We demonstrate improved operation of exchange-coupled semiconductor quantum dots by substantially reducing the sensitivity of exchange operations to charge noise. The method involves biasing a double ...dot symmetrically between the charge-state anticrossings, where the derivative of the exchange energy with respect to gate voltages is minimized. Exchange remains highly tunable by adjusting the tunnel coupling. We find that this method reduces the dephasing effect of charge noise by more than a factor of 5 in comparison to operation near a charge-state anticrossing, increasing the number of observable exchange oscillations in our qubit by a similar factor. Performance also improves with exchange rate, favoring fast quantum operations.
Silicon is more than the dominant material in the conventional microelectronics industry: it also has potential as a host material for emerging quantum information technologies. Standard fabrication ...techniques already allow the isolation of single electron spins in silicon transistor-like devices. Although this is also possible in other materials, silicon-based systems have the advantage of interacting more weakly with nuclear spins. Reducing such interactions is important for the control of spin quantum bits because nuclear fluctuations limit quantum phase coherence, as seen in recent experiments in GaAs-based quantum dots. Advances in reducing nuclear decoherence effects by means of complex control still result in coherence times much shorter than those seen in experiments on large ensembles of impurity-bound electrons in bulk silicon crystals. Here we report coherent control of electron spins in two coupled quantum dots in an undoped Si/SiGe heterostructure and show that this system has a nuclei-induced dephasing time of 360 nanoseconds, which is an increase by nearly two orders of magnitude over similar measurements in GaAs-based quantum dots. The degree of phase coherence observed, combined with fast, gated electrical initialization, read-out and control, should motivate future development of silicon-based quantum information processors.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We report on a quantum dot device design that combines the low disorder properties of undoped SiGe heterostructure materials with an overlapping gate stack in which each electrostatic gate has a ...dominant and unique function-control of individual quantum dot occupancies and of lateral tunneling into and between dots. Control of the tunneling rate between a dot and an electron bath is demonstrated over more than nine orders of magnitude and independently confirmed by direct measurement within the bandwidth of our amplifiers. The inter-dot tunnel coupling at the charge configuration anti-crossing is directly measured to quantify the control of a single inter-dot tunnel barrier gate. A simple exponential dependence is sufficient to describe each of these tunneling processes as a function of the controlling gate voltage.
Like modern microprocessors today, future processors of quantum information may be implemented using all-electrical control of silicon-based devices. A semiconductor spin qubit may be controlled ...without the use of magnetic fields by using three electrons in three tunnel-coupled quantum dots. Triple dots have previously been implemented in GaAs, but this material suffers from intrinsic nuclear magnetic noise. Reduction of this noise is possible by fabricating devices using isotopically purified silicon. We demonstrate universal coherent control of a triple-quantum-dot qubit implemented in an isotopically enhanced Si/SiGe heterostructure. Composite pulses are used to implement spin-echo type sequences, and differential charge sensing enables single-shot state readout. These experiments demonstrate sufficient control with sufficiently low noise to enable the long pulse sequences required for exchange-only two-qubit logic and randomized benchmarking.
Epitaxial growth of InAs(001) is investigated using kinetic Monte Carlo simulations based on calculations employing ab initio density functional theory (DFT). Simulated islands are elongated along ...the $ \bar 1 $10 direction. Steps along the $ \bar 1 $10 direction (A‐steps) are straight whereas steps in the perpendicular direction (B‐steps) are rough. The observed anisotropy in step structure and island morphology is correlated to the detailed microscopic processes of nucleation and growth and the underlying reconstruction of the surface. The supporting calculations employing DFT reveal low energy for steps along $ \bar 1 $10 direction (A‐steps).
