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In this thesis, we explore the possibility of optimizing general superconducting circuits via autodifferentiation. Following a summary of the essential components of superconducting circuits leading to a generalized expression for their Hamiltonian, we use an analytical solution for the gradient of the eigenvalues and eigenvectors to fill in a missing gradient step and form a general computational graph for arbitrary smooth loss functions that depend on circuit parameter values and its eigenvalues or eigenvectors. After verifying numerically that the resultant gradients predicted match a first-order approximation to high precision, we leverage knowledge of the gradient to perform optimization over key metrics including the fundamental resonant frequency of the circuit, its anharmonicity, charge and flux sensitivities, and both its longitudinal and dephasing coherence times. We demonstrate by comparison of these key metrics before and after that the minimization of our objective loss functions corresponds to the intended improvement in circuit characteristics. We demonstrate concurrent optimization of each of these objectives in the flux-tunable transmon and fluxonium circuit topologies, then show that randomly sampling parameter values within some fixed range can lead to optimization on-par with SOTA experimental devices. Finally, we assess how to address the problem of allocating truncation numbers for fixed computational resources, to maximize the convergence of the circuit eigenspectrum. Using this means of truncation number allocation, we undertake a preliminary investigation of a circuit with N = 3 inductive (Josephson junction) elements, showing that its overall performance for a small set of random samples can outperform that of both kinds of circuits with only N = 2 single-loop inductive elements. We conclude with an outlook on further applications of the tools and methodologies developed here, particularly with regards to designing better qubits for design and experimentation in-lab.

The Casimir effect between type-II superconducting plates in the coexisting phase of a superconducting phase and a normal phase is investigated. The dependence of the optical conductivity of the superconducting plates on the external magnetic field is described in terms of the penetration depth of the incident electromagnetic field, and the permittivity along the imaginary axis is represented by a linear combination of the permittivities for the plasma model and Drude models. The characteristic frequency in each model is determined using the force parameters for the motion of the magnetic field vortices. The Casimir force between parallel YBCO plates in the mixed state is calculated, and the dependence on the applied magnetic field and temperature is considered.

This paper describes the microfabrication and electrical characterization of aluminum-coated superconducting through-silicon vias (TSVs) with sharp superconducting transition above 1 K. The sharp superconducting transition was achieved by means of fully conformal and void-free DC-sputtering of the TSVs with Al, and is here demonstrated in up to $500~\mu \text{m}$ -deep vias. Full conformality of Al sputtering was made possible by shaping the vias with a tailored hourglass profile, which allowed a metallic layer as thick as 430 nm to be deposited in the center of the vias. Single-via electric resistance as low as 160 $\text{m}\Omega $ at room temperature and superconductivity at 1.27 K were measured by a three-dimensional (3D) cross-bridge Kelvin resistor structure. This work establishes a CMOS-compatible fabrication process suitable for arrays of superconducting TSVs and 3D integration of superconducting silicon-based devices. [2020-0354]

With everchanging technology and constantly increasing demand of energy, superconducting materials can play a crucial role as by offering lowest possible electrical resistance. In this quest, Niobium (Nb) thin films synthesized by magnetron sputtering has been analyzed at diverse sputtering constraints. Different parameters as Nitrogen (N2) partial pressure, power densities, magnetization are explored by Usadel equation. Usadel equation integrating Bardeen-Cooper-Schrieffer theory of superconductivity, analyzed dependence of superconducting transition temperature on varied sputtering parameters by computational technique. It has been found that Nioubium Nitride (NbN) operating at elevated temperature exhibits superconductive properties. Optimal growth is found at N2 partial pressure ~15 Standard Cubic Centimeters per Minute (SCCM) where power densities ranges from 200 to 500W at a critical temperature of 14.5K.

We have determined the electron–phonon interaction in type II Dirac semimetallic 1T-PdTe2 by means of helium atom scattering. While 1T-PdTe2 is isostructural with 1T-PtTe2, only the former is superconductor. The difference can be traced to the substantially larger value of the electron–phonon coupling in 1T-PdTe2, λ = 0.58, obtained from the Debye-Waller attenuation of the He specular peak. With this value and the surface Debye temperature, ΘD = 106.2 K, we have figured out the superconducting critical temperature, Tc = 1.83 K given by the BCS theory, which is in good agreement with Tc = (1.95 ± 0.03) K obtained with low-temperature scanning tunneling microscopy. The value of the effective mass related to ΘD indicates that the large electron–phonon coupling in 1T-PdTe2 is due to coupling, not only with the zone-center optical mode O2 at 9.2 meV, as proposed in a recent theoretical study, but also with the zone-boundary acoustic mode LA. Our results suggest that the topological states of a Dirac cone play a negligible role on the onset of superconductivity.

A quarter wavelength superconducting aluminum film coplanar waveguide resonator with a high-quality factor was designed and fabricated. Furthermore, its resonant frequency and quality as a function of temperatures and exciting powers were investigated. The experimental results showed that the resonance frequency decreases about 1% with the increase of temperature from 50 mk to 1 K, and the load quality factor of the resonator also decreases. The resonant frequency decreases with power from − 120 to − 70 dBm. The quality factor increases with increased excitation power due to the loss decreases and nonlinearity. The results are consistent with the theoretical analysis of the surface impedance model.

At the electron accelerator for beams with high brilliance and low emittance (ELBE), the second version of a superconducting radio-frequency (SRF) photoinjector was brought into operation in 2014. After a period of commissioning, a gradual transfer to routine operation took place in 2017 and 2018, so that more than 3000h of user beam have already been generated since 2019. During this time, a total of 20 cathodes (2 Cu, 12 Mg, 6 Cs₂Te) were used, but no serious cavity degradation was observed. In this paper, we summarize the operational experience of the last 6 years of SRF gun operation, with special emphasis on the main RF properties of the cavity. This includes the evolution of QvsE, dark current, multipacting, but also mechanical properties such as Lorentz force detuning, helium pressure sensitivity as well as microphonics. The latter is closely connected to an active compensation by a so-called low-level RF feedback loop, which is also briefly presented., Proceedings of the 20th International Conference on RF Superconductivity, SRF2021, East Lansing, MI, USA

Understanding the behaviour of vortices under nanoscale confinement in superconducting circuits is of importance for development of superconducting electronics and quantum technologies. Using numerical simulations based on the Ginzburg-Landau theory for non-homogeneous superconductivity in the presence of magnetic fields, we detail how lateral confinement organises vortices in a long superconducting nanostripe, and present a phase diagram of vortex configurations as a function of the stripe width and magnetic field. We discuss why average vortex density is reduced and reveal that confinement also has profound influence on vortex dynamics in the dissipative regime under sourced electrical current, mapping out transitions between asynchronous and synchronous vortex rows crossing the nanostripe as the current is varied. Synchronous crossings are of particular interest, since they cause single-mode modulations in the voltage drop along the stripe in a high (typically GHz-to-THz) frequency range., Comment: 17 pages, 13 figures

