JETC (Japanese Technology Evaluation Center) Panel Report on High Temperature Superconductivity in Japan

The Japanese regard success in R and D in high temperature superconductivity as an important national objective. The results of a detailed evaluation of the current state of Japanese high temperature superconductivity development are provided. The analysis was performed by a panel of technical experts drawn from U.S. industry and academia, and is based on reviews of the relevant literature and visits to Japanese government, academic and industrial laboratories. Detailed appraisals are presented on the following: Basic research; superconducting materials; large scale applications; processing of superconducting materials; superconducting electronics and thin films. In all cases, comparisons are made with the corresponding state-of-the-art in the United States

Readout and Control Beyond a Few Qubits: Scaling-up Solid State Quantum Systems

Quantum entanglement and superposition, in addition to revealing interesting physics in their own right, can be harnessed as computational resources in a machine, enabling a range of algorithms for classically intractable problems. In recent years, experiments with small numbers of qubits have been demonstrated in a range of solid-state systems, but this is far from the numbers required to realise a useful quantum computer. In addition to the qubits themselves, quantum operation requires a host of classical electronics for control and readout, and current techniques used in few-qubit systems are not scalable. This thesis presents a series of techniques for control and readout of solid-state qubits, working towards scalability by integrating classical control with the quantum technology. Two techniques for reducing the footprint associated with readout of gallium arsenide spin qubits are demonstrated. Gate electrodes, used to define the quantum dot, are also shown to be sensitive state detectors. These gate-sensors, and the more conventional Quantum Point Contacts, are then multiplexed in the frequency domain, where three-channel qubit readout and ten-channel QPC readout are demonstrated. Two types of superconducting devices are also explored. The loss in superconducting coplanar waveguide resonators is measured, and a suppression of coupling to the parasitic electromagnetic environment is demonstrated. The thesis also details software for the simulation of Josephson-junction based circuits including features beyond what is available in commercial products. Finally, an architecture for managing control of a scalable machine is proposed where classical components are distributed throughout a cryostat and cryogenic switches route control pulses to the appropriate qubits. A simple implementation of the architecture is demonstrated that incorporates a double quantum dot, a gallium arsenide switch matrix, frequency multiplexed readout, and cryogenic classical computation

A SCANNING SQUID MICROSCOPE FOR IMAGING HIGH-FREQUENCY MAGNETIC FIELDS

This thesis examines the design and operation of a large-bandwidth scanning SQUID microscope for spatially imaging high frequency magnetic fields. Towards this end, I present results on a cryo-cooled 4.2 K scanning SQUID microscope with a bandwidth of dc to 2 GHz and a sensitivity of about 52.4 nT per sample. By using a thin-film hysteretic Nb dc-SQUID and a pulsed sampling technique, rather than a non-hysteretic SQUID and a flux-locked loop, the bandwidth limitation of existing scanning SQUID microscopes is overcome. The microscope allows for non-contact images of time-varying magnetic field to be taken of room-temperature samples with time steps down to 50 ps and spatial resolution ultimately limited by the size of the SQUID to about 10 micrometers. The new readout scheme involves repeatedly pulsing the bias current to the dc SQUID while the voltage across the SQUID is monitored. Using a fixed pulse amplitude and applying a fixed dc magnetic flux allows the SQUID to measure the applied magnetic flux with a sampling time set by the pulse length of about 400 ps. To demonstrate the capabilities of the microscope, I imaged magnetic fields from 0 Hz (static fields) up to 4 GHz. Samples included a magnetic loop, microstrip transmission lines, and microstrip lines with a break in order to identify and isolate electrical opens in circuits. Finally, I discuss the operation and modeling of the SQUID and how to further increase the bandwidth of the microscope to allow bandwidth of upwards of 10 GHz

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

Design of a Large Bandwidth Scanning SQUID Microscope using a Cryocooled Hysteretic dc SQUID

I present the design and analysis of a large bandwidth scanning Superconducting Quantum Interference Device (SQUID) microscope. Currently available SQUID microscopes are limited to detecting magnetic fields with frequencies less than 1 MHz. However, for observing nanosecond time scale phenomena such as logic operations in today's computer chips, SQUID microscopes with 1 GHz bandwidth and larger are required. The major limitation in SQUID microscope bandwidth is not the SQUID itself but the electronics and readout technique. To increase bandwidth, the fast transition of a hysteretic dc SQUID from the zero voltage state to the resistive state can be used as the detection element in a new SQUID readout technique, referred to as pulsed SQUID sampling. The technique involves pulsing the bias current to the dc SQUID while monitoring the voltage across it. As the pulse length shortens, the SQUID measures the applied external magnetic flux with shorter sampling time, which increases the bandwidth. Experimental tests of the technique have demonstrated the possibility of following signals with frequencies up to 1 GHz using a dc SQUID with Nb-AlOx-Nb Josephson junctions at around 4 K. Ringing in the pulse profile permitted the effective bandwidth of the sampling technique to be much greater than the nominal value suggested by the pulse length setting on the generator. I identify additional means of increasing bandwidth: redesigning the dc SQUID, implementing transmission line wiring, adding high speed superconducting circuits, etc. which should allow bandwidths to reach 40 GHz and higher. Towards creating a large bandwidth SQUID microscope, I also assembled and tested with collaborators a fully functional 4 K scanning SQUID microscope. With the microscope, which used a nonhysteretic niobium dc SQUID with conventional flux-locked-loop SQUID electronics, I was able to obtain the magnetic field image of a current carrying circuit

Decoherence, control, and encoding of coupled solid-state quantum bits

In this thesis the decoherence properties, gate performance, control of solid-state quantum bits (qubits), and novel design proposals for solid-state qubits analogous to quantum optics are investigated. The qubits are realized as superconducting nanocircuits or quantum dot systems. The thesis elucidates both very appealing basic questions, like the generation and detection of deeply nonclassical states of the electromagnetic field, i.e., single photon Fock states, in the solid-state, but also presents a broad range of different strategies to improve the scalability and decoherence properties of solid-state qubit setups