1,216 research outputs found
Fidelity and visibility reduction in Majorana qubits by entanglement with environmental modes
We study the dynamics and readout of topological qubits encoded by
zero-energy Majorana bound states in a topological superconductor. We take into
account bosonic modes due to the electromagnetic environment which couple the
Majorana manifold to above-gap continuum quasi-particles. This coupling causes
the degenerate ground state of the topological superconductor to be dressed in
a polaron-like manner by quasi-particle states and bosons, and the system to
become gapless. Topological protection and hence full coherence is only
maintained if the qubit is operated and read out within the low-energy spectrum
of the dressed states. We discuss reduction of fidelity and/or visibility if
this condition is violated by a quantum-dot readout that couples to the bare
(undressed) Majorana modes. For a projective measurement of the bare Majorana
basis, we formulate a Bloch-Redfield approach that is valid for weak
Majorana-environment coupling and takes into account constraints imposed by
fermion-number-parity conservation. Within the Markovian approximation, our
results essentially confirm earlier theories of finite-temperature decoherence
based on Fermi's golden rule. However, the full non-Markovian dynamics reveals,
in addition, the fidelity reduction by a projective measurement. Using a
spinless nanowire model with -wave pairing, we provide quantitative results
characterizing these effects.Comment: 18 pages, 10 figure
Design, theory, and measurement of a polarization insensitive absorber for terahertz imaging
We present the theory, design, and realization of a polarization-insensitive
metamaterial absorber for terahertz frequencies. We derive
geometrical-independent conditions for effective medium absorbers in general,
and for resonant metamaterials specically. Our fabricated design reaches and
absorptivity of 78% at 1.145 ThzComment: 6 Pages, 5 figures; figures update
Parity-to-charge conversion in Majorana qubit readout
We study the time-dependent effect of Markovian readout processes on Majorana
qubits whose parity degrees of freedom are converted into the charge of a
tunnel-coupled quantum dot. By applying a recently established effective
Lindbladian approximation [1-3], we obtain a completely positive and trace
preserving Lindblad master equation for the combined dot-qubit dynamics,
describing relaxation and decoherence processes beyond the rotating-wave
approximation. This approach is applicable to a wide range of weakly coupled
environments representing experimentally relevant readout devices. We study in
detail the case of thermal decay in the presence of a generic Ohmic bosonic
bath, in particular for potential fluctuations in an electromagnetic circuit.
In addition, we consider the nonequilibrium measurement environment for a
parity readout using a quantum point contact capacitively coupled to the dot
charge.Comment: References updated in v2. 21 pages, 9 figure
NASA SPLICE Project: Development and Testing of Precision Landing GN&C Technologies
NASA's technology advancement needs for entry, descent and landing call for high-precision, high-rate sensors that can improve navigation accuracy and vehicle control performance. Higher landing accuracy is required for any future human lander missions, and likely, for most robotic missions 1,2. Sensors and algorithms that significantly reduce navigation errors and can image the local terrain will enable landing at locations of high scientific interest that would otherwise pose significant risk to the vehicle. The Safe and Precise Landing-Integrated Capabilities Evolution project, or SPLICE, is developing precision landing and hazard avoidance (PL&HA) technologies for NASA and for potential commercial space flight missions. SPLICE technologies include sensors, algorithms, advanced space flight computing capabilities, and simulation tools used to integrate and study guidance, navigation, and control (GN&C) system performance. SPLICE efforts include hardware-in-the-loop (HWIL) simulation testing, ground testing, and flight testing, including reuse of hardware from the CoOperative Blending of Autonomous Landing Technologies (COBALT) suborbital flight-test payload3,4. Two of the precise navigation sensors that are being developed and matured within SPLICE are LiDARs. Since 2006, NASA Langley has been developing a Navigation Doppler LiDAR (NDL) for precise velocity measurements, and SPLICE is building an NDL engineering test unit (ETU) that will be brought up to TRL 6 following environmental and high-speed1,2 testing. NASA Goddard is developing a Hazard Detection LiDAR (HD LiDAR) engineering development unit (EDU) for SPLICE that has relevance to future human and robotic lander missions. The HD LiDAR will be flight test and matured to TRL 5