One of the biggest issues in the design of a superconducting magnet is to be able to protect the magnet during an accidental quench. The quench is the transition between the superconducting state and the resistive state of a superconducting magnet, generating an important amount of energy due to Joule effect. This quench and its propagation by heat conduction will lead to an important temperature increase. It can totally burn the magnet and degrade irreversibly its properties. It is then mandatory to detect a quench if it occurs to avoid any degradation risk. In the framework of the MADMAX project, a European project around dark matter research, the protection asks for a particular attention. In fact, with a conductor of Cable-In-Conduit-Conductor (CICC) with a copper profile, cooled with stagnant superfluid helium, and without a superfluid helium bath around the coils to cool down the coils, the quench behavior appears as unpredictable. Then, in order to study the quench behavior, a mock-up coil (MACQU), that has the same MADMAX's features, has been designed and tested at CEA Saclay, in the JT60-SA testing facility. A large testing temperature range (from 1.75 K to 2.01 K), with a large testing current (from 10 kA to 17 kA) were tested. It allowed to demonstrate the safety of the detection system, and to study a particular quench propagation behavior: the Thermal Hydraulic Quench Back. This physical phenomenon has been also observed with THEA®, a numerical simulation tool. It appeared that the quench propagation was divided in three main phases. The first phase is the quasi-linear phase, where the quench propagates at constant speed. The quench propagation speed is driven by the velocity of the helium that, due to the Joule energy deposition of the quench, flows in the conduit. The thermal conduction in the copper stabilizer is not the main phenomena of the quench propagation. The second phase is the acceleration phase, where the quench accelerates progressively. The velocity of the helium increasing with the normal length, the velocity of the helium still drives the quench propagation speed and contributes to accelerate it. At the same time, the friction forces pre-heat the coil upfront the quench propagation front and the temperature of the non-quenched zone increases. The third phase starts when, due to the friction forces, an important part of the non-quenched zone instantaneously transits and generates a second quench, much faster than the first one. This phase is called the breaking phase, characterized by the important break of the normal length, corresponding to this characteristic acceleration., Une des problématiques majeures autour de la conception d'aimant supraconducteur est d'être capable de protéger l'aimant en cas de quench accidentel. Le quench est la transition de l'état supraconducteur vers l'état résistif d'un aimant supraconducteur, ce qui engendre une grande quantité d'énergie par effet Joule. Ce quench ainsi que sa propagation par conduction vont mener à une forte montée de température, pouvant dégrader voire endommager irrémédiablement l'aimant. Il est donc nécessaire de détecter un quench lorsqu'il se produit afin d'éviter tout risques de dégradation. Dans le cadre du projet MADMAX, projet européen autour de la recherche de la matière noire, un type de conducteur inédit a été développé, la protection demande donc une attention toute particulière. En effet, avec un conducteur de type Cable-In-Conduit-Conductor (CICC) avec un profilé en cuivre, refroidi avec de l'hélium superfluide stagnant à l'intérieur du conduit, et sans bain d'hélium superfluide autour des bobines afin de les refroidir, le comportement du quench est imprévisible. Ainsi, afin de mieux appréhender le comportement du quench, une maquette d'aimant supraconducteur (MACQU), reprenant en grande partie les caractéristiques de MADMAX, a été conçue puis testée au CEA Saclay, au sein de la station d'essais JT60-SA. Une large plage de température a été testée, allant de 1.75 K à 2.01 K, ainsi qu'une large plage de courant allant de 10 kA à 17 kA. Elle a permis de démontrer la sécurité du système de détection, tout en permettant d'étudier un phénomène de propagation de quench particulier: le Quench Back Thermo Hydraulique. Ce phénomène physique a été aussi observé via THEA®, outil de simulation numérique. Il est apparu que la propagation du quench se faisait en 3 phases principales. La première phase est la phase quasi-linéaire où le quench se propage à vitesse constante. La vitesse de propagation de quench est pilotée par la vitesse d'expansion de l'hélium qui, suite au dépot d'énergie du quench, s'écoule dans le conduit. La conduction thermique dans la goulotte en cuivre n'est donc pas le phénomène dominant de la propagation du quench. La deuxième phase est la phase d'accélération, où le quench accélère progressivement. La vitesse de l'hélium augmentant avec la longueur transitée, la vitesse de l'helium pilote toujours celle du quench et contribue à l'accélérer. Dans le même temps, les forces de friction préchauffent l'aimant loin du front de propagation et la temperature de l'aimant augmente dans la zone supraconductrice. La troisième phase démarre lorsque sous l'effet des forces de friction, une grande partie de la zone supraconductrice restante transite instantanément et génère un second quench beaucoup plus rapide que le précédent. Cette phase est appelée la phase de cassure, caractérisée par une importante cassure de la longueur transitée, correspondant à cette caractéristique accélération.

The Gottesman-Kitaev-Preskill (GKP) error-correcting code uses one or more bosonic modes to encode a finite-dimensional logical space, allowing a low-error logical qubit to be encoded in a small number of resonators. In this thesis, I propose new methods to implement logical gates and measurements with GKP codes and analyse their performance. The logical gate scheme uses the single-qubit Clifford frame to greatly reduce the number of gates needed to implement an algorithm without increasing the hardware requirements. The logical measurement scheme uses one ancilla mode to achieve a 0.1% logical error rate over a measurement time of 630 ns when the measurement efficiency is as low as 75%. Finally, I provide a subsystem decomposition which can be used to analyse GKP codes efficiently even as the Fock space distribution of the codestates goes to infinity.

Here, we report results of the investigation of the lattice dynamics of the sylvanite mineral AuAgTe4 in a wide temperature and pressure range by Raman spectroscopy, together with the first-principle calculations. At ambient pressure, the experimental spectrum agrees well with the calculation data. The temperature behavior of the phonon self-energies (frequencies and linewidths) are described by an anharmonic mechanism and imply negligible contributions of electron–phonon interaction at low temperatures. A structural phase transition was recorded in the pressure range of 4–6 GPa, which is in accordance with theoretical predictions. At higher pressures, evidence of local structural disorder was found that made it possible to experimentally observe the spectrum of the density of vibrational states of AuAgTe4, which becomes superconducting under pressure.

Superconducting electronics (SCE) is uniquely suited to implement neuromorphic systems. As a result, SCE has the potential to enable a new generation of neuromorphic architectures that can simultaneously provide scalability, programmability, biological fidelity, on-line learning support, efficiency and speed. Supporting all of these capabilities simultaneously has thus far proven to be difficult using existing semiconductor technologies. However, as the fields of computational neuroscience and artificial intelligence (AI) continue to advance, the need for architectures that can provide combinations of these capabilities will grow. In this paper, we will explain how superconducting electronics could be used to address this need by combining analog and digital SCE circuits to build large scale neuromorphic systems. In particular, we will show through detailed analysis that the available SCE technology is suitable for near term neuromorphic demonstrations. Furthermore, this analysis will establish that neuromorphic architectures built using SCE will have the potential to be significantly faster and more efficient than current approaches, all while supporting capabilities such as biologically suggestive neuron models and on-line learning. In the future, SCE-based neuromorphic systems could serve as experimental platforms supporting investigations that are not feasible with current approaches. Ultimately, these systems and the experiments that they support would enable the advancement of neuroscience and the development of more sophisticated AI.

The hybrid architecture of the driven qubit–photon–magnon system has recently emerged as a promising candidate for novel quantum technologies. In this paper, we introduce the effective wave-function of a superconducting single qubit and a magnon mode contained within a cavity resonator and an external field. The non-classicality of the magnon and resonator modes are investigated by using the negative values of the Wigner function. Additionally, we discuss the non-classicality of the qubit state via the Wigner–Yanase skew information. We find that the mixture angle of the qubit–resonator plays a controllable role in non-classicality. However, the strength of the magnon–photon increases the non-classical behaviour of the system.

