6,648 research outputs found
Decoherence of Majorana qubits by noisy gates
We propose and study a realistic model for the decoherence of topological
qubits, based on Majorana fermions in one-dimensional topological
superconductors. The source of decoherence is the fluctuating charge on a
capacitively coupled gate, modeled by non-interacting electrons. In this
context, we clarify the role of quantum fluctuations and thermal fluctuations
and find that quantum fluctuations do not lead to decoherence, while thermal
fluctuations do. We explicitly calculate decay times due to thermal noise and
give conditions for the gap size in the topological superconductor and the gate
temperature. Based on this result, we provide simple rules for gate geometries
and materials optimized for reducing the negative effect of thermal charge
fluctuations on the gate
Carbon nanotubes in electric and magnetic fields
We derive an effective low-energy theory for metallic (armchair and
non-armchair) single-wall nanotubes in the presence of an electric field
perpendicular to the nanotube axis, and in the presence of magnetic fields,
taking into account spin-orbit interactions and screening effects on the basis
of a microscopic tight binding model. The interplay between electric field and
spin-orbit interaction allows us to tune armchair nanotubes into a helical
conductor in both Dirac valleys. Metallic non-armchair nanotubes are gapped by
the surface curvature, yet helical conduction modes can be restored in one of
the valleys by a magnetic field along the nanotube axis. Furthermore, we
discuss electric dipole spin resonance in carbon nanotubes, and find that the
Rabi frequency shows a pronounced dependence on the momentum along the
nanotube
Modeling and experimental investigations of the stress-softening behavior of soft collagenous tissues
This paper deals with the formulation of a micro-mechanically based dam-age model for soft collagenous tissues. The model is motivated by (i) a sliding filament model proposed in the literature [1] and (ii) by experimental observations from electron microscopy (EM) images of human abdominal aorta specimens, see [2]. Specifically, we derive a continuum damage model that takes into account statistically distributed pro- teoglycan (PG) bridges. The damage model is embedded into the constitutive framework proposed by Balzani et al. [3] and adjusted to cyclic uniaxial tension tests of a hu- man carotid artery. Furthermore, the resulting damage distribution of the model after a circumferential overstretch of a simplified
arterial section is analyzed in a finite element calculation
Helical modes in carbon nanotubes generated by strong electric fields
Helical modes, conducting opposite spins in opposite directions, are shown to
exist in metallic armchair nanotubes in an all-electric setup. This is a
consequence of the interplay between spin-orbit interaction and strong electric
fields. The helical regime can also be obtained in chiral metallic nanotubes by
applying an additional magnetic field. In particular, it is possible to obtain
helical modes at one of the two Dirac points only, while the other one remains
gapped. Starting from a tight-binding model we derive the effective low-energy
Hamiltonian and the resulting spectrum
Climate model and proxy data constraints on ocean warming across the Paleocene-Eocene Thermal Maximum
Constraining the greenhouse gas forcing, climatic warming and estimates of climate sensitivity across ancient large transient warming events is a major challenge to the palaeoclimate research community. Here we provide a new compilation and synthesis of the available marine proxy temperature data across the largest of these hyperthermals, the Paleocene-Eocene Thermal Maximum (PETM). This includes the application of consistent temperature calibrations to all data, including the most recent set of calibrations for archaeal lipid-derived palaeothermometry. This compilation provides the basis for an informed discussion of the likely range of PETM warming, the biases present in the existing record and an initial assessment of the geographical pattern of PETM ocean warming. To aid interpretation of the geographic variability of the proxy-derived estimates of PETM warming, we present a comparison of this data with the patterns of warming produced by high pCO2 simulations of Eocene climates using the Hadley Centre atmosphere-ocean general circulation model (AOGCM) HadCM3L. On the basis of this comparison and taking into account the patterns of intermediate-water warming we estimate that the global mean surface temperature anomaly for the PETM is within the range of 4 to 5Ā°C
Adiabatic quantum simulations with driven superconducting qubits
We propose a quantum simulator based on driven superconducting qubits where
the interactions are generated parametrically by a polychromatic magnetic flux
modulation of a tunable bus element. Using a time-dependent Schrieffer-Wolff
transformation, we analytically derive a multi-qubit Hamiltonian which features
independently tunable and -type interactions as well as local bias
fields over a large parameter range. We demonstrate the adiabatic simulation of
the ground state of a hydrogen molecule using two superconducting qubits and
one tunable bus element. The time required to reach chemical accuracy lies in
the few microsecond range and therefore could be implemented on currently
available superconducting circuits. Further applications of this technique may
also be found in the simulation of interacting spin systems.Comment: 11 pages, 6 figure
- ā¦