132 research outputs found
Majorana bound states in a coupled quantum-dot hybrid-nanowire system
Hybrid nanowires combining semiconductor and superconductor materials appear
well suited for the creation, detection, and control of Majorana bound states
(MBSs). We demonstrate the emergence of MBSs from coalescing Andreev bound
states (ABSs) in a hybrid InAs nanowire with epitaxial Al, using a quantum dot
at the end of the nanowire as a spectrometer. Electrostatic gating tuned the
nanowire density to a regime of one or a few ABSs. In an applied axial magnetic
field, a topological phase emerges in which ABSs move to zero energy and remain
there, forming MBSs. We observed hybridization of the MBS with the end-dot
bound state, which is in agreement with a numerical model. The ABS/MBS spectra
provide parameters that are useful for understanding topological
superconductivity in this system.Comment: Article and Supplementary Materia
Anharmonicity of a Gatemon Qubit with a Few-Mode Josephson Junction
Coherent operation of gate-voltage-controlled hybrid transmon qubits
(gatemons) based on semiconductor nanowires was recently demonstrated. Here we
experimentally investigate the anharmonicity in epitaxial InAs-Al Josephson
junctions, a key parameter for their use as a qubit. Anharmonicity is found to
be reduced by roughly a factor of two compared to conventional metallic
junctions, and dependent on gate voltage. Experimental results are consistent
with a theoretical model, indicating that Josephson coupling is mediated by a
small number of highly transmitting modes in the semiconductor junction
Response to recharge variation of thin rainwater lenses and their mixing zone with underlying saline groundwater
In coastal zones with saline groundwater, fresh groundwater lenses may form due to infiltration of rain water. The thickness of both the lens and the mixing zone, determines fresh water availability for plant growth. Due to recharge variation, the thickness of the lens and the mixing zone are not constant, which may adversely affect agricultural and natural vegetation if saline water reaches the root zone during the growing season. In this paper, we study the response of thin lenses and their mixing zone to variation of recharge. The recharge is varied using sinusoids with a range of amplitudes and frequencies. We vary lens characteristics by varying the Rayleigh number and Mass flux ratio of saline and fresh water, as these dominantly influence the thickness of thin lenses and their mixing zone. Numerical results show a linear relation between the normalised lens volume and the main lens and recharge characteristics, enabling an empirical approximation of the variation of lens thickness. Increase of the recharge amplitude causes increase and the increase of recharge frequency causes a decrease in the variation of lens thickness. The average lens thickness is not significantly influenced by these variations in recharge, contrary to the mixing zone thickness. The mixing zone thickness is compared to that of a Fickian mixing regime. A simple relation between the travelled distance of the centre of the mixing zone position due to variations in recharge and the mixing zone thickness is shown to be valid for both a sinusoidal recharge variation and actual records of daily recharge data. Starting from a step response function, convolution can be used to determine the effect of variable recharge in time. For a sinusoidal curve, we can determine delay of lens movement compared to the recharge curve as well as the lens amplitude, derived from the convolution integral. Together the proposed equations provide us with a first order approximation of lens characteristics using basic lens and recharge parameters without the use of numerical models. This enables the assessment of the vulnerability of any thin fresh water lens on saline, upward seeping groundwater to salinity stress in the root zone
Supercurrent transport through 1-periodic full-shell Coulomb islands
We experimentally investigate supercurrent through Coulomb islands, where
island and leads are fabricated from semiconducting nanowires with fully
surrounding superconducting shells. Applying flux along the wire yields a
series of destructive Little-Parks lobes with reentrant supercurrent. We find
Coulomb blockade with 2 peak spacing in the zeroth lobe and 1 average
spacing, with regions of significant even-odd modulation, in the first lobe.
Evolution of Coulomb-peak amplitude through the first lobe is consistent with a
theoretical model of supercurrent carried predominantly by zero-energy states
in the leads and the island.Comment: 11 pages, 5+5 figure
A globally applicable framework for compound flood hazard modeling
Coastal river deltas are susceptible to flooding from pluvial,
fluvial, and coastal flood drivers. Compound floods, which result from the
co-occurrence of two or more of these drivers, typically exacerbate impacts
compared to floods from a single driver. While several global flood models
have been developed, these do not account for compound flooding. Local-scale
compound flood models provide state-of-the-art analyses but are hard to
scale to other regions as these typically are based on local datasets.