The demands for high performance concealed object detection in mass transit, public events or industrial facilities have been increasing in light of increased risks. Concealed object detection at high throughput rates in places where lives can be jeopardized or where there is a risk of smuggling prohibited objects is a major challenge using conventional methods. Terahertz radiation has attracted attention for utilization in security imaging applications, due to its ability to penetrate dielectric materials, such as common clothing and packaging materials, while being strongly reflected from metals and strongly absorbed by water. Terahertz radiation also strongly interacts with organic matter, enabling spectroscopic techniques for the detection of explosives and narcotics, for example. The utilization of terahertz imaging helps to solve the problems of conventional physical security requiring straight body human contact. Terahertz radiation is also inherently safe when compared to alternative technologies such as X-ray imaging, due to its non-ionising nature. The diffraction-limited spatial resolution of sub-millimeter wave imaging (in the order of 15 millimeters) does not reveal anatomical details, guaranteeing the privacy of the scanned people.Meanwhile, ever cheaper, more compact and easier to use cryocoolers capable of reaching temperatures down to less than 10 Kelvin (-263 °C) enable the utilization of superconducting electronics and detectors. These low temperatures allow for much lower thermal noise in detectors when compared to their room temperature equivalents, enabling much higher sensitivity. These high sensitivity imaging detectors are able to operate passively, without the need for bulky, intrusive and expensive high-power coherent terahertz sources, detecting the natural thermal blackbody terahertz radiation. The low temperatures also allow exploiting superconducting materials and their phenomena, such as exceptionally low conduction losses and high kinetic inductance.Enabled by modern microfabrication technologies, VTT's (Technical Research Centre of Finland) superconducting kinetic inductance bolometers provide a solution for implementing a large focal plane array of detector pixels, enabling a high-performance passive terahertz imaging system for security imaging of human-sized objects at a close range and video frame rates. The high responsivity and inherent multiplexability negate the need for expensive and complex cryogenic signal amplification or high-bandwidth RF components in the system.In this thesis, the superconducting properties of these kinetic inductance bolometers were characterized as a function of their temperature and geographical location on the wafer and assessed considering the theoretical predictions of both the two-fluid and the Mattis-Bardeen models. Further measurements were made in an attempt to characterize the noise and thermal properties of the bolometers and to determine the superconducting parameters of the thin superconducting films. The obtained data provides the basis for understanding the performance of the detectors and improving the design and fabrication process for bolometers in the future.

The demands for high performance concealed object detection in mass transit, public events or industrial facilities have been increasing in light of increased risks. Concealed object detection at high throughput rates in places where lives can be jeopardized or where there is a risk of smuggling prohibited objects is a major challenge using conventional methods. Terahertz radiation has attracted attention for utilization in security imaging applications, due to its ability to penetrate dielectric materials, such as common clothing and packaging materials, while being strongly reflected from metals and strongly absorbed by water. Terahertz radiation also strongly interacts with organic matter, enabling spectroscopic techniques for the detection of explosives and narcotics, for example. The utilization of terahertz imaging helps to solve the problems of conventional physical security requiring straight body human contact. Terahertz radiation is also inherently safe when compared to alternative technologies such as X-ray imaging, due to its non-ionising nature. The diffraction-limited spatial resolution of sub-millimeter wave imaging (in the order of 15 millimeters) does not reveal anatomical details, guaranteeing the privacy of the scanned people.Meanwhile, ever cheaper, more compact and easier to use cryocoolers capable of reaching temperatures down to less than 10 Kelvin (-263 °C) enable the utilization of superconducting electronics and detectors. These low temperatures allow for much lower thermal noise in detectors when compared to their room temperature equivalents, enabling much higher sensitivity. These high sensitivity imaging detectors are able to operate passively, without the need for bulky, intrusive and expensive high-power coherent terahertz sources, detecting the natural thermal blackbody terahertz radiation. The low temperatures also allow exploiting superconducting materials and their phenomena, such as exceptionally low conduction losses and high kinetic inductance.Enabled by modern microfabrication technologies, VTT's (Technical Research Centre of Finland) superconducting kinetic inductance bolometers provide a solution for implementing a large focal plane array of detector pixels, enabling a high-performance passive terahertz imaging system for security imaging of human-sized objects at a close range and video frame rates. The high responsivity and inherent multiplexability negate the need for expensive and complex cryogenic signal amplification or high-bandwidth RF components in the system.In this thesis, the superconducting properties of these kinetic inductance bolometers were characterized as a function of their temperature and geographical location on the wafer and assessed considering the theoretical predictions of both the two-fluid and the Mattis-Bardeen models. Further measurements were made in an attempt to characterize the noise and thermal properties of the bolometers and to determine the superconducting parameters of the thin superconducting films. The obtained data provides the basis for understanding the performance of the detectors and improving the design and fabrication process for bolometers in the future.

Sulphur has one of the most complex high pressure-temperature phase diagrams of the elements, with many solid phases and multiple liquid phases reported up to 40 GPa (400 kbar). The high temperature stability of solid phases is poorly understood and the melting curve above 12 GPa has been largely unstudied. As pressure and temperature increase, optical and electronic characteristics of sulphur change significantly in both solid and liquid phases. At present there are four generally accepted solid phases up to 83 GPa, above which solid sulphur becomes metallic. Sulphur is thought to be present in multiple planetary cores and is a constituent of H3S, a high pressure-temperature superconductor (TC = 203 K), so understanding its behaviour at extremes is of broad interest. In this thesis, multiple probe techniques have been used to examine phase transformations and transport properties of sulphur at high pressure-temperature conditions, including optical spectroscopy and x-ray diffraction measurements. High pressure conditions have been achieved using the diamond anvil cell, and three heating techniques have been implemented; resistive heating, near-IR (1070 nm) laser heating and mid-IR (10.6 µm) laser heating. Pressures were determined using the ruby fluorescence effect, the diamond edge scale, the equation of state of gold during x-ray studies and by comparing Raman band frequencies to the literature. Stokes to anti-Stokes intensity ratio behaviour was used to measure temperatures alongside thermal emission measurements, and a thermocouple during resistive heating experiments. Sulphur samples were heated to temperatures of ∼1900 K, and compressed to 49.9 GPa. Solid and liquid phase changes were identified using Raman spectra, optical observations, anomalies in laser power vs. temperature dependencies, and x-ray diffraction patterns. The melting curve has been extended up to 45.6 GPa, and an extensive study of the stability regions of solid phases has been made. Multiple liquid phases have been detected, exhibiting different optical absorption properties. The time-domain thermoreflectance technique was also investigated for high pressure samples heated using laser pulses to measure transport (thermal conduction).

We analyze the subgap excitations and phase diagram of a quantum dot (QD) coupled to a semiconducting nanowire fully wrapped by a superconducting (S) shell. We take into account how a Little-Parks (LP) pairing fluxoid (a winding in the S phase around the shell) influences the proximity effect on the dot. We find that under axially symmetric QD-S coupling, shell fluxoids cause the induced pairing to vanish, producing instead a level renormalization that pushes subgap levels closer to zero energy and flattens fermionic parity crossings as the coupling strength increases. This fluxoid-induced stabilization mechanism has analoges in symmetric S-QD-S Josephson junctions at phase $\pi$, and can naturally lead to patterns of near-zero modes weakly dispersing with parameters in all but the zero-th lobe of the LP spectrum., Comment: 17 pages, 8 figures. v2: final version with a new figure

High-density power conversion and energy storage solutions were and are being explored for use in Electric Aircraft (EA). A superconducting magnetic energy storage (SMES) system is a promising candidate due to its fast response and ability to satisfy large pulse loads as is expected from EA. For the SMES, Dual Active Bridge (DAB) converters can offer high-density power conversion and provide galvanic isolation. This paper proposes a design methodology for a bidirectional DC/DC converter using DAB converter and a MOSFET-based chopper to interface SMES. This paper includes analysis, performance prediction, and needs for implementation in EA. The DAB/SMES system has been successfully designed, analyzed, and is being optimized for application in EA.

A quantum computer can outperform classical computers for certain tasks. The general challenge to realize a quantum computer is to solve decoherence, which is due to coupling of a quantum state with the local environment. One possible way to overcome decoherence is to use a topological quantum state. Topologically protected states are expected to have long coherence times. In a topological quantum computer, the information is carried by nonlocal Majorana states. Such states can be engineered in a semiconductor nanowire, which has strong spin-orbit interactions, coupled to a superconductor. The first signatures of Majorana states have been observed recently. In order to substantiate the existence of Majorana states and use them as qubits, an exchange - or braiding - operation of these states has to be performed. The current challenge is to improve the quality of the materials and interfaces. Recent progress toward improved Majorana signals using materials science advances is reviewed in this article.

