Search

Silicon-based microelectronics technology is extremely mature, yet this profoundly important material is now also poised to become a foundation for quantum information processing technologies. In this article, we review the properties of silicon that have made it the material of choice for semiconductor-based qubits with an emphasis on the role that modeling and simulation have played in understanding and quantifying these properties. We also address some of the challenges that remain for silicon qubits for which modeling can play an important role and we offer our perspective on the future of this technology.

Silicon-quantum-dot qubits must contend with low-lying valley excited states that are sensitive functions of the quantum-well heterostructure and disorder; quantifying and maximizing the energies of these states are critical to improving device performance. We describe a spectroscopic method for probing excited states in isolated $\mathrm{Si}$/$\mathrm{Si}$-$\mathrm{Ge}$ double quantum dots using standard baseband pulsing techniques, easing the extraction of energy spectra in multiple-dot devices. We use this method to measure dozens of valley excited-state energies spanning multiple wafers, quantum dots, and orbital states, which are crucial for evaluating the dependence of valley splitting on quantum well width and other epitaxial conditions. Our results suggest that narrower wells can be beneficial for increasing valley splittings, but this effect can be confounded by variations in growth and fabrication conditions. These results underscore the importance of valley-splitting measurements for guiding the development of $\mathrm{Si}$ qubits.

Singlet-triplet spin states of a quantum dot support promising semiconductor-based qubits, yet often suffer from poor state preparation and measurement, due to low-lying excited states. Engineering a large energy splitting is impeded by the inability to accurately measure both large and small energy splittings with the device biased to nominal operation. The authors present a measurement and fitting approach that accurately extracts both large and small splittings in this regime. They also find evidence that both orbital and valley degrees of freedom may set this energy separation, significantly affecting which paths to pursue in device design.

Treatment of congenital adrenal hyperplasia (CAH) in childhood focuses on growth and development and adult final height (FH) is a measure of effective treatment. We hypothesized that shorter adults will have more severe underlying disease and worse health outcomes.This was a cross-sectional analysis of 199 adults with CAH. FH and quality of life were expressed as z-scores adjusted for midparental target height or UK population height.FH correlated inversely with age (men, r = -0.38; women, r = -0.26, P.01). Men and women had z-scores adjusted for midparental target height of -2 and -1, respectively, and both groups had UK population height z-scores of -1 below the UK population (P.01). In women, FH was shorter in non-salt-wasting than salt-wasting classic CAH (P.05) and in moderately affected genotype group B women than either more severely affected groups null and A (P.01) or the mildest group C (P.001). Short stature and a higher prevalence of hypertension were observed in classic CAH patients diagnosed late (after 1 y) compared with those diagnosed early and in women treated with glucocorticoid only compared with those treated with both glucocorticoids and mineralocorticoids (P.05). FH did not associate with insulin sensitivity, lipid profile, adiposity, or quality of life.Adult CAH patients remain short, although height prognosis has improved over time. The shortest adults are those diagnosed late with moderate severity CAH and are at increased risk of adult hypertension; we hypothesize that these patients are exposed in childhood to high androgens and/or excessive glucocorticoids with potential programming of hypertension. Another possibility is inadequate mineralocorticoid treatment early in life in the late-diagnosed patient group. Prospective studies are now required to examine these hypotheses.

Electrically defined silicon-based qubits are expected to show improved quantum memory characteristics in comparison with GaAs-based devices due to reduced hyperfine interactions with nuclear spins. Silicon-based qubit devices have proved more challenging to build than their GaAs-based counterparts, but recently several groups have reported substantial progress in single-qubit initialization, measurement, and coherent operation. We report coherent control of electron spins in two coupled quantum dots in an undoped Si/SiGe heterostructure, forming two levels of a singlet-triplet qubit. We measure a nuclei-induced T2* of 365 {plus minus} 11 ns, an increase over similar measurements in GaAs-based quantum dots by nearly two orders of magnitude. This value for T2* is consistent with theoretical expectations for our estimated dot sizes and a natural abundance of 29Si.

