way towards large-scale integration of all-electrical and ultrafast spin qubits

A compact and versatile cryogenic probe station for quantum device testing

Fast feedback from cryogenic electrical characterization measurements is key for the development of scalable quantum computing technology. At room temperature, high-throughput device testing is accomplished with a probe-based solution, where electrical probes are repeatedly positioned onto devices for acquiring statistical data. In this work we present a probe station that can be operated from room temperature down to below 2$\,$K. Its small size makes it compatible with standard cryogenic measurement setups with a magnet. A large variety of electronic devices can be tested. Here, we demonstrate the performance of the prober by characterizing silicon fin field-effect transistors as a host for quantum dot spin qubits. Such a tool can massively accelerate the design-fabrication-measurement cycle and provide important feedback for process optimization towards building scalable quantum circuits

Silicon quantum dot devices with a self-aligned second gate layer

We implement silicon quantum dot devices with two layers of gate electrodes using a self-alignment technique, which allows for ultra-small gate lengths and intrinsically perfect layer-to-layer alignment. In a double quantum dot system, we investigate hole transport and observe current rectification due to Pauli spin blockade. Magnetic field measurements indicate that hole spin relaxation is dominated by spin-orbit interaction, and enable us to determine the effective hole $g$-factor $\simeq1.6$. From an avoided singlet-triplet crossing, occurring at high magnetic field, the spin-orbit coupling strength $\simeq0.27$meV is obtained, promising fast and all-electrical spin control

Selective hepatitis B and D virus entry inhibitors from the group of pentacyclic lupane-type betulin-derived triterpenoids

Current treatment options against hepatitis B and D virus (HBV/HDV) infections have only limited curative effects. Identification of Na+/taurocholate co-transporting polypeptide (NTCP) as the high-affinity hepatic receptor for both viruses in 2012 enables target-based development of HBV/HDV cell-entry inhibitors. Many studies already identified appropriate NTCP inhibitors. However, most of them interfere with NTCP's physiological function as a hepatic bile acid transporter. To overcome this drawback, the present study aimed to find compounds that specifically block HBV/HDV binding to NTCP without affecting its transporter function. A novel assay was conceptualized to screen for both in parallel; virus binding to NTCP (measured via binding of a preS1-derived peptide of the large HBV/HDV envelope protein) and bile acid transport via NTCP. Hits were subsequently validated by in vitro HDV infection studies using NTCP-HepG2 cells. Derivatives of the birch-derived pentacyclic lupane-type triterpenoid betulin revealed clear NTCP inhibitory potency and selectivity for the virus receptor function of NTCP. Best performing compounds in both aspects were 2, 6, 19, and 25. In conclusion, betulin derivatives show clear structure-activity relationships for potent and selective inhibition of the HBV/HDV virus receptor function of NTCP without tackling its physiological bile acid transport function and therefore are promising drug candidates.Peer reviewe