In the quench protection (QP) process of superconducting fusion devices, the operating reliability and efficiency of power components are affected by stray parameters of the fast discharge resistor (FDR) system, especially the system stray inductance. In this paper, the fundamental condition of the QP operating process and the large power FDR system structure layout for the Large-scale Superconductor Test Facility (LSTF) are presented. The negative affected VCB, CPC circuit action, and the energy discharging process are further analyzed. The stray inductance optimization method of the resistor module and structure connections are proposed to reduce the stray inductance value. Finally, the optimization results are presented by the Q3D module of the FEA software.

Topological band theory has contributed to some of the most astonishing developments in solid-state physics. The unique attributes that arise from topological effects are at the focus of modern experimental and theoretical research. Weyl point, a topological defect at the Fermi surface, enables topological transitions and transport phenomena. Its existence is considerably restricted in natural materials due to the tuning and dimension constraint. Recently, The Weyl points have been predicted to accommodate within superconducting nanostructures in the spectrum of Andreev bound states. Theoretically, one can easily manipulate the dimensionality and the tuning process through elementary approaches with specially designed structures. This opens up a new window for explorations in a higher dimension, high-order topological effects,Majorana states, and other complications even though it may be still experimentally challenging. One realization of such structures is the multi-terminal Josephson junction. The parameters are the superconducting phase differences of the terminals and theWeyl points reside at low energies within the superconducting gap. Chapter 2 of this thesis investigates the topological effect in the quantized transconductance of such a structure considering the presence of the continuous spectrum that is intrinsic to superconductors. This research is based on scattering formalism and relates the Landauer conductance to the continuous spectrumas a background field in the regular topological charge picture. Chapter 3 is based on a very generic superconducting nanostructure setup so long as it hosts Weyl points in it. The research proposes a unit that tunnel-couples such a setup with a quantum dot. The distinct feature of the spectrum, especially the distinction between its spin-singlet and spin-doublet due to spin-orbit coupling, leads to an exploration of the state manipulation. Eventually, through adiabatic and diabatic approaches, one can feasibly realize a full unitary transformation of the spectrum. Because of this, the unit could easily find its promising application in entangled qubits. Chapter 4 also relies on the generic low-energy Weyl point setup in the superconducting nanostructure, but instead, it is weakly tunnel-coupled to regularmetallic leads. We know that spintronics explores the intrinsic spin degree of freedom. It is usually realized on magnetic materials. In the setup of this research, the energy spectrum contains a natural spin-orbit that creates a minimalistic magnetic state in the vicinity of theWeyl point. The spin structure of the spectrum allows fine-controls over the spin and switch between magnetic/non-magnetic state. Hence this chapter’s research focuses on the possible spintronics features based on master equations. Chapter 5 furthers the research of chapter 4. It considers a universal energy scale sets up by the tunnel coupling strength. In the language of the Green’s function, this chapter studies the topological effect through the response function. This set up is a suitable example of low energy Weyl points situated in the presence of a low-energy continuous spectrum brought by electrons in the leads. We have seen in Chapter 1 how the continuous spectrum above the gap modifies the topology leading to a non-quantized contribution to the transconductance. The peculiarity of couplingWeyl points to a low energy continuous spectrum is that the dissipation gives rise to a redefinition of the Berry curvature, whichmanifests as a continuous density of topological charge instead of a pointlike one. This unusual characteristic can be captured by the tunnel current and thus can assist the detection ofWeyl points experimentally.

We have investigated the effect of initial microstructures on the change in critical current density (Jc) of Nb-46.5 wt%Ti (NbTi) superconducting material. It is well known that α-Ti phases distributed in NbTi material act as a flux pinning center, resulting in an improvement in critical current density. Therefore, it is crucial to obtain the grain-refined microstructure, which is strongly related with precipitation of uniformly distributed fine α-Ti phases and higher volume faction of α-Ti phases, as α-Ti phases are precipitated at the grain boundaries and triple points during heat treatments. Therefore, in order to characterize the effect of initial microstructure of NbTi on critical current density, different initial microstructures were obtained by applying equal channel angular pressing (ECAP) and hot rolling with different strains. It was revealed experimentally that hot rolling with a higher strain is efficient for obtaining the initial microstructure, which has equiaxed fine grains of β-NbTi with the aid of dynamic recrystallization, and which is helpful for precipitating fine α-Ti phases during intermediate heat treatment. Furthermore, it was confirmed that critical current density can be enhanced by obtaining a smaller α-Ti phase, a higher aspect ratio of α-Ti phase, a higher volume fraction of α-Ti phase and a ribbon-like folded α-Ti phase.

A superconducting microstrip half-wavelength resonator is proposed as a suitable band-pass filter for broadband moderate spectral resolution spectroscopy for terahertz (THz) astronomy. The proposed filter geometry has a free spectral range of an octave of bandwidth without introducing spurious resonances, reaches a high coupling efficiency in the pass-band and shows very high rejection in the stop-band to minimize reflections and cross-talk with other filters. A spectrally sparse prototype filter-bank in the band 300-400 GHz has been developed employing these filters as well as an equivalent circuit model to anticipate systematic errors. The fabricated chip has been characterized in terms of frequency response, reporting an average peak coupling efficiency of 27% with an average spectral resolution of 940., 11 pages, 17 figures

Lidar measurement are widely used today for remote sensing and atmosphere monitoring. Indeed, depending on the specific wavelength adopted, it is possible to determine the presence of specific molecules and pollutants in the atmosphere. Several studies claim that the use of Lidar systems with wavelength larger than 1 µm would provide a special tool for aerosols and carbon dioxide observations. Aerosols measurements are typically performed up to the wavelength ? = 1064 nm, while for longer wavelengths they are limited by the detector noise. In this work the results of a preliminary Lidar measurement at ? = 1064 nm by using a Superconducting Nanowire Single Photon Detector (SNSPD) are presented. SNSPDs are characterized by high efficiency in the infrared wavelength domain (IR), low noise and dead time, which can in turn enhance the signal quality in Lidar atmospheric measurements at ? > 1 µm, an interesting range for environmental applications. Preliminary measurements have been performed at the University of Naples Federico II by using a NbTiN SNSPD cooled at a temperature of 4.2 K and the Lidar system MALIA. The experimental results at 1064 nm demonstrate the feasibility of the approach for aerosol measurements in the IR, opening the way to further investigations on the use of these detectors for Lidar atmospheric measurements in the IR. The backscattering coefficient, ?, has been also estimated and its value is around 10-6 m-1sr-1.