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 (0,2)(1,1) 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., Comment: 4 pages, 5 figures

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 anti-crossings, 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 five in comparison to operation near a charge-state anti-crossing, increasing the number of observable exchange oscillations in our qubit by a similar factor. Performance also improves with exchange rate, favoring fast quantum operations., 10 pages, 8 figures

We report the first experimental demonstration of a lateral graphene heterostructure field-effect transistor (HFET) at wafer scale, where the graphene heterostructure channel consists of epitaxial graphene (Gr)/fluorographene (GrF)/graphene (Gr). GrF is a widebandgap material, providing a potential barrier to lateral carrier transport. Gate bias modulation of the Gr/GrF/Gr barrier via an electric field effect results in normally-off enhancement-mode graphene HFETs with an ON-OFF switching ratio of 105 at room temperature. These devices also demonstrate excellent current-voltage saturation, providing a potential path for active RF applications.

Three coupled quantum dots in isotopically purified silicon enable all-electrical qubit control with long coherence time., 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.

The structure of InAs(0 0 1)-(2×4) surfaces equilibrated under typical MBE conditions is studied by scanning tunneling microscopy (STM). Depending on the magnitude of the As flux, typical surfaces are found to contain a mixture of α2(2×4) and β2(2×4) reconstructions. The relative populations of the α2 and β2 reconstructions are found to depend on substrate temperature and the magnitude of the As flux. The atomic-scale details of the reconstructed units on these mixed-phase surfaces are definitively determined by comparing atomic-resolution dual-bias STM images to first-principles calculations. The imaging mechanism for revealing atomic-scale details, particularly the trench dimer, is found to be qualitatively similar to that for GaAs, although the effect is less pronounced. Additionally, a significant population of ad-atom related structures are observed on quenched surfaces, apparently unrelated to any equilibrium ad-atom population.

Current treatment in Europe for adrenal insufficiency in neonates and infants is unsatisfactory as unlicensed adult dosage formulations are used. These are difficult to administer and may give rise to inconsistencies in dose as the content uniformity of the dosage form cannot be verified. As there is no licensed hydrocortisone formulation for children < 6 years hydrocortisone is often compounded by pharmacies using adult hydrocortisone tablets. In a recent study of compounded hydrocortisone up to 20% of the batches did not meet European Pharmacopeial accuracy and precision criteria (ECE 2014 Abstract #1278). This medication safety study investigated hydrocortisone individually and extemporaneously compounded for paediatric use in adrenally insufficient patients, meaning the current therapy is inadequate in up to every 5th child treated. Thus, there is a need for specifically designed and licensed hydrocortisone formulations for this vulnerable paediatric patient group especially neonates and infants.

We demonstrate double quantum dots fabricated in undoped Si/SiGe heterostructures relying on a double top-gated design. Charge sensing shows that we can reliably deplete these devices to zero charge occupancy. Measurements and simulations confirm that the energetics are determined by the gate-induced electrostatic potentials. Pauli spin blockade has been observed via transport through the double dot in the two electron configuration, a critical step in performing coherent spin manipulations in Si., Comment: 4 pages, 4 figures

We report on the fabrication and characterization of a few-electron quantum dot controlled by a single gate electrode. Our device has a double-quantum-well design, in which the doping controls the occupancy of the lower well while the upper well remains empty under the free surface. A small air-bridged gate contacts the surface, and is positively biased to draw laterally confined electrons into the upper well. Electrons tunneling between this accumulation-mode dot and the lower well are detected using a quantum point contact (QPC), located slightly offset from the dot gate. The charge state of the dot is measured by monitoring the differential transconductance of the QPC near pinch-off. Addition spectra starting with N=0 were observed as a function of gate voltage. DC sensitivity to single electrons was determined to be as high as 8.6%, resulting in a signal-to-noise ratio of ~9:1 with an equivalent noise bandwidth of 12.1 kHz. Analysis of random telegraph signals associated with the zero to one electron transition allowed a measurement of the lifetimes for the filled and empty states of the one-electron dot: 0.38 ms and 0.22 ms, respectively, for a device with a 10 nm AlInAs tunnel barrier between the two wells., 3 pages, 3 figures