In high glucose protein kinase C-ζ activation is required for mesangial cell generation of reactive oxygen species
In high glucose protein kinase C-ζ activation is required for mesangial cell generation of reactive oxygen species.BackgroundWe postulated that in mesangial cells exposed to high glucose, protein kinase C-ζ (PKC-ζ) is necessary for the generation of reactive oxygen species (ROS) by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and that the requirement of PKC-ζ for filamentous (F)-actin disassembly may involve ROS. To identify signaling mechanisms relevant to PKC-ζ activation and ROS generation, including phosphoinositide 3 kinase (PI3 kinase), we examined mesangial cell stimulation with platelet-derived growth factor (PDGF).MethodsIn primary rat mesangial cells cultured in 5.6mmol/L or 30mmol/L D-glucose, PKC-ζ expression was identified with immunoblotting and activity was analyzed in cell membrane immunoprecipitates and by confocal immunofluorescence imaging. ROS generation was measured by dichlorofluorescein fluorescence using confocal microscopy and was inhibited by transfection of antisense against NADPH subunits p22phox or p47phox or with Tempol. F-actin disassembly was observed by dual-channel confocal fluorescence imaging. PI3 kinase activity was detected by immunoblotting of phosphorylated Akt.ResultsIn high glucose, generation of NADPH oxidase-dependent ROS was dependent on PKC-ζ. Conversely, sustained PKC-ζ activity was dependent on ROS generation, suggesting a positive feedback. PKC-ζ-dependent F-actin disassembly in high glucose required ROS generation. PDGF stimulated NADPH oxidase generation of ROS through a PKC-ζ mechanism that was independent of Akt phosphorylation and remained unchanged in high glucose.ConclusionIn high glucose, mesangial cell PKC-ζ is required for ROS generation from NADPH oxidase similar to PDGF stimulation of PKC-ζ-dependent ROS generation through a pathway independent of PI3 kinase. F-actin disassembly in high glucose also requires ROS. A positive feedback loop occurs between ROS and the activation of PKC-ζ in high glucose
Rosiglitazone Prevents High Glucose-Induced Vascular Endothelial Growth Factor and Collagen IV Expression in Cultured Mesangial Cells
Peroxisome proliferator-activated receptor (PPARγ), a ligand-dependent transcription factor, negatively modulates high glucose effects. We postulated that rosiglitazone (RSG), an activator of PPARγ prevents the upregulation of vascular endothelial growth factor (VEGF) and collagen IV by mesangial cells exposed to high glucose. Primary cultured rat mesangial cells were growth-arrested in 5.6 mM (NG) or 25 mM D-glucose (HG) for up to 48 hours. In HG, PPARγ mRNA and protein were reduced within 3 h, and enhanced ROS generation, expression of p22phox, VEGF and collagen IV, and PKC-ζ membrane association were prevented by RSG. In NG, inhibition of PPARγ caused ROS generation and VEGF expression that were unchanged by RSG. Reduced AMP-activated protein kinase (AMPK) phosphorylation in HG was unchanged with RSG, and VEGF expression was unaffected by AMPK inhibition. Hence, PPARγ is a negative modulator of HG-induced signaling that acts through PKC-ζ but not AMPK and regulates VEGF and collagen IV expression by mesangial cells
Probing Solar Convection
In the solar convection zone acoustic waves are scattered by turbulent sound
speed fluctuations. In this paper the scattering of waves by convective cells
is treated using Rytov's technique. Particular care is taken to include
diffraction effects which are important especially for high-degree modes that
are confined to the surface layers of the Sun. The scattering leads to damping
of the waves and causes a phase shift. Damping manifests itself in the width of
the spectral peak of p-mode eigenfrequencies. The contribution of scattering to
the line widths is estimated and the sensitivity of the results on the assumed
spectrum of the turbulence is studied. Finally the theoretical predictions are
compared with recently measured line widths of high-degree modes.Comment: 26 pages, 7 figures, accepted by MNRA
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