In this project, the fabrication of single-photon detectors based on superconducting nanowires is presented, with great focus on extending their operation range to the mid infrared. In particular, Niobium Titanium Nitride (NbTiN) and Molybdenum Silicide (MoSi), superconducting materials with different properties, are presented, studied and used as fabrication platforms. Different approaches are followed, mainly adjusting the nanowire width and thickness to achieve near unity quantum efficiency at mid infrared wavelengths. With the vision of using these devices for atmospheric LIDAR and sensing experiments, saturation at 2050 nm is studied that corresponds to the absorption peak of CO2. For the best device made on NbTiN thin films, unity quantum efficiency is shown at 2050 nm with a time jitter of 116 ps at 1550 nm. Simulations using the transfer matrix method and the commercial software Lumerical are carried out, concluding that the devices made in NbTiN could have 23.1-26.7% system detection efficiency at 2050 nm on a Silicon SiO2/Si platform. Further improvements show that the detection efficiency could reach between 52-62% (for 0.33 and 0.5 fill factor, respectively calculated with FDTD simulations) by engineering optical cavities. I detta projekt presenteras en fabrikations process för enstaka foton detektorer baserade på supraledande nanotrådar. Fokuset har legat på att utöka våglängds regionen där detektorernas kan detektera till mid-infrarött ljus. Två specifika supraledande material, Niobium Titan (NbTiN) och Molybdenum Silicide (MoSi), med olika egenskaper har studerats och använts som material. Dimensionerna på nanotrådarna, framför allt tjockleken och bredden, har optimerats för att uppnå nära enhetlig kvant-effektivitet vid mid-infraröda våglängder. Med visionen att detektorerna ska användas för atmosfäriska LiDAR mätningar har de studerats för satruering vid 2050 nm som motsvarar ett absorbtions maximum för CO2. Detektorerna tillverkade med NbTinN uppnådde 100% kvant effektivitet för 2050 nm ljus med ett tids jitter på 116 ps vid 1550 nm ljus. Simuleringar med överförings matrisen metoden och den kommersiella mjukvaran Lumerical visar att NbTiN detektorer placerade på en SiO2/Si platform kan ha en 23.1-26.7% effektivitet vid 2050 nm. Ytterligare simuleringas visar att effektiviteten kan nå upp till 52-62% (för 0.33 och 0.5 fyllnadsfaktor, respektive beräknad med FDTD) genom att inkludera optiska kaviteter.

Particle accelerators exist in different variants serving different purposes in scientific, industrial and medical applications. In the field of high energy physics, hadron colliders are often seen as “exploration machines” and lepton colliders as facilities for precise measurements of the particles. The Large-Hadron-Collider (LHC) is such a hadron collider at which the Higgs boson was found, that couldn’t be characterized completely [et.20], yet. It is a storage ring collider where the colliding particles are accelerated in forward direction by the radio frequency cavities and perpendicularly, to form a closed ring, by bending magnets. This results in strong radiation, called synchrotron radiation named after the synchrotron circular accelerator in which it was discovered [et.20]. For colliders of light particles this is a big disadvantage and was one of the limiting factors of LHC’s predecessor, the Large Electron-Positron collider (LEP), because the radiation power losses scale with 1/m4 [Hue, Wie07]. Hence, there are concepts for linear colliders as the next large high energy physics experiment machine, namely the International Linear Collider (ILC) and the Compact Linear Collider (CLIC) [Eur20, The18]. Nonetheless, the synchrotron radiation can also be used for scientific purposes. Nowadays, dedicated synchrotrons exist that utilise this broadband and intense light. The Karlsruhe Institute of Technology (KIT) has got a storage ring called KArlsruhe Research Accelerator (KARA) providing its radiation for a variety of use cases ranging from imaging with hard x-rays to lithography, and THz and infrared spectroscopy [ANK14]. Modern synchrotron light sources make use of insertion devices (ID) that provide much higher intensities than bending magnets do. One kind of such an ID is a so-called wiggler. Its spectrum is similar broadband as that of a bending magnet, but much stronger enabling hard x-ray imaging at KARA’s IMAGE beamline. Though in general synchrotron radiation is not wanted at high energy physics accelerators, the next generation of colliders will make use of it, too. They produce it on purpose before accelerating the beam to its final energy to actively reduce the transverse momentum. Thereby the luminosity and thus the statistics of the collision processes can be increased. For this purpose very strong wigglers, then called damping wiggler, are used. For the CLIC project it is planned to have two damping rings each with two straight sections with 26 2-m-long superconducting wiggler with a magnetic field of 3 T and a period length of 0.05 m [The18] each. The layout of the CLIC facility and one damping ring is sketched in Fig. 1.1. After the injection and pre-acceleration the electron beams’ oscillations are damped in the pre damping rings (labelled PDR in the figure) and damping rings (DR) with the damping wigglers, before the beams are longitudinally compressed (in BC1, BC2), guided (in TA, BDS), and accelerated (in booster linac, main linac) untill they collide in the interaction point (IP). A novel cooling technique for superconducting wigglers was developed by the Budker Institute of Nuclear Physics (BINP) to make individual wigglers more accessible to maintenance by using conduction cooling instead of bath cooling. This and the expected high heat load from the upstream wigglers are challenging for devices with these high fields and relatively short period length. Also, damping rings with superconducting wigglers have not been built yet, so the demand to test a prototype of such a wiggler emerged. The Conseil Européen pour la Recherche Nucléaire (CERN), the KIT and the BINP have established a collaboration to investigate the maturity of the technology and to investigate beam dynamic effects caused by a prototype of such a wiggler build by BINP and installed in KARA. The wiggler serves as a prototype for CERN and as a light source for KIT’s IMAGE beamline at KARA. In this thesis, the question if damping wigglers can be used in such large scale, as planned for damping rings, will be tackeled. Can the beam dynamics of the wigglers be simulated properly and can the effects be experimentally confirmed? Or does it turn out that beam dynamics of damping wigglers are not understood sufficiently well to rely the next generation collider physics accelerators thereon when ca. 50 % of the damping rings’ circumference are damping wigglers? Can one find effects appearing in real devices that are not covered by simulations so far? These are questions, this thesis tries to answer by simulating the damping wiggler prototype and doing measurements with it in KARA. For the CLIC project emerging from a conceptual design study to a technical design study it is of interest if this prototype can fulfill the expectations and work reliably in a real accelerator. This is the topic of this thesis. As collective effects showed to be of importance for the CLIC damping rings [The18] in the past, it is also of interest to do first experiments with this wiggler with regard to collective effects. At KARA there is also a strong research on one collective effect, the so-called “micro-bunching instability” which is under investigation in theory, simulation and experiments. Therefore it makes sense to experimentally investigate the wiggler’s influence on this particular collective effect within this work. After the necessary theoretical background on accelerator physics, beam dynamics, synchrotron radiation and insertion devices—in Chapter 2—, the experimental setup of the accelerator and its insertion devices and then the simulation and measurement techniques are presented in Chapter 3. Because often IDs are simulated as many alternating dipoles or quadrupoles only acting in the vertical plane no common approach exists that satisfies our needs for the simulation, in particular higher order multipoles represented in actual field data. So different approaches for including the wiggler into the storage ring models are evaluated and compared against each other. This encompasses, firstly, the transformation of magnetic field data to Fourier components as input for different wiggler implementation of the particle tracking code elegant. And, secondly, the selection of the actual implementation of the wiggler model. The available options and choices made will be discussed in Chapter 4. The basic functionality, features and influence of the wiggler on beam dynamics are tested experimentally in the 2.50 GeV operation mode. In this mode also heat load studies were conducted. Some further beam dynamics investigations and experiments were carried out in the short-bunch, low-α mode at 1.30 GeV beam-energy to better understand the mechanisms of coherent synchrotron radiation and the so-called micro-bunching instability. Accordingly, the development of two different models of the storage ring is presented in Chapter 5. Chapter 6 describes the experimental characterisation of the wiggler. This includes the findings of the heat load studies as well as the beam dynamics investigations that yielded additional octupole components of the integrated magnetic field. The wiggler was not only used to investigate its transverse beam dynamics, but it was also used to investigate the influence of the damping time on the THz radiation emitted by the electron beam. This happened in the special short-bunch, low-α mode. The optics manipulation needed to operate the wiggler in this special operation mode along with the experimental results of these efforts are presented in Chapter 7.

