Quantum interference between single photons from a single atom and a cold atomic ensemble

Master'sMASTER OF SCIENC

Microwave magnon damping in YIG films at millikelvin temperatures

Magnon systems used in quantum devices require low damping if coherence is to be maintained. The ferrimagnetic electrical insulator yttrium iron garnet (YIG) has low magnon damping at room temperature and is a strong candidate to host microwave magnon excitations in future quantum devices. Monocrystalline YIG films are typically grown on gadolinium gallium garnet (GGG) substrates. In this work, comparative experiments made on YIG waveguides with and without GGG substrates indicate that the material plays a significant role in increasing the damping at low temperatures. Measurements reveal that damping due to temperature-peak processes is dominant above 1 K. Damping behaviour that we show can be attributed to coupling to two-level fluctuators (TLFs) is observed below 1 K. Upon saturating the TLFs in the substrate-free YIG at 20 mK, linewidths of 1.4 MHz are achievable: lower than those measured at room temperature.Comment: 5 pages, 4 figure

Mitigation of frequency collisions in superconducting quantum processors

The reproducibility of qubit parameters is a challenge for scaling up superconducting quantum processors. Signal crosstalk imposes constraints on the frequency separation between neighboring qubits. The frequency uncertainty of transmon qubits arising from the fabrication process is attributed to deviations in the Josephson junction area, tunnel barrier thickness, and the qubit capacitor. We decrease the sensitivity to these variations by fabricating larger Josephson junctions and reduce the wafer-level standard deviation in resistance down to 2%. We characterize 32 identical transmon qubits and demonstrate the reproducibility of the qubit frequencies with a 40 MHz standard deviation (i.e. 1%) with qubit quality factors exceeding 2 million. We perform two-level-system (TLS) spectroscopy and observe no significant increase in the number of TLSs causing qubit relaxation. We further show by simulation that for our parametric-gate architecture, and accounting only for errors caused by the uncertainty of the qubit frequency, we can scale up to 100 qubits with an average of only 3 collisions between quantum-gate transition frequencies, assuming 2% crosstalk and 99.9% target gate fidelity.Comment: 8 figures, 18 pages (8 pages main text), units fixed in Fig. 1

Simplified Josephson-junction fabrication process for reproducibly high-performance superconducting qubits

We introduce a simplified fabrication technique for Josephson junctions and demonstrate superconducting Xmon qubits with T1 relaxation times averaging above 50 μs (Q > 1.5 7 1 0 6). Current shadow-evaporation techniques for aluminum-based Josephson junctions require a separate lithography step to deposit a patch that makes a galvanic, superconducting connection between the junction electrodes and the circuit wiring layer. The patch connection eliminates parasitic junctions, which otherwise contribute significantly to dielectric loss. In our patch-integrated cross-type junction technique, we use one lithography step and one vacuum cycle to evaporate both the junction electrodes and the patch. This eliminates a key bottleneck in manufacturing superconducting qubits by reducing the fabrication time and cost. In a study of more than 3600 junctions, we show an average resistance variation of 3.7% on a wafer that contains forty 0.5 7 0.5-cm2 chips, with junction areas ranging between 0.01 and 0.16 μm2. The average on-chip spread in resistance is 2.7%, with 20 chips varying between 1.4% and 2%. For the junction sizes used for transmon qubits, we deduce a wafer-level transition-frequency variation of 1.7%-2.5%. We show that 60%-70% of this variation is attributed to junction-area fluctuations, while the rest is caused by tunnel-junction inhomogeneity. Such high frequency predictability is a requirement for scaling-up the number of qubits in a quantum computer

