Graphics Processing Unit-Accelerated Numerical Simulations and Theoretical Study of Qubit Dynamics in Realistic Systems

Quantum computers are thought to be the future of computation, using the properties of quantum mechanics to solve problems intractable to classical computers. Quantum computing leverages non-classical properties, such as entanglement, to achieve an exponential improvement in computational power. A quantum computer would enable us to address many real-world problems, such as how to synthesize fertilizers more efficiently; how to combat global warming; or to simulate protein folding in biological systems. Although much work has been done to describe the use and implementation of entanglement generation theoretically, it is still a challenge to develop such protocols experimentally. The bulk of this work is focused on creating Graphics Processing Unit (GPU)-accelerated computer simulations of quantum systems with advanced numerical and analytical techniques. Simulations can guide experiments attempting to create building blocks of quantum computers - qubits and their control devices. However, simulation of more realistic device setups in two dimensional systems has been facing problems owing to the space and time domain scaling associated with the solutions of the many-particle time dependent Schrodinger equation (TDSE). Nevertheless, recent advances in computer hardware performance has made previously intractable two-particle problems readily solvable. I have developed custom GPU-accelerated software based on a staggered-leapfrog algorithm that opens up new possibilities of simulating two-dimensional two-particle systems accurately. I focus on three research projects. Firstly, optimally defining a charge-based solid state qubit, and controlling it in a simple and experimentally achievable way, while accounting for imperfections of the waveform generators. I simulate the physical qubit on a fine-grained lattice, and propose an innovative control scheme that accounts for finite rise/fall time of the experimental apparatus, while being relatively fast and resulting in very high operation fidelity. An optimal pulsing scheme with rise time-dependent parameters is found, and shown to be able to achieve an arbitrary qubit rotation. Since the proposed pulse sequence reduces to sine waves to minimize total pulse duration, it is straightforward to implement experimentally, and easily generalisable to different systems. I also show how the fidelity remains sufficiently high independently of the initial qubit state. The proposed sequence can even reduce errors caused by charge noise under certain conditions. Readout techniques are discussed as well, and found to not present significant issues. Secondly, I aid the effort to create a Surface Acoustic Wave quantum computer prototype by describing how to produce an universal quantum gate set with a Root-of-SWAP operation used as a physical two-qubit gate. Using realistic parameters, it is shown how this operation can be performed with high fidelity. Previous work has been done to simulate a proposed Root-of-SWAP method in one dimension - this work focuses on extending this to two dimensions. We find that the method of generating Root-of-SWAP mentioned above breaks down in two dimensions- unwanted excitations are introduced in the extra dimension, causing a phase difference to appear, and thus ruining coherence of the state. I propose to implement the Root-of-SWAP operation via a tunneling interaction across the effective double dot instead. This was previously considered, however was thought to be unstable against variations in tunnel barrier height, which has exponential impact on the speed of the quantum operation. Using newly available computing power, we were able to run detailed two dimensional simulations investigating this method and its robustness against variations in the double dot potential. We find that the method produces high fidelity Root-of-SWAP states, and is robust against small variations in the tunnel barrier. Additionally, we find a relation between the tunnel barrier height and spin measurement probability, providing a way for experimentalists to estimate an actual device barrier indirectly. Finally, I theoretically model and simulate transport through a single electron transistor (SET) device. It is shown that a single donor structure can reliably be engineered from doped quantum dots by taking advantage of the tunability of the electron tunneling rates as well as the interplay, at low temperatures, between disorder conferred by randomness in dopant distribution and electron-electron interaction originating from the high doping concentration. It is possible to electrostatically isolate a single donor from the large ensemble of dopants. I investigate how such a complex system is expected to conduct, and verify a hypothesis that two donors take part in the transport by numerically reproducing the experimental measurements. Finally, it is shown that this device can be used as a single atom detector of the charge occupancy of a nearby capacitively coupled double quantum dot. While this final part does not make use of the GPU-accelerated software, it is still closely related to the rest of this work, and the theme of modeling realistic quantum devices.Project for Developing Innovation Systems of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Engineering and Physical Sciences Research Council (EPSRC) and Hitachi via CASE studentships RG 9463

Pierwotny tętniak zapalny tętnicy szyjnej wspólnej : opis przypadku

Background: Aneurysms of the common carotid artery are relatively rare. Mycotic aneurysms in such location occur even less frequently. Causative factors can be sepsis, recent surgery, or dental procedures. Immunosuppression may also be a burdening factor. Case report: The authors present the case of a 70-year-old male patient with primary mycotic carotid artery aneurysm. The patient was diagnosed with ultrasound and multidetector spiral computed tomography and treated operatively with a PTFE prosthesis. Culture-appropriate antibiotic therapy was administered during the perioperative period. Discussion: The rarity of extracranial carotid artery aneurysms might cause diagnostic and treatment difficulties. Open surgery is the recommended method, although the endovascular approach is gaining more acceptance. The choice of treatment modality might be facilitated by a detailed morphologic description of the aneurysm and its surroundings. Available methods of imaging are described

