Hall Effect Gyrators and Circulators

The electronic circulator, and its close relative the gyrator, are invaluable tools for noise management and signal routing in the current generation of low-temperature microwave systems for the implementation of new quantum technologies. The current implementation of these devices using the Faraday effect is satisfactory, but requires a bulky structure whose physical dimension is close to the microwave wavelength employed. The Hall effect is an alternative non-reciprocal effect that can also be used to produce desired device functionality. We review earlier efforts to use an ohmically-contacted four-terminal Hall bar, explaining why this approach leads to unacceptably high device loss. We find that capacitive coupling to such a Hall conductor has much greater promise for achieving good circulator and gyrator functionality. We formulate a classical Ohm-Hall analysis for calculating the properties of such a device, and show how this classical theory simplifies remarkably in the limiting case of the Hall angle approaching 90 degrees. In this limit we find that either a four-terminal or a three-terminal capacitive device can give excellent circulator behavior, with device dimensions far smaller than the a.c. wavelength. An experiment is proposed to achieve GHz-band gyration in millimetre (and smaller) scale structures employing either semiconductor heterostructure or graphene Hall conductors. An inductively coupled scheme for realising a Hall gyrator is also analysed.Comment: 18 pages, 15 figures, ~5 MB. V3: sections V-VIII revisited plus other minor changes, Fig 2 added. Submitted to PR

Cryogenic Control Beyond 100 Qubits

Quantum computation has been a major focus of research in the past two decades, with recent experiments demonstrating basic algorithms on small numbers of qubits. A large-scale universal quantum computer would have a profound impact on science and technology, providing a solution to several problems intractable for classical computers. To realise such a machine, today's small experiments must be scaled up, and a system must be built which provides control and measurement of many hundreds of qubits. A device of this scale is challenging: qubits are highly sensitive to their environment, and sophisticated isolation techniques are required to preserve the qubits' fragile states. Solid-state qubits require deep-cryogenic cooling to suppress thermal excitations. Yet current state-of-the-art experiments use room-temperature electronics which are electrically connected to the qubits. This thesis investigates various scalable technologies and techniques which can be used to control quantum systems. With the requirements for semiconductor spin-qubits in mind, several custom electronic systems, to provide quantum control from deep cryogenic temperatures, are designed and measured. A system architecture is proposed for quantum control, providing a scalable approach to executing quantum algorithms on a large number of qubits. Control of a gallium arsenide qubit is demonstrated using a cryogenically operated FPGA driving custom gallium arsenide switches. The cryogenic performance of a commercial FPGA is measured, as the main logic processor in a cryogenic quantum control system, and digital-to-analog converters are analysed during cryogenic operation. Recent work towards a 100-qubit cryogenic control system is shown, including the design of interconnect solutions and multiplexing circuitry. With qubit fidelity over the fault-tolerant threshold for certain error correcting codes, accompanying control platforms will play a key role in the development of a scalable quantum machine

Quantum acoustics with superconducting circuits

The past 20 years has seen rapid developments in circuit quantum electrodynamics, where superconducting qubits and resonators are used to control and study quantum light-matter interaction at a fundamental level. The development of this field is strongly influenced by quantum information science and the prospect of realizing quantum computation, but also opens up opportunities for combinations of different physical systems and research areas. Superconducting circuits in the microwave domain offer a versatile platform for interfacing with other quantum systems thanks to strong nonlinearities and zero-point fluctuations, as well as flexibility in design and fabrication. Hybrid quantum systems based on circuit quantum electrodynamics could enable novel functionalities by exploiting the strengths of the individual components.This thesis covers experiments coupling superconducting circuits to surface acoustic waves (SAWs), mechanical waves propagating along the surface of a solid. Strong coupling can be engineered using the piezoelectric properties of GaAs substrates, and our experiments exploit this to investigate phenomena in quantum field-matter interaction. A key property of surface acoustic waves is the slow propagation speed, typically five orders of magnitude slower than light in vacuum, and the associated short wavelength. This enables the giant atom regime where the artificial atom in the form of a superconducting circuit is large compared to the wavelength of interacting SAW radiation, a condition which is difficult to realize in other systems. Experiments described in this thesis use these properties to demonstrate electromagnetically induced transparency for a mechanical mode, as well as non-Markovian interactions between an artificial giant atom and the SAW field. When the SAW field is confined to a resonant cavity, the short wavelength allows multimode spectra suitable for interacting with a frequency comb. We use a multimode SAW resonator to characterize the ensemble of microscopic two-level system defects with a two-tone spectroscopy approach. Finally, we introduce a hybrid superconducting-SAW resonator with applications in quantum information processing in mind. Experiments with this device demonstrate entanglement of SAW modes, and show promising results on the way to engineer cluster states for quantum computation in continuous variables

