This thesis deals with reducing quasiparticle generation in superconducting circuits caused by stray photons by utilizing electromagnetic absorbers. This thesis deals with reducing quasiparticle generation in superconducting circuits caused by stray photons by utilizing electromagnetic absorbers. \nQuasiparticle generation is a known problem in several superconducting circuit applications such as quantum computing research. \nIn order to quantify successful radiation reduction, a superconducting resonator was used as a sensor and a cylindrical shield was designed for testing the absorbers. \nFour different absorbers were tested, three of them are commercially available microwave absorbers and one was designed by our group. \nInternal differences were observed between the absorbers, however, compared to reference measurements, no improvements were observed. \nThe results also indicates that the resonators properties changed in-between measurements and more tests should be performed in order to draw final conclusions.
This thesis deals with reducing quasiparticle generation in superconducting circuits caused by stray photons by utilizing electromagnetic absorbers. This thesis deals with reducing quasiparticle generation in superconducting circuits caused by stray photons by utilizing electromagnetic absorbers. \nQuasiparticle generation is a known problem in several superconducting circuit applications such as quantum computing research. \nIn order to quantify successful radiation reduction, a superconducting resonator was used as a sensor and a cylindrical shield was designed for testing the absorbers. \nFour different absorbers were tested, three of them are commercially available microwave absorbers and one was designed by our group. \nInternal differences were observed between the absorbers, however, compared to reference measurements, no improvements were observed. \nThe results also indicates that the resonators properties changed in-between measurements and more tests should be performed in order to draw final conclusions.
Superconducting resonators with high quality factors are of great interest in many areas. However, the quality factor of the resonator can be weakened by many dissipation channels including trapped magnetic flux vortices and nonequilibrium quasiparticles which can significantly impact the performance of superconducting microwave resonant circuits and qubits at millikelvin temperatures. Quasiparticles result in excess loss, reducing resonator quality factors and qubit lifetimes. Vortices trapped near regions of large microwave currents also contribute excess loss. However, vortices located in current-free areas in the resonator or in the ground plane of a device can actually trap quasiparticles and lead to a reduction in the quasiparticle loss. In this thesis, we will describe experiments involving the controlled trapping of vortices for reducing quasiparticle density in the superconducting resonators. We provide a model for the simulation of reduction of nonequilibrium quasiparticles by vortices. In our experiments, quasiparticles are generated either by stray pair-breaking radiation or by direct injection using normal-insulator-superconductor (NIS)-tunnel junctions.
Superconducting materials are useful for a wide range of applications, ranging from particle detectors to quantum computing. The presence of excitations (quasiparticles) in the superconductor can be a resource in some devices and a disturbance in others - in kinetic inductance detectors, the generation of quasiparticles by an incoming photon can be read out and used to calculate the energy of the photon, while the tunnelling of quasiparticles is a cause of decoherence in quantum computing. While early research mostly focused on superconductors close to the critical temperature, temperatures that are far below the critical temperatures are often used for current applications. At these low temperatures, the energy exchange between quasiparticles and phonons becomes slow, and even small disturbances may drive the energy distribution of the quasiparticles out of thermal equilibrium. In fact, experiments usually show a residual number of quasiparticles even at milikelvin temperatures, for which the thermal density of quasiparticles would be negligible. This thesis consists of two parts: In the first part, I model the energy distribution of quasiparticles in superconducting resonators: Quasiparticles absorb energy by absorbing photons and release it as phonons, which drives the superconductor out of thermal equilibrium. I solve a set of rate equations, both numerical and analytical, to calculate the energy distribution of the quasiparticles. For parameters typically used in kinetic inductance detectors made of aluminum, the microwave signal used to read out the resonator dominates the energy distribution of the quasiparticles, while their density is determined by other processes. At not too low temperatures, quasiparticles are mainly produced by thermal phonons, and my calculations reproduce recently measured quality factors of resonators at temperatures above 250 mK without free parameters. I show that even a very small number of photons of higher energy, which I ascribe to noise from the microwave generator, would dominate the quasiparticle density at lower temperatures. Under this assumption, my calculations also reproduce the quality factors at lower temperatures. In the second part of this thesis, I investigate the effects of spatial fluctuations of the superconducting energy gap on the dynamics of quasiparticles. Recently, it has been proposed that such fluctuations may lead to the localization of quasiparticles in states below the energy gap, which could cause a finite quasiparticle density even at very low temperatures. Using the Keldysh-Usadel formalism, I derive rates for the different processes (recombination/(de-) localization/generation) of quasiparticles in such systems. Using these rate equations I derive new upper limits for the quasiparticle density that could result from these localized states. These upper limits show that localization caused by weak magnetic impurities in typical aluminum systems does not lead to a relevant quasiparticle density. This contradicts previous theories, which only consider the recombination between two localized quasiparticles without examining their dynamics in detail.