Microwave magnonics at millikelvin temperatures

This thesis reports on three recent experimental studies of microwave magnons in yttrium iron garnet (YIG) systems at millikelvin temperatures. We begin with an introduction to the emerging field of quantum magnonics and its underpinning motivations. Basic theory of dipolar spin-wave or magnon dynamics in various sample geometries is presented. This introduction is followed by a brief description of the specificities of our experimental setup and properties of YIG --- the magnetic material used in our studies. The first experiment involves a hybrid system combining a YIG sphere and a niobium-based superconducting planar resonator. The device is measured at millikelvin temperatures and signals of strong magnon-photon coupling are observed when the excitation energy is at the level of single photons. The superconducting resonator is shown to maintain a good quality factor even under sizeable in-plane magnetic field that is required to support the excitation of magnons in YIG. The second experiment demonstrates the operation of a magnonic crystal based on an etched YIG film at millikelvin temperatures. A magnonic bandgap is successfully observed both under continuous- and pulsed- microwave excitation. High magnon damping is observed in the device at low temperature. The third experiment involves the measurement of magnon damping in YIG films. Comparisons between results from different samples and at different temperatures provide insight into the role of the substrate and two-level fluctuators at low temperature. A brief review of the known damping processes in bulk YIG from room temperature down to millikelvin temperature is also presented. The final chapter summarises results in this thesis and suggests possible future research directions.</p

Microwave magnonics at millikelvin temperatures

This thesis reports on three recent experimental studies of microwave magnons in yttrium iron garnet (YIG) systems at millikelvin temperatures. We begin with an introduction to the emerging field of quantum magnonics and its underpinning motivations. Basic theory of dipolar spin-wave or magnon dynamics in various sample geometries is presented. This introduction is followed by a brief description of the specificities of our experimental setup and properties of YIG --- the magnetic material used in our studies. The first experiment involves a hybrid system combining a YIG sphere and a niobium-based superconducting planar resonator. The device is measured at millikelvin temperatures and signals of strong magnon-photon coupling are observed when the excitation energy is at the level of single photons. The superconducting resonator is shown to maintain a good quality factor even under sizeable in-plane magnetic field that is required to support the excitation of magnons in YIG. The second experiment demonstrates the operation of a magnonic crystal based on an etched YIG film at millikelvin temperatures. A magnonic bandgap is successfully observed both under continuous- and pulsed- microwave excitation. High magnon damping is observed in the device at low temperature. The third experiment involves the measurement of magnon damping in YIG films. Comparisons between results from different samples and at different temperatures provide insight into the role of the substrate and two-level fluctuators at low temperature. A brief review of the known damping processes in bulk YIG from room temperature down to millikelvin temperature is also presented. The final chapter summarises results in this thesis and suggests possible future research directions.</p

Fast analytic and numerical design of superconducting resonators in flip-chip architectures

In superconducting quantum processors, the predictability of device parameters is of increasing importance as many labs scale up their systems to larger sizes in a 3D-integrated architecture. In particular, the properties of superconducting resonators must be controlled well to ensure high-fidelity multiplexed readout of qubits. Here we present a method, based on conformal mapping techniques, to predict a resonator's parameters directly from its 2D cross-section, without computationally heavy simulation. We demonstrate the method's validity by comparing the calculated resonator frequency and coupling quality factor with those obtained through 3D finite-element-method simulation and by measurement of 15 resonators in a flip-chip-integrated architecture. We achieve a discrepancy of less than 2% between designed and measured frequencies, for 6-GHz resonators. We also propose a design method that reduces the sensitivity of the resonant frequency to variations in the inter-chip spacing.Comment: 12 pages, 11 figures. Major correction of equation (17), with one minor correction in Appendix C. This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice, after which this version may no longer be accessibl

Transmon qubit readout fidelity at the threshold for quantum error correction without a quantum-limited amplifier

High-fidelity and rapid readout of a qubit state is key to quantum computing and communication, and it is a prerequisite for quantum error correction. We present a readout scheme for superconducting qubits that combines two microwave techniques: applying a shelving technique to the qubit that effectively increases the energy-relaxation time, and a two-tone excitation of the readout resonator to distinguish among qubit populations in higher energy levels. Using a machine-learning algorithm to post-process the two-tone measurement results further improves the qubit-state assignment fidelity. We perform single-shot frequency-multiplexed qubit readout, with a 140ns readout time, and demonstrate 99.5% assignment fidelity for two-state readout and 96.9% for three-state readout - without using a quantum-limited amplifier