Summary form only given. Molecular beam epitaxial growth on III-V substrates has been optimized for [100] oriented growth. However, spintronic devices may benefit from growth of quantum structures on [110] oriented substrates because the D'yakanov-Perel spin relaxation mechanism is suppressed or absent in this orientation for 2-D systems. Consistent with this hypothesis, we have recently measured a significant enhancement of the spin relaxation time in a [110]-oriented, short period, InAs/GaSb superlattice relative to a similar [001]-oriented structure. The spin dynamics were measured in the two superlattices using 100 fs mid-infrared pulses from a synchronously pumped PPLN optical parametric oscillator and polarization-sensitive, differential transmission. Results at /spl sim/120 K, indicate that the spin relaxation time in the [110] superlattice is twenty times longer than that measured for the [001] structure. We are currently investigating the growth and optimization of 6.1 /spl Aring/ heterostructures on InAs [110] and GaSb [110] oriented substrates. We have noted interesting differences in the growth behavior between the [100] and [110] orientations with respect to group V dependence and antimonide on arsenide terminated interfaces. In addition, the bandgap of corresponding InAs/GaSb superlattices are consistently observed at much lower energies on [110] versus [100] orientations. The origin of these differences will be discussed with reference to band structure calculations.

We report on an experimental demonstration of graphene-metal ohmic contacts with contact resistance below 100 Ω µm. These have been fabricated on graphene wafers, both with and without hydrogen intercalation, and measured using the transmission line method. Specific contact resistivities of 3 × 10−7 to 1.2 × 10−8 Ω cm2 have been obtained. The ultra-low contact resistance yielded short-channel (source-drain distance of 0.45 µm) HfO2/graphene field effect transistors (FETs) with a low on-resistance (Ron) of 550 Ω µm and a high current density of 1.7 A/mm at a source-drain voltage of 1 V. These values represent state-of-the-art (SOA) performance in graphene-metal contacts and graphene FETs. This ohmic contact resistance is comparable to that of SOA high-speed III–V high electron mobility transistors.

We have demonstrated few-electron quantum dots in Si/SiGe and InGaAs, with occupation number controllable from N = 0. These display a high degree of spatial symmetry and identifiable shell structure. Magnetospectroscopy measurements show that two Si-based devices possess a singlet N =2 ground state at low magnetic field and therefore the two-fold valley degeneracy is lifted. The valley splittings in these two devices were 120 and 270 {\mu}eV, suggesting the presence of atomically sharp interfaces in our heterostructures., Comment: 3 pages, 3 figures

We investigate the end grafted polyelectrolyte brush in the poor solvent regime of the corresponding neutral polymer system. Using Poisson-Boltzmann theory for the electrostatics and Flory-Huggins-mean-field theory for the excluded volume and Van-der-Waals-like monomer interactions, we find the existence of a first order phase transition to a collapsed state for moderate to highly charged polyelectrolytes in the poor solvent regime. Irreversibilities in the disjoining pressure between planar grafted surfaces are predicted. For polyelectrolytes grafted to spherical and cylindrical surfaces with small radii of curvature, the phase transition is predicted to become second order in the infinite molecular weight limit. A phase diagram for the entire poor solvent regime is given.

Two patients underwent resection and replacement of the ascending aorta using a low-porosity Teflon graft anastomosed with silk suture. In both patients false aneurysms developed that required operation 13 and 23 years postoperatively. The clinical courses of these patients, along with data from the literature, suggest that the combination of a low-porosity Teflon graft and a silk suture anastomosis presents a major potential hazard for the development of anastomotic false aneurysm.