The ideal superconductor provides a pristine environment for the delicate states of a quantum computer: because there is an energy gap to excitations, there are no spurious modes with which the qubits can interact, causing irreversible decay of the quantum state. As a practical matter, however, there exists a high density of excitations out of the superconducting ground state even at ultralow temperature; these are known as quasiparticles. Observed quasiparticle densities are of order 1 μm^{-3}, tens of orders of magnitude greater than the equilibrium density expected from theory. Nonequilibrium quasiparticles extract energy from the qubit mode and can induce dephasing. Here we show that a dominant mechanism for quasiparticle poisoning is direct absorption of high-energy photons at the qubit junction. We use a Josephson junction-based photon source to controllably dose qubit circuits with millimeter-wave radiation, and we use an interferometric quantum gate sequence to reconstruct the charge parity of the qubit. We find that the structure of the qubit itself acts as a resonant antenna for millimeter-wave radiation, providing an efficient path for photons to generate quasiparticles. A deep understanding of this physics will pave the way to realization of next-generation superconducting qubits that are robust against quasiparticle poisoning.
Identifying, quantifying, and suppressing decoherence mechanisms in qubits are important steps towards the goal of engineering a quantum computer or simulator. Superconducting circuits offer flexibility in qubit design; however, their performance is adversely affected by quasiparticles (broken Cooper pairs). Developing a quasiparticle mitigation strategy compatible with scalable, high-coherence devices is therefore highly desirable. Here we experimentally demonstrate how to control quasiparticle generation by downsizing the qubit, capping it with a metallic cover, and equipping it with suitable quasiparticle traps. Using a flip-chip design, we shape the electromagnetic environment of the qubit above the superconducting gap, inhibiting quasiparticle poisoning. Our findings support the hypothesis that quasiparticle generation is dominated by the breaking of Cooper pairs at the junction, as a result of photon absorption by the antenna-like qubit structure. We achieve record low charge-parity switching rate (<1 Hz). Our aluminium devices also display improved stability with respect to discrete charging events.
Micro-fabricated superconducting circuits have emerged as a leading candidate for quantum computers. However, typical error rates of 10-3 for quantum gates remain a challenge for performing error correction and need to be further decreased for achieving practical quantum computation. Improving qubit coherence times for superconducting qubits is a continuous endeavor in order to decrease error rates. As part of this effort, a topic addressed in this dissertation is the study of the generation mechanism and dynamics of non-equilibrium quasiparticles (QPs), which are electron-like excitations of a superconductor. This source of decoherence was studied by measuring the rate of single-electron changes in the parity state of multiple charge-sensitive transmon qubits under different conditions. The presence of non-equilibrium QPs in superconducting resonators and qubits operating at millikelvin temperature has been known for decades. Understanding the sources and dynamics of these QPs is crucial to evaluate and improve the performance of the devices. Specifically, for superconducting qubits, a single QP tunneling across the Josephson junction changes the total charge parity from even to odd or vice versa, and can induce energy relaxation and dephasing, limiting the coherence of the device. To study the dynamics of non-equilibrium QPs, I utilized a Ramsey-like pulse sequence to measure the rate of single-electron parity changes in transmon qubits, which had a weak charge dispersion, with different shunting capacitor geometries and materials. I started out by measuring a charge parity rate of ~ 33 s-1 for a grounded "x-mon" qubit and ~ 17 s-1 for a galvanically isolated "two-pads" qubit with our standard setup. These rates decreased to ~ 5 s-1 and ~ 2 s-1 for the x-mon and two-pads respectively when IR-filters and extra layers of shielding were installed. From my measurements, I found that the charge parity rates are sensitive to the geometry of the shunting capacitor, weakly depend on the amount of shielding surrounding the device, and independent of using Al or Ta for the superconducting shunting capacitor. To distinguish between two possible mechanisms for the parity switching, I measured the parity rates after cooling one of the qubit devices in an external magnetic field to induce vortices in the superconducting electrodes. After increasing the magnetic field above 23 μT, I measured a faster QP trapping rate and a much smaller T1, suggesting that at least one vortex was added to the superconducting electrode near the Josephson junction. On the other hand, the measured charge parity rates exhibited no clear dependence on the background QP trapping rates. These data suggest that a large portion of parity switching events occurs from QPs being produced