Josephson junctions (JJs) with Josephson energy $E_J \lesssim 1K$ are widely employed as non-linear elements in superconducting circuits for quantum computing, operating at milli-Kelvin temperatures. Here we experimentally study incoherent phase slips (IPS) in low-$E_J$ Aluminum-based JJs at $T, 10 pages, 6 figures

Nowadays, there exist many physical realizations of qubits for quantum computing. Some modern and prominent examples of types of quantum computer platforms are the superconducting qubits, spin qubits, trapped-ion qubits, photonic qubits and topological qubits. In this document, we overview the underlying physical principles of three of these platforms: superconducting qubits, spin qubits in semiconductors and trapped-ion qubits. This work is also meant to serve as an introduction to these technologies. This document is organized as follows. First, we introduce the so called DiVincenzo criteria [3] that de ne the requirements that a system has to meet to be useful as a quantum computer, with a brief explanation of the decoherence and the Bloch sphere. Second, we go through the three quantum computer platforms. The second part begins with an introduction to the superconducting qubits by explaining some basic concepts of superconductivity such as the Meissner effect and the flux quantization on a superconducting ring. This is done in order to understand the basics of the Josephson junctions used in this type of qubits. Then, the three main types of superconducting qubits are explained. Superconducting qubits can be divided on three main types depending on the variable chosen to encode the qubit state: charge qubits, phase qubits and flux qubits. In the sections dedicated to the charge qubits, the Cooper pair box, which is the predecessor to the transmon qubit, is explained in order to understand the transmon qubits. We focus our attention mainly on the transmon qubit because it seems to be the most popular type of superconducting qubit. In the following sections, the spin qubits and the trapped-ion qubits are also explained. There are also different possibilities for implementing spin qubits like, for example, in NV-centers in a diamond lattice, color centers, spins on molecules, shallow dopants or electrostatically confined quantum dots in semiconductors. We focus on the latter. For trapped-ion qubits there are different state encoding possibilities depending on the energy levels used as computational subspace so, for example, we have Zeeman, hyperfine, optical and fine-structure trapped-ion qubits. Each section corresponding to the different qubit implementations explained on this work follows the same basic scheme. The explanation of each implementation is done by showing how it satisfies the five DiVincenzo criteria.

We report the experimental nondemolition measurement of coherence, predictability and concurrence on a system of two qubits. The quantum circuits proposed by De Melo et al. (Phys Rev Lett 98(25):250501, 2007) are implemented on IBM Q (superconducting circuit) and IonQ (trapped ion) quantum computers. Three criteria are used to compare the performance of the different machines on this task: measurement accuracy, nondemolition of the observable, and quantum state preparation. We find that the IonQ quantum computer provides constant state fidelity through the nondemolition process, outperforming IBM Q systems on which the fidelity consequently drops after the measurement. Our study compares the current performance of these two technologies at different stages of the nondemolition measurement of bipartite complementarity.

Quantum memories and networks for distributed quantum information processing require links between the microwave, mechanical, and optical domains. Coherent integration of long-lived superconducting qubits (SCQs) with optomechanical and photonic devices (OMPDs) remains an outstanding challenge. We present a step towards coherent integration using a suspended trace air-gap resonator (STAR): a superconducting resonator on a silicon-on-insulator (SOI) substrate with the signal trace suspended by silicon tethers above and between galvanically connected ground metal planes. As a result, the electric field energy is closely confined within the microwave structure, yielding lower crosstalk compared to conventional coplanar waveguides (CPW). An order of magnitude improvement in the quality factors for STAR over previous work on SOI is achieved, in a transverse cross-sectional area that is an order of magnitude more compact. Electric field participation in lossy bulk dielectrics, a dominant source of energy leakage in previous measurements of aluminum CPW resonators on SOI, is virtually eliminated in STAR. The loss from the metal-air interface now dominates, but can be reduced by several factors using superconductors with better surface properties. Most importantly, STAR fabrication is compatible with Josephson junction and air-bridge deposition for highly coherent integration of SCQs and OMPDs to realize proposals for quantum information storage and networking.

International audience; In spite of their complex geometry, CICCs have to be modelled with a rather simple description for coupling losses when operating in transient regime. Difficulties to predict AC losses in superconducting cable have already been shown in previous models such as the new analytical one developed at CEA named COLISEUM (after COupling Losses analytIcal Staged cablEs Unified Model) and CEA heuristic one MPAS. In this paper, we present a parametric analysis for coupling losses in superconducting strands and cables subjected to time-varying transverse magnetic field by using the recently developed COLISEUM. This analysis aims at understanding trends of the model in a broad domain of investigation and their associated limits of application. We show that for a wide range of parameters, it is possible to reduce this model of four time constants to a smaller subset. This reduction brings simplifications to the current COLISEUM and enables it to be consistent with the number of time constants considered in the MPAS model from CEA.

International audience; As part of the Jefferson Lab 12 GeV upgrade, the Hall B CLAS12 system requires two superconducting iron-free magnets — a torus and a solenoid. The physics requirements to maximize space for the detectors guided engineers toward particular coil designs for each of the magnets which, in turn, led to the choice of using conduction cooling. The torus consists of 6 trapezoidal NbTi coils connected in series with an operating current of 3770 A. The solenoid is an actively shielded 5 T magnet consisting of 5 NbTi coils connected in series operating at 2416 A. Within the hall, the two magnets are located in close proximity to each other and are completely covered both inside and outside by particle detectors. Stringent size limitations were imposed for both magnets and introduced particular design and fabrication challenges. This paper describes the design, construction, installation, commissioning, and operation of the two magnets.

We describe a microfabrication process that, thanks to a specifically tailored sidewall profile, enables for the first-time wafer-scale arrays of high-aspect ratio through-silicon vias (TSVs) coated with DC-sputtered Aluminum, achieving at once superconducting and CMOS-compatible 3D interconnects. Void-free conformal coating of up to $500\ \mu\mathrm{m}$ -deep and $50\ \mu\mathrm{m}$ -wide vias with a mere $2\ \mu\mathrm{m}$ -thick layer of Al, a widely available metal in for IC manufacturing, was demonstrated. Single-via electric resistance as low as $468 \mathrm{m}\Omega$ at room temperature and superconductivity at 1.25 K were measured by a cross-bridge Kelvin resistor structure. This work establishes the fabrication of functional superconducting interposers suitable for 3D integration of high-density silicon-based quantum computing architectures.

Light does not typically scatter light, as witnessed by the linearity of Maxwell’s equations. In this work, we demonstrate two superconducting circuits, in which microwave photons have well-defined energy and momentum, but their lifetime is finite due to decay into lower energy photons. The circuits we present are formed with Josephson junction arrays where strong quantum phase-slip fluctuations are present either in all of the junctions or in only a single junction. The quantum phase-slip fluctuations are shown to result in the strong inelastic photon-photon interaction observed in both circuits. The phenomenon of a single photon decay provides a new way to study multiple long-standing many-body problems important for condensed matter physics. The examples of such problems, which we cover in this work include superconductor to insulator quantum phase transition in one dimension and a general quantum impurity problem. The photon lifetime data can be treated as a rare example of a verified and useful quantum many-body simulation.

International audience; In spite of their complex geometry, CICCs have to be modelled with a rather simple description for coupling losses when operating in transient regime. Difficulties to predict AC losses in superconducting cable have already been shown in previous models such as the new analytical one developed at CEA named COLISEUM (after COupling Losses analytIcal Staged cablEs Unified Model) and CEA heuristic one MPAS. In this paper, we present a parametric analysis for coupling losses in superconducting strands and cables subjected to time-varying transverse magnetic field by using the recently developed COLISEUM. This analysis aims at understanding trends of the model in a broad domain of investigation and their associated limits of application. We show that for a wide range of parameters, it is possible to reduce this model of four time constants to a smaller subset. This reduction brings simplifications to the current COLISEUM and enables it to be consistent with the number of time constants considered in the MPAS model from CEA.