Hardware and Methods for Scaling Up Quantum Information Experiments

Quantum computation promises to solve presently intractable problems, with hopes of yielding solutions to pressing issues to society. Despite this, current machines are limited to tens of qubits. The field is in a state of continuous scaling, with groups around the world working on all aspects of this problem. The work of this thesis aims to contribute to this effort. It is motivated by the goal of increasing both the speed and bandwidth of experiments conducted within our laboratory. Low-loss radio-frequency multiplexers were characterised at cryogenic temperatures, with some shown to operate at below 7mK. The Analog Devices ADG904 was one of these, and its insertion loss was measured at <0.5dB up to 2GHz. Their heat load was measured, and it was found that a switching speed of 10 MHz with an RF signal power of -30dB dissipates 43uW. Installing these switches yields a benefit over installing extra cabling in our cryostat for a switching speed of up to 2MHz and RF power of -30dBm. A switch matrix was prototyped for cryogenic operation, enabling re-routing of wiring inside a cryostat with a minimally increased thermal load. This could be used to significantly increase the scale of high frequency experiments. This switch has also been embedded within a calibration routine, facilitating measurement of a specific feature of interest at millikelvin temperatures. As the field of quantum engineering scales, such measurements will be crucial to close the loop, providing feedback to fabrication and semiconductor growth efforts. Finally, a rapid-turnaround test rig has been developed which has 32 high frequency and 100 DC lines, enabling tests of significant scale in liquid helium. This reduces the time per experiment at 4.2 K to hours rather than days, enabling tests such as thermal cycling, as well as the evaluation of on-chip structures or active electronics and classical computing hardware; which are all necessary elements of any solid state quantum computing architecture

Integrated Readout at the Quantum-Classical Interface of Semiconductor Qubits

Quantum computing promises to deliver uniquely powerful information processing machines by exploiting the quantum phenomena of superposition and entanglement. In solid-state systems, there has been significant progress in the isolation and control of the fundamental units needed to build such machines, known as qubits. However, scaling-up the number of qubits to the point where sophisticated algorithms can be performed presents considerable experimental challenges. In particular, it is becoming increasingly apparent that a new class of tools will be required to interface between fragile quantum systems, and the classical readout and control hardware of the outside world. This thesis presents experimental investigations towards the development of a scalable readout architecture for semiconductor qubit platforms. Fast readout of a GaAs-AlGaAs double quantum dot in the few-electron regime is first demonstrated via an embedded dispersive gate sensor (DGS), alleviating the burden of requiring separate charge sensors for every qubit. The sensitivity and bandwidth of this technique are extracted and benchmarked against well-established readout methods. Dispersive gate sensing of quantum point contacts (QPCs) is then presented, probing charge rearrangement within the local electrostatic environment of quasi one-dimensional channels. A low-loss, lumped-element, LC resonant circuit is also implemented for frequency multiplexed readout. The second set of experiments concern the design and characterisation of miniaturised, on-chip circulators based on the quantum Hall effect, and the quantum anomalous Hall effect. Microwaves are first capacitively coupled into edge magnetoplasmon modes in a mesoscopic GaAs-AlGaAs droplet. Non-reciprocal forward transmission comparable to off-the-shelf components is observed, which is accounted for within an interferometric picture. This circulator design is then extended to thin films of the three-dimensional topological insulator, Cr-doped (Bi,Sb)2Te3, wherein similar non-reciprocity is demonstrated in the absence of an external magnetic field

Fault and Defect Tolerant Computer Architectures: Reliable Computing With Unreliable Devices

This research addresses design of a reliable computer from unreliable device technologies. A system architecture is developed for a fault and defect tolerant (FDT) computer. Trade-offs between different techniques are studied and yield and hardware cost models are developed. Fault and defect tolerant designs are created for the processor and the cache memory. Simulation results for the content-addressable memory (CAM)-based cache show 90% yield with device failure probabilities of 3 x 10(-6), three orders of magnitude better than non fault tolerant caches of the same size. The entire processor achieves 70% yield with device failure probabilities exceeding 10(-6). The required hardware redundancy is approximately 15 times that of a non-fault tolerant design. While larger than current FT designs, this architecture allows the use of devices much more likely to fail than silicon CMOS. As part of model development, an improved model is derived for NAND Multiplexing. The model is the first accurate model for small and medium amounts of redundancy. Previous models are extended to account for dependence between the inputs and produce more accurate results

MOCAST 2021

The 10th International Conference on Modern Circuit and System Technologies on Electronics and Communications (MOCAST 2021) will take place in Thessaloniki, Greece, from July 5th to July 7th, 2021. The MOCAST technical program includes all aspects of circuit and system technologies, from modeling to design, verification, implementation, and application. This Special Issue presents extended versions of top-ranking papers in the conference. The topics of MOCAST include:Analog/RF and mixed signal circuits;Digital circuits and systems design;Nonlinear circuits and systems;Device and circuit modeling;High-performance embedded systems;Systems and applications;Sensors and systems;Machine learning and AI applications;Communication; Network systems;Power management;Imagers, MEMS, medical, and displays;Radiation front ends (nuclear and space application);Education in circuits, systems, and communications

Approaches to Building a Quantum Computer Based on Semiconductors

Throughout this Ph.D., the quest to build a quantum computer has accelerated, driven by ever-improving fidelities of conventional qubits and the development of new technologies that promise topologically protected qubits with the potential for lifetimes that exceed those of comparable conventional qubits. As such, there has been an explosion of interest in the design of an architecture for a quantum computer. This design would have to include high-quality qubits at the bottom of the stack, be extensible, and allow the layout of many qubits with scalable methods for readout and control of the entire device. Furthermore, the whole experimental infrastructure must handle the requirements for parallel operation of many qubits in the system. Hence the crux of this thesis: to design an architecture for a semiconductor-based quantum computer that encompasses all the elements that would be required to build a large scale quantum machine, and investigate the individual these elements at each layer of this stack, from qubit to readout to control

SUSTAINABLE ENERGY HARVESTING TECHNOLOGIES – PAST, PRESENT AND FUTURE

Chapter 8: Energy Harvesting Technologies: Thick-Film Piezoelectric Microgenerato