Noncommuting conserved charges in quantum thermodynamics and beyond

Thermodynamic systems typically conserve quantities ("charges") such as energy and particle number. The charges are often assumed implicitly to commute with each other. Yet quantum phenomena such as uncertainty relations rely on observables' failure to commute. How do noncommuting charges affect thermodynamic phenomena? This question, upon arising at the intersection of quantum information theory and thermodynamics, spread recently across many-body physics. Charges' noncommutation has been found to invalidate derivations of the thermal state's form, decrease entropy production, conflict with the eigenstate thermalization hypothesis, and more. This Perspective surveys key results in, opportunities for, and work adjacent to the quantum thermodynamics of noncommuting charges. Open problems include a conceptual puzzle: Evidence suggests that noncommuting charges may hinder thermalization in some ways while enhancing thermalization in others.Comment: 9.5 pages (3 figures) + appendices (10 pages

Quantum advantage in postselected metrology

Abstract: In every parameter-estimation experiment, the final measurement or the postprocessing incurs a cost. Postselection can improve the rate of Fisher information (the average information learned about an unknown parameter from a trial) to cost. We show that this improvement stems from the negativity of a particular quasiprobability distribution, a quantum extension of a probability distribution. In a classical theory, in which all observables commute, our quasiprobability distribution is real and nonnegative. In a quantum-mechanically noncommuting theory, nonclassicality manifests in negative or nonreal quasiprobabilities. Negative quasiprobabilities enable postselected experiments to outperform optimal postselection-free experiments: postselected quantum experiments can yield anomalously large information-cost rates. This advantage, we prove, is unrealizable in any classically commuting theory. Finally, we construct a preparation-and-postselection procedure that yields an arbitrarily large Fisher information. Our results establish the nonclassicality of a metrological advantage, leveraging our quasiprobability distribution as a mathematical tool

Using the concept of Dynamic Aperture to model the pseudo-diffusive effects in beams and estimate non-burnoff intensity losses

Overall, using the concept of dynamic aperture to predict intensity losses seems to be successful, with fits giving good agreement with the data. Comparison of three suggested models was made, and it was suggested that model 11 is optimal in terms of number of parameters and computation time

Quantum advantage in postselected metrology

Abstract: In every parameter-estimation experiment, the final measurement or the postprocessing incurs a cost. Postselection can improve the rate of Fisher information (the average information learned about an unknown parameter from a trial) to cost. We show that this improvement stems from the negativity of a particular quasiprobability distribution, a quantum extension of a probability distribution. In a classical theory, in which all observables commute, our quasiprobability distribution is real and nonnegative. In a quantum-mechanically noncommuting theory, nonclassicality manifests in negative or nonreal quasiprobabilities. Negative quasiprobabilities enable postselected experiments to outperform optimal postselection-free experiments: postselected quantum experiments can yield anomalously large information-cost rates. This advantage, we prove, is unrealizable in any classically commuting theory. Finally, we construct a preparation-and-postselection procedure that yields an arbitrarily large Fisher information. Our results establish the nonclassicality of a metrological advantage, leveraging our quasiprobability distribution as a mathematical tool

Quantum advantage in postselected metrology.

In every parameter-estimation experiment, the final measurement or the postprocessing incurs a cost. Postselection can improve the rate of Fisher information (the average information learned about an unknown parameter from a trial) to cost. We show that this improvement stems from the negativity of a particular quasiprobability distribution, a quantum extension of a probability distribution. In a classical theory, in which all observables commute, our quasiprobability distribution is real and nonnegative. In a quantum-mechanically noncommuting theory, nonclassicality manifests in negative or nonreal quasiprobabilities. Negative quasiprobabilities enable postselected experiments to outperform optimal postselection-free experiments: postselected quantum experiments can yield anomalously large information-cost rates. This advantage, we prove, is unrealizable in any classically commuting theory. Finally, we construct a preparation-and-postselection procedure that yields an arbitrarily large Fisher information. Our results establish the nonclassicality of a metrological advantage, leveraging our quasiprobability distribution as a mathematical tool

Quantum advantage in postselected metrology

© 2020, The Author(s). In every parameter-estimation experiment, the final measurement or the postprocessing incurs a cost. Postselection can improve the rate of Fisher information (the average information learned about an unknown parameter from a trial) to cost. We show that this improvement stems from the negativity of a particular quasiprobability distribution, a quantum extension of a probability distribution. In a classical theory, in which all observables commute, our quasiprobability distribution is real and nonnegative. In a quantum-mechanically noncommuting theory, nonclassicality manifests in negative or nonreal quasiprobabilities. Negative quasiprobabilities enable postselected experiments to outperform optimal postselection-free experiments: postselected quantum experiments can yield anomalously large information-cost rates. This advantage, we prove, is unrealizable in any classically commuting theory. Finally, we construct a preparation-and-postselection procedure that yields an arbitrarily large Fisher information. Our results establish the nonclassicality of a metrological advantage, leveraging our quasiprobability distribution as a mathematical tool