A preliminary report is presented of a prospective randomized trial conducted by eight cooperative institutions under the auspices of the National Heart and Lung Institute to compare the effectiveness of medical and surgical therapy in the management of the acute stages of unstable angina pectoris. To date 150 patients have been included in the randomized trial, 80 assigned to medical and 70 to surgical therapy; the clinical presentation, coronary arterial anatomy and left ventricular function in the two groups are similar. Some physicians have been reluctant to prescribe medical or surgical therapy by a random process, and the ethical basis of the trial has been questioned. Since there are no hard data regarding the natural history and outcome of therapy for unstable angina pectoris, randomization appears to provide a rational way of selecting therapy. Furthermore, subsets of patients at high risk may emerge during the process of randomization. The design of this randomized trial is compared with that of another reported trial. Thus far, the study has shown that it is possible to conduct a randomized trial in patients with unstable angina pectoris, and that the medical and surgical groups have been similar in relation to the variables examined. The group as a whole presented with severe angina pectoris, either as a crescendo pattern or as new onset of angina at rest, and 84 percent had recurrence of pain while in the coronary care unit and receiving vigorous medical therapy. It is anticipated that sufficient patients will have been entered into the trial within the next 12 months to determine whether medical or surgical therapy is superior in the acute stages of unstable angina pectoris.

Attention is called to the development of total obstruction to right ventricular outflow in patients with cyanotic congenital cardiac disease after systemic to pulmonary arterial anastomoses. The clinical and necropsy features are described in 4 patients in whom right ventricular outflow obstruction developed (in 3 at valve level-“acquired pulmonic atresia”-and in 1 at infundibulum) from 13 to 19 years after pulmonary to systemic shunt operations. The mechanism of development of right ventricular outflow obstruction in these patients is discussed.

Ventricular function was evaluated in 18 patients prior to left ventriculography and selective coronary arteriography. Simultaneous left ventricular pressure (catheter-tip manometer) and dP/dt were recorded at resting heart rates and during tachycardia induced by right atrial pacing. Pressure-velocity curves were constructed from which V max and maximum measured contractile element velocity (max V CE ) were obtained. V max and max V CE initially increased with pacing-induced tachycardia in 17 of the 18 patients. Eight patients developed evidence of myocardial ischemia during atrial pacing. During the period of myocardial ischemia there was a decrease in V max and max V CE in all eight patients despite constant or increasing heart rate. In the 10 patients who did not develop evidence of myocardial ischemia with pacing-induced tachycardia, V max and max V CE continued to increase or remained constant with increasing rate. Peak left ventricular dP/dt increased coincident with the onset of myocardial ischemia in six of eight patients despite a fall in V max and max V CE in all eight patients during the ischemic period. A highly significant difference was demonstrated between V max values of patients with normal ejection fractions and patients with low ejection fractions, both at rest and during pacing-induced tachycardia.

Infarct site, extent, and the degree of associated asynergy are major determinants of the hemodynamic consequences of myocardial infarction. Although conventional electrocardiography and vectorcardiography are routinely employed in assessing the location and size of infarction, they are relatively nonspecific. The newer techniques of high-frequency electrocardiography and isopotential mapping offer promise but have yet to undergo systematic evaluation. A rough measure of the extent of infarction is obtained from serum enzyme measurements. However, they furnish no information with regard to localization. The region of infarction may be detected by precordial scanning following the intravenous or intracoronary injection of a radioisotope. The infarct may be revealed as an area of decreased perfusion (cold spot) or as an area to which a specific radioactive label is bound (hot spot). With the availability of newer radionuclides such as 43 potassium and the use of computer techniques, a more precise means of localizing and quantifying myocardial infarction may become available. Optimal definition of asynergy is obtained with contrast angiography. However, the risk of this procedure has limited its use, to date, in acute myocardial infarction. Apex- and kinetocardiography, chest X-ray, and fluoroscopy often suggest regional ventricular dysfunction, but these techniques are not sufficiently specific. Newer noninvasive methods for objectively evaluating regional ventricular dysfunction are ECG-gated cardiac scintiphotography and radarkymography. With ECG-gated scintiphotography, end-diastolic and end-systolic cardiac isotope images are obtained following intravenous injection of 99m technetium-albumin. From these images, assessment of asynergy and extent and location of infarct can be made. With radarkymography, heart-wall motion is assessed and quantitated by tracking segments of the cardiac silhouette visualized on a cinefluorogram. These techniques are ideally suited to the acutely ill patient. Echocardiography is another noninvasive technique with potential application to the study of asynergy. However, at present, only posterior-wall motion can be measured. At the time of surgery regions of infarction may be localized by means of chemical indicators (fluoroscein), isotope techniques, or epicardial electrocardiographic mapping. Recently much has been learned about the hemodynamics of myocardial infarction. Through the use of the techniques described, further insight into regional ventricular abnormality and extent and localization of myocardial infarction could be obtained. With this information better approaches to therapy and prognosis could be developed. Many students of the coronary circulation must have noted that the ventricular zone affected by ligating a large coronary branch not only appears cyanotic and dilated, but that it seems to alter in its mode of contraction. The detailed and sequential changes in contraction are not easily followed by the unaided eye and so far have not been recorded myographically. The reasons for this were the lack of an adequate and suitable myograph and a technique for the application of one to a limited ventricular surface so that records obtained represent, at least reasonably well, changes in muscle length and not predominantly artifacts due to position changes, thrusts and vibrations of the vigorously beating ventricle. 1