Chemical solution deposition (CSD) of YBa2Cu3O7−δ (YBCO) nanocomposites from colloidal precursor solutions containing double metal oxide preformed nanocrystals is a promising, cost effective and reproducible approach to produce superconducting films with high critical current density (Jc) and enhanced pinning. Here, the influence of the preformed nanocrystal composition on the microstructure and superconducting properties of the YBCO nanocomposite films is studied, with a focus on establishing a simple and scalable process to grow nanocomposites that can be transferred to grow nano-added coated conductors. Colloidal stable BaZrO3, BaHfO3, BaTiO3 and SrZrO3 nanocrystals (3-6 nm in diameter) were synthesized and added to an environment-friendly low-fluorine YBCO precursor solution. High-quality superconducting layers were grown on LaAlO3 single-crystal substrates from these four nanocomposite precursor solutions in a single deposition process, without the need of a seed layer, yielding Jc of 4-5 MA/cm² at 77 K in self-field. The different YBCO microstructures produced by the four types of nanocrystals and the resulting microstrain of the films are compared and related with the magnetic-field and angular dependence of Jc. We demonstrate the BaHfO3-containing nanocomposite as the best-performing with a homogeneous distribution of nanoparticles with 7 nm in average diameter and a high density of stacking faults, which leads to some of the best superconducting properties ever achieved via low-fluorine CSD. The Jc exhibits a much smoother decay in applied magnetic fields and a much more isotropic behaviour for non-parallel magnetic fields, and the pinning force is increased by a factor of 3.5 at 77 K and 1 T with respect to the pristine film.

In this paper we present a new method to determine separately inter and intragranular magnitudes of polycrystalline superconducting materials, such as: intrinsic effective anisotropy, $$t=\rho _c/\rho _{ab}$$ , the slope of the linear part in the temperature dependence of the ab-planes resistivity, $$A_{ab}=\Delta \rho _{ab}/\Delta T$$ , the weak links resistivity, $$\rho _{wl}$$ , and the orientation probability of the grains’ a-axes along a certain preferential direction, $$\gamma _{xa}$$ . Here, $$\rho _{ab}$$ , $$\rho _{c}$$ and T are the main values of the resistivitity tensor and the measurement temperature, respectively. The application of the method, illustrated by two (Bi,Pb)2223 polycrystalline samples, allows comparing the parameters obtained for the grains with those reported in single crystals. Moreover, we have demonstrated that different compacting pressures in the fabrication of the pellets change not only the inter, but also the intragranular properties of the ceramics. The method could be used to follow the different steps during the synthesis of polycrystalline high $$T_{c}$$ superconductors in order to optimise the process.

In condensed matter systems, coherent backscattering and quantum interference in the presence of time-reversal symmetry lead to well-known phenomena such as weak localization (WL) and universal conductance fluctuations (UCF). Here we use multi-pass Landau-Zener transitions at the avoided crossing of a highly-coherent superconducting qubit to emulate these phenomena. The average and standard deviation of the qubit transition rate exhibit a dip and peak when the driving waveform is time-reversal symmetric, analogous to WL and UCF, respectively. The higher coherence of this qubit enabled the realization of both effects, in contrast to earlier work arXiv:1204.6428, which successfully emulated UCF, but did not observe WL. This demonstration illustrates the use of non-adiabatic control to implement quantum emulation with superconducting qubits., 14 pages, 7 figures; two reference added; figure 4 changed; 2 more figures added in Suppl. Info

Zukünftige Beschleunigerprojekte und Nutzerexperimente erfordern für ihren Betrieb einen hochbrillanten Elektronenstrahl mit hohem mittlerem Strom. Eine Elektronenquelle mit dem Potential die Anforderungen erfüllen, ist ein supraleitender Hochfrequenz (SHF) Photoinjektor im Dauerstrichbetrieb. Die Strahldynamik eines solchen Photoinjektor Systems bestimmt die maximal zu erreichende Strahlbrillanz und wird ihrerseits von den Design und Betriebsparametern des Photoinjektors beeinflusst. Ziel ist immer die entscheidenden Design- und Betriebsparameter der Elektronenquelle hinsichtlich einer maximalen Strahlbrillanz zu wählen. Diese Aufgabe verlangt ein detailliertes Verständnis der Strahldynamik-Prozesse. Ferner ist es notwendig, eine Optimierung des Photoinjektors als Ganzes, mit dem Ziel einer maximalen Strahlqualität bei hohem mittlerem Strom, vorzunehmen. Dieses ermöglicht auch, die physikalischen Grenzen eines gegebenen Designs zu ermitteln und im Betrieb vollständig auszunutzen. Diese Doktorarbeit befasst sich mit der Strahldynamik in einem SHF Photoinjektor, unter Berücksichtigung interner Raumladungseffekte. Die Erkenntnisse zur Strahldynamik werden für die Entwicklung eines Optimierungsprogramms verwendet, um die Leistung des Injektors hinsichtlich der Strahlbrillanz zu verbessern. Die entwickelte Methode basiert auf Pareto-Optimierung mehrerer Zielfunktionen, unter Verwendung eines generischen Algorithmus. Das zentrale Ergebnis dieser Arbeit umfasst ein universelles Optimierungsprogramm, das für Photoinjektoren unabhängig von ihrem Design und Anwendungsgebiet genutzt werden kann. Für den Betrieb mit hoher Strahlbrillanz ist es möglich aus den erhaltenen Pareto-optimalen Lösungen einen stabilen Satz an Einstellwerten für den Photoinjektor zu extrahieren. Durch die allgemeine Optimierungsstrategie lässt sich das entwickelte Programm auch für andere Beschleunigerabschnitte, oder die Optimierung einer ganzen Anlage mit erweiterter Zielsetzung anpassen. An increasing number of future accelerator projects, light sources and user experiments require high brightness, high average current electron beams for operation. Superconducting radio-frequency (SRF) photoinjectors running in continuous-wave (cw) mode hold the potential to serve as an electron source that generates electron beams of high brightness. Different operation and design parameters of the SRF photoinjector impact the beam dynamics and, thus, the beam brightness. Therefore, an in-depth understanding of the beam dynamics processes in an SRF photoinjector and the dependency of the beam dynamics on the photoinjector set parameters is crucial. A high brightness beam operation requires a global optimization of the SRF photoinjector that allows to find suitable photoinjector settings and to figure out and extend the physical performance limits of the investigated injector design. The dissertation at hand offers a detailed analysis of the beam dynamics in an SRF photoinjector regarding internal space charge effects. Furthermore, the impact of the photoinjector elements on the electron beam is discussed. The lessons learned from this theoretical view are implemented in the development of an optimization tool to achieve a high brightness performance. A universal multi-objective optimization program based on a generic algorithm was developed to extract stable, optimum gun parameter from Pareto-optimum solutions. This universal tool is able to optimize and find the physical performance limit of any (S)RF photoinjector independent from the individual application of the electron source (energy recovery linac, free electron laser, ultra-fast electron diffraction). This thesis thereby verifies and complements existing theoretical considerations regarding photoinjector-beam interactions. The global optimization strategy can be introduced to variable optimization objectives as well as it can be extended to an optimization of further parts of the accelerator facility.