Hence, there is a need for globally applicable compound flood hazard
modeling. We develop, validate, and apply a framework for compound flood
hazard modeling that accounts for interactions between all drivers. It
consists of the high-resolution 2D hydrodynamic Super-Fast INundation of CoastS (SFINCS) model, which is
automatically set up from global datasets and coupled with a global
hydrodynamic river routing model and a global surge and tide model. To test
the framework, we simulate two historical compound flood events, Tropical
Cyclone Idai and Tropical Cyclone Eloise in the Sofala province of Mozambique, and compare
the simulated flood extents to satellite-derived extents on multiple days
for both events. Compared to the global CaMa-Flood model, the
globally applicable model generally performs better in terms of the critical
success index (−0.01–0.09) and hit rate (0.11–0.22) but worse in
terms of the false-alarm ratio (0.04–0.14). Furthermore, the simulated flood
depth maps are more realistic due to better floodplain connectivity and
provide a more comprehensive picture as direct coastal flooding and pluvial flooding
are simulated. Using the new framework, we determine the dominant flood
drivers and transition zones between flood drivers. These vary significantly
between both events because of differences in the magnitude of and time lag
between the flood drivers. We argue that a wide range of plausible events
should be investigated to obtain a robust understanding of compound flood
interactions, which is important to understand for flood adaptation,
preparedness, and response. As the model setup and coupling is automated,
reproducible, and globally applicable, the presented framework is a
promising step forward towards large-scale compound flood hazard modeling.</p
Top-transmon: hybrid superconducting qubit for parity-protected quantum computation
Qubits constructed from uncoupled Majorana fermions are protected from
decoherence, but to perform a quantum computation this topological protection
needs to be broken. Parity-protected quantum computation breaks the protection
in a minimally invasive way, by coupling directly to the fermion parity of the
system --- irrespective of any quasiparticle excitations. Here we propose to
use a superconducting charge qubit in a transmission line resonator (a socalled
transmon) to perform parity-protected rotations and read-out of a topological
(top) qubit. The advantage over an earlier proposal using a flux qubit is that
the coupling can be switched on and off with exponential accuracy, promising a
reduced sensitivity to charge noise.Comment: 7 pages, 5 figure
Introduction to topological superconductivity and Majorana fermions
This short review article provides a pedagogical introduction to the rapidly
growing research field of Majorana fermions in topological superconductors. We
first discuss in some details the simplest "toy model" in which Majoranas
appear, namely a one-dimensional tight-binding representation of a p-wave
superconductor, introduced more than ten years ago by Kitaev. We then give a
general introduction to the remarkable properties of Majorana fermions in
condensed matter systems, such as their intrinsically non-local nature and
exotic exchange statistics, and explain why these quasiparticles are suspected
to be especially well suited for low-decoherence quantum information
processing. We also discuss the experimentally promising (and perhaps already
successfully realized) possibility of creating topological superconductors
using semiconductors with strong spin-orbit coupling, proximity-coupled to
standard s-wave superconductors and exposed to a magnetic field. The goal is to
provide an introduction to the subject for experimentalists or theorists who
are new to the field, focusing on the aspects which are most important for
understanding the basic physics. The text should be accessible for readers with
a basic understanding of quantum mechanics and second quantization, and does
not require knowledge of quantum field theory or topological states of matter.Comment: 21 pages, 5 figure
Charge transport through single molecules, quantum dots, and quantum wires
We review recent progresses in the theoretical description of correlation and
quantum fluctuation phenomena in charge transport through single molecules,
quantum dots, and quantum wires. A variety of physical phenomena is addressed,
relating to co-tunneling, pair-tunneling, adiabatic quantum pumping, charge and
spin fluctuations, and inhomogeneous Luttinger liquids. We review theoretical
many-body methods to treat correlation effects, quantum fluctuations,
nonequilibrium physics, and the time evolution into the stationary state of
complex nanoelectronic systems.Comment: 48 pages, 14 figures, Topical Review for Nanotechnolog
Determining Electric Fields in Thunderclouds With the Radiotelescope LOFAR
An analysis is presented of electric fields in thunderclouds using a recently proposed method based on measuring radio emission from extensive air shower events during thunderstorm conditions. This method can be regarded as a tomography of thunderclouds using cosmic rays as probes. The data cover the period from December 2011 till August 2014. We have developed an improved fitting procedure to be able to analyze the data. Our measurements show evidence for the main negative-charge layer near the −10° isotherm. This we have seen for a winter as well as for a summer cloud where multiple events pass through the same cloud and also the vertical component of the electric field could be reconstructed. On the day of measurement of some cosmic-ray events showing evidence for strong fields, no lightning activity was detected within 100 km distance. For the winter events, the top heights were between 5 and 6 km, while in the summer, typical top heights of 9 km were seen. Large horizontal components in excess of 70 kV/m of the electric fields are observed in the middle and top layers
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