Global impact of the COVID-19 pandemic on subarachnoid haemorrhage hospitalisations, aneurysm treatment and in-hospital mortality: 1-year follow-up

Background: Prior studies indicated a decrease in the incidences of aneurysmal subarachnoid haemorrhage (aSAH) during the early stages of the COVID-19 pandemic. We evaluated differences in the incidence, severity of aSAH presentation, and ruptured aneurysm treatment modality during the first year of the COVID-19 pandemic compared with the preceding year. Methods: We conducted a cross-sectional study including 49 countries and 187 centres. We recorded volumes for COVID-19 hospitalisations, aSAH hospitalisations, Hunt-Hess grade, coiling, clipping and aSAH in-hospital mortality. Diagnoses were identified by International Classification of Diseases, 10th Revision, codes or stroke databases from January 2019 to May 2021. Results: Over the study period, there were 16 247 aSAH admissions, 344 491 COVID-19 admissions, 8300 ruptured aneurysm coiling and 4240 ruptured aneurysm clipping procedures. Declines were observed in aSAH admissions (-6.4% (95% CI -7.0% to -5.8%), p=0.0001) during the first year of the pandemic compared with the prior year, most pronounced in high-volume SAH and high-volume COVID-19 hospitals. There was a trend towards a decline in mild and moderate presentations of subarachnoid haemorrhage (SAH) (mild: -5% (95% CI -5.9% to -4.3%), p=0.06; moderate: -8.3% (95% CI -10.2% to -6.7%), p=0.06) but no difference in higher SAH severity. The ruptured aneurysm clipping rate remained unchanged (30.7% vs 31.2%, p=0.58), whereas ruptured aneurysm coiling increased (53.97% vs 56.5%, p=0.009). There was no difference in aSAH in-hospital mortality rate (19.1% vs 20.1%, p=0.12). Conclusion: During the first year of the pandemic, there was a decrease in aSAH admissions volume, driven by a decrease in mild to moderate presentation of aSAH. There was an increase in the ruptured aneurysm coiling rate but neither change in the ruptured aneurysm clipping rate nor change in aSAH in-hospital mortality

Global Impact of the COVID-19 Pandemic on Stroke Volumes and Cerebrovascular Events: One-Year Follow-up.

BACKGROUND AND OBJECTIVES Declines in stroke admission, intravenous thrombolysis, and mechanical thrombectomy volumes were reported during the first wave of the COVID-19 pandemic. There is a paucity of data on the longer-term effect of the pandemic on stroke volumes over the course of a year and through the second wave of the pandemic. We sought to measure the impact of the COVID-19 pandemic on the volumes of stroke admissions, intracranial hemorrhage (ICH), intravenous thrombolysis (IVT), and mechanical thrombectomy over a one-year period at the onset of the pandemic (March 1, 2020, to February 28, 2021) compared with the immediately preceding year (March 1, 2019, to February 29, 2020). METHODS We conducted a longitudinal retrospective study across 6 continents, 56 countries, and 275 stroke centers. We collected volume data for COVID-19 admissions and 4 stroke metrics: ischemic stroke admissions, ICH admissions, intravenous thrombolysis treatments, and mechanical thrombectomy procedures. Diagnoses were identified by their ICD-10 codes or classifications in stroke databases. RESULTS There were 148,895 stroke admissions in the one-year immediately before compared to 138,453 admissions during the one-year pandemic, representing a 7% decline (95% confidence interval [95% CI 7.1, 6.9]; p<0.0001). ICH volumes declined from 29,585 to 28,156 (4.8%, [5.1, 4.6]; p<0.0001) and IVT volume from 24,584 to 23,077 (6.1%, [6.4, 5.8]; p<0.0001). Larger declines were observed at high volume compared to low volume centers (all p<0.0001). There was no significant change in mechanical thrombectomy volumes (0.7%, [0.6,0.9]; p=0.49). Stroke was diagnosed in 1.3% [1.31,1.38] of 406,792 COVID-19 hospitalizations. SARS-CoV-2 infection was present in 2.9% ([2.82,2.97], 5,656/195,539) of all stroke hospitalizations. DISCUSSION There was a global decline and shift to lower volume centers of stroke admission volumes, ICH volumes, and IVT volumes during the 1st year of the COVID-19 pandemic compared to the prior year. Mechanical thrombectomy volumes were preserved. These results suggest preservation in the stroke care of higher severity of disease through the first pandemic year. TRIAL REGISTRATION INFORMATION This study is registered under NCT04934020