Thrombomodulin Deficiency in Human Diabetic Nerve Microvasculature
Charlene E. Hafer-Macko 1 2 3 ,
Frederick M. Ivey 2 3 ,
Kymberly A. Gyure 4 ,
John D. Sorkin 2 3 and
Richard F. Macko 1 2 3
1 ...Department of Neurology, University of Maryland School of Medicine, Baltimore, Maryland
2 Department of Gerontology, University of Maryland School of Medicine, Baltimore, Maryland
3 Baltimore Veterans Administration Medical Center - Geriatrics Research, Education, and Clinical Center, Baltimore, Maryland
4 Department of Pathology, University of Maryland School of Medicine, Baltimore, Maryland
Abstract
Human diabetic neuropathy is multifactorial in etiology, with ischemia as a final common pathology. Although impaired vascular
endothelial cell function in diabetic microvascular injury is established, the role of thrombomodulin (TM)-dependent protein
C antithrombotic mechanism in the pathogenesis of neuropathy is unclear. This neuropathologic case-control study investigated
whether vascular endothelial TM expression is deficient in peripheral nerve microvessels in diabetic neuropathy. Sural nerve
biopsies from 7 patients with diabetic neuropathy and 10 with axonal neuropathy without vasculopathy were immunostained with
anti-TM and anti–von Willebrand factor (vWF; an endothelial cell marker) antibodies. The proportion of TM-positive microvessels
was expressed relative to total vWF-staining vessels, according to vessel caliber and regional distribution within the nerve.
In diabetic nerves compared with reference controls, the proportion of TM-positive endoneurial microvessels was 15-fold lower
(0.02 vs. 0.30 in diabetic nerves vs. controls, P < 0.004), and the proportion of small-caliber epineurial microvessels was 10-fold lower (0.04 vs. 0.43, P < 0.001). No TM expression was detected at the perineurium in diabetic or control nerves. We demonstrate a substantial reduction
of vascular endothelial TM expression throughout human diabetic neuropathy. These findings suggest that an impaired native
TM-dependent protein C antithrombotic mechanism may contribute to microvascular ischemia in the pathogenesis of diabetic neuropathy.
Footnotes
Address correspondence and reprint requests to Charlene Hafer-Macko, MD, Department of Neurology, University of Maryland School
of Medicine, 22 South Greene St., Baltimore, MD 21201-1595. E-mail: cmacko{at}grecc.umaryland.edu .
Received for publication 17 October 2001 and accepted in revised form 14 March 2002.
AGE, advanced glycation end product; APC, activated protein C; IL-1β, interleukin-1β; PAI-1, plasminogen activator inhibitor-1;
TM, thrombomodulin; TNF-α, tumor necrosis factor-α; VEGF, vascular endothelial growth factor; vWF, von Willebrand factor.
DIABETES
We adapt the level set method to simulate the growth of thin films described by the motion of island boundaries. This island dynamics model involves a continuum in the lateral directions, but retains ...atomic scale discreteness in the growth direction. Several choices for the island boundary velocity are discussed, and computations of the island dynamics model using the level set method are presented.
This paper describes a solid state system in which a qubit is realized as the spin of a single trapped electron in a quantum dot and read functionality is via an adjacent quantum wire with a single ...or a small number of conductive states. Because of the limited design window for this system, simulation is an important guide to an experimental search for successful designs. We use a semianalytic approximation that is accurate enough to provide meaningful results and computationally simple enough to allow high throughput, as needed for design and optimization. In particular, we find designs that achieve double pinchoff (i.e., a single trapped electron in the dot and a single conductive state in the wire). After relaxing the design requirements to allow for a small number of conductive states in the wire, we find successful designs that are optimally robust, in the sense that their success is unlikely to be affected by fabrication errors.
The development of new semiconductor device materials is currently dominated by costly and time-consuming experimental investigations in which growth parameters are varied and correlated, ex situ, to ...device characteristics. Model-based process development has the potential to assist greatly in optimizing growth parameters and providing models that are suitable for use in in-situ process control. Researchers have used this approach sparingly, however, because robust, predictive models that describe the growth of semiconductor materials are neither well-developed nor validated. This situation is changing, however, due to the availability of high-resolution microscopy and improved modeling capabilities resulting from greatly increased computer power and more efficient numerical algorithms. The focus in this paper is on III-V semiconductor molecular beam epitaxy (MBE), which is the preferred technique for growing complex heterostructure device materials with many critical interfaces. The challenge is to understand the connection between macroscopic reactor conditions and microscopic film properties such as interface thickness and morphology.
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