Evidence of nematic effects in the mixed superconducting phase of slightly underdoped Ba(Fe1-xCox)2As2 is reported. We have found strong in-plane resistivity anisotropy for crystals in different strain conditions. For these compositions, there is no magnetic long-range order, so the description may be ascribed to the interplay between the superconducting and nematic order parameters. A piezoelectric-based apparatus is used to apply tensile or compressive strain to tune nematic domain orientation in order to examine intrinsic nematicity. Measurements are done under a rotating magnetic field, and the analysis of the angular dependence of physical quantities identifies the cases in which the sample is detwinned. Furthermore, the angular dependence of the data allows us to evaluate the effects of nematicity on the in-plane superconductor stiffness. Our results show that although nematicity contributes in a decisive way to the conduction properties, its contributions to the anisotropy properties of the stiffness of the superconducting order parameter is not as significant in these samples., Instituto de Física de Líquidos y Sistemas Biológicos

International audience; A future circular collider (FCC) with a center-of-mass energy of 100 TeV and a circumference of around 100 km, or an energy upgrade of the LHC (HE-LHC) to 27 TeV require bending magnets providing 16 T in a 50-mm aperture. Several development programs for these magnets, based on Nb$_3$Sn technology, are being pursued in Europe and in the U.S. In these programs, cos-theta, block-type, common-coil, and canted-cos-theta magnets are explored; first model magnets are under manufacture; limits on conductor stress levels are studied; and a conductor with enhanced characteristics is developed. This paper summarizes and discusses the status, plans, and preliminary results of these programs.

ISOLDE is a facility dedicated to the production of a large variety of Radioactive Ion Beams. The facility is located at the European Organization for Nuclear Research (CERN). In addition to two target stations followed by low energy separators, the facility includes a post-accelerating linac with both normal conducting (REX) and superconducting (HIE-ISOLDE) sections. The HIE-ISOLDE section consists of four cryomodules with five SRF cavities each that need to be phased individually. In this paper, we will describe the procedure and the software applications developed to phase each of the cavities as well as improvements that will be introduced in the near future to reduce the time it takes to complete the process., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

Recent progress in the development of superconducting nanowire single-photon detectors (SNSPDs) has delivered excellent performances, and has had a great impact on a range of research fields. The timing jitter, which denotes the temporal resolution of the detection, is a crucial parameter for many applications. Despite extensive work since their apparition, the lowest jitter achievable with SNSPDs is still not clear, and the origin of the intrinsic limits is not fully understood. Understanding its intrinsic behaviour and limits is a mandatory step toward improvements. Here, we report our experimental study on the intrinsically-limited timing jitter in molybdenum silicide (MoSi) SNSPDs. We show that to reach intrinsic jitter, several detector properties such as the latching current and the kinetic inductance of the devices have to be understood. The dependence on the nanowire cross-section and the energy dependence of the intrinsic jitter are exhibited, and the origin of the limits are explicited. System timing jitter of 6.0 ps at 532 nm and 10.6 ps at 1550 nm photon wavelength have been obtained.

Quantum computing is a promising field regarding computation capabilities for several important tasks. One of the most promising solid-state quantum technology, which is being developed at Qutech (TU Delft), is superconducting qubits. In superconducting quantum processors, qubits are arranged along a 2D grid that only permits information transmission between adjacent qubits. In order to run a quantum algorithm on such a processor a mapping strategy of scheduling-placement-routing is needed. Quantum computers hold the promise for solving efficiently important problems in computational sciences that are intractable nowadays by exploiting quantum phenomena such are superposition and entanglement. Research in quantum computing is mainly driven by the development of quantum devices and quantum algorithms. Quantum algorithms can be described by quantum circuits, which are hardware agnostic -- e.g it is assumed that any arbitrary interaction between qubits is possible. However, real quantum processors have a series of constraints that must be complied to when running a quantum algorithm. Therefore, a mapping process that adapts the quantum circuit to chip's constraints is required. The mapping process will, in general, increase the number of gates and/or the circuit depth. As qubits and gates are error prone, it will result in an increment of the failure rate of computation while running the adapted quantum algorithm in a given quantum device. Most of the current mapping models optimize and are assessed based on two metrics: circuit depth (or latency) and number of (movement) operations added; they should be as minimal as possible. However, these metrics are not giving any information about how the mapping process is affecting the reliability of the algorithm. In other words, can still the algorithm produce `good' results after being mapped? The aim of this thesis is to propose some new mapping metrics that allow to study the impact of the mapping process on the algorithm's reliability. These are, quantum fidelity, probability of success of the algorithm and quantum volume. They could be used not only to assess the quality of the mapping procedure but also as parameters to be optimized by the mapping. To this purpose, different quantum algorithms have been mapped into the superconducting quantum processor, called Surface-17, developed at QuTech.

In this paper we summarize the evolution and contributions of superconducting magnets to particle accelerators as chronicled over the last 50 years of Particle Accelerator Conferences (PAC, NA-PAC and IPAC). We begin with an historical overview based primarily on PAC Proceedings augmented with references to key milestones in the development of superconducting magnets for particle accelerators. We then provide some illustrative examples of applications that have occurred over the past 50 years, focusing on those that have either been realized in practice or provided technical development for other projects, with discussion of possible future applications.

2D materials possess wide‐tuning properties ranging from semiconducting and metallization to superconducting, etc., which are determined by their structure, empowering them to be appealing in optoelectronic and photovoltaic applications. Pressure is an effective and clean tool that allows modifications of the electronic structure, crystal structure, morphologies, and compositions of 2D materials through van der Waals (vdW) interaction engineering. This enables an insightful understanding of the variable vdW interaction induced structural changes, structure–property relations as well as contributes to the versatile implications of 2D materials. Here, the recent progress of high‐pressure research toward 2D materials and heterostructures, involving graphene, boron nitride, transition metal dichalcogenides, 2D perovskites, black phosphorene, MXene, and covalent–organic frameworks, using diamond anvil cell is summarized. A detailed analysis of pressurized structure, phonon dynamics, superconducting, metallization, doping together with optical property is performed. Further, the pressure‐induced optimized properties and potential applications as well as the vision of engineering the vdW interactions in heterostructures are highlighted. Finally, conclusions and outlook are presented on the way forward., Pressure can modify the electronic structures, crystal structures, morphologies, and compositions of 2D materials, which contributes to their versatile implications. Here, the recent progress of high‐pressure research of 2D materials and associated heterostructures is summarized. The structure, phonon dynamics, superconducting, metallization, doping, optical property, optimized properties, and future research directions of them are discussed in detail.

A detailed knowledge of the resistivity of high-temperature superconductors in the overcritical current regime is important to achieve reliable numerical simulations of applications such as superconducting fault current limiters. We have previously shown that the combination of fast pulsed current measurements and finite element analysis allows accounting for heating effects occurring during the current pulses. We demonstrated that it is possible to retrieve the correct current and temperature dependence of the resistivity data points of the superconductor material. In this contribution, we apply this method to characterize the resistivity vs. current and temperature of commercial REBCO tapes in the overcritical current regime, between 77 and 90 K and in self-field conditions. The self-consistency of the overcritical resistivity model ρ O C ( I , T ) is verified by comparing DC fault measurements with the results of numerical simulations using this model as input. We then analyze by numerical simulation to what extent using the ρ O C ( I , T ) model instead of the widely used power-law model ρ P W L ( I , T ) affects the thermal and electrical performance of the tapes in the practical case of a superconducting fault current limiter. A remarkable difference is observed between the measured overcritical current resistivity model ρ O C ( I , T ) and the power-law resistivity model ρ P W L ( I , T ) . In particular, the simulations using the power-law model show that the device quenches faster than with the overcritical resistivity model. This information can be used to optimize the architecture of the stabilizer in superconducting fault current limiters.

This paper presents the fabrication and electrical characterization of superconducting high-aspect ratio through-silicon vias DC-sputtered with aluminum. Fully conformal and void-free coating of $300~\mu \text{m}$ -deep and $50~\mu \text{m}$ -wide vias with Al, a CMOS-compatible and widely available superconductor, was made possible by tailoring a funneled sidewall profile for the axisymmetric vias. Single-via electric resistance as low as 80.44 $\text{m}\Omega $ at room temperature and superconductivity below 1.28 K were measured by a cross-bridge Kelvin resistor structure. This work thus demonstrates the fabrication of functional superconducting interposer layers, suitable for high-density 3D integration of silicon-based quantum computing architectures.