Indicators can be used to measure myocardial blood flow if the myocardial circulation can be isolated from other segments of the systemic circulation. Methods of measuring myocardial blood flow can be divided into groups according to the technic employed to provide "isolation" of the myocardium. There is no practical method of proved accuracy whereby coronary blood flow can be measured in man in absolute terms of ml./min. All the so-called clearance methods measure myocardial blood flow in flow/volume units or ml./min./100 Gm. Myocardial blood flow measurements in these flow/volume units have not proved useful in separation of normal individuals from those with ischemic heart disease at rest, but have been useful in the study of acute intervention such as exercise or the administration of drugs.

In 11 patients with hypertension associated with acute myocardial infarction intravenous phentolamine decreased the elevated left ventricular filling pressure. Cardiac index increased with a small decrease in arterial pressure, and therefore the same or increased stroke work was achieved at a lower filling pressure. In patients with acute hypertension myocardial oxygen demand was decreased and this may tend to increase infarct size. The potentially beneficial effects were less great in patients with chronic hypertension antedating the myocardial infarction.

In three patients with the tetralogy of Fallot cardiac catheterization has revealed increased pulmonary vascular resistance four to ten years following construction of systemic to pulmonary anastomoses. The continuous murmur disappeared in two patients and became barely audible in another. The initial clinical improvement was reversed in all three. As the pulmonary artery pressure rose, the pressure gradient between the right ventricle and pulmonary artery diminished in four patients and disappeared in one. In one patient cyanosis had not yet developed and the continuous murmur was audible, although pulmonary hypertension was present. In another patient massive pulmonary artery dilatation was present and the pulmonary artery pressure rose, but pulmonary vascular resistance was not strikingly increased.

The pharmacodynamics of nitroglycerin have been studied in dogs and man. In dogs, intravenous nitroglycerin (TNG) caused a decrease in left ventricular work, myocardial blood flow (MFB), and myocardial oxygen consumption, and no significant change in coronary vascular resistance. Intracoronary artery TNG in dogs caused an immediate increase in MBF and a decrease in coronary vascular resistance that persisted until arterial pressure fell. In man, sublingual TNG (0.4 mg.) caused a decrease in left ventricular work, MBF, and myocardial oxygen consumption, and no significant decrease in coronary vascular resistance in patients with and without arteriographically proven coronary artery disease. The injection of doses of 0.1 to 0.2 mg. of TNG directly into the coronary artery in man caused an immediate increase in MBF and a decrease in coronary vascular resistance in patients with and without arteriographically proven coronary artery disease. The hypothesis proposed for the mechanism of action of TNG in the relief of angina pectoris is first, a decrease in coronary vascular resistance due to its effect on the coronary circulation, and, secondly, a decrease in cardiac work due to its effect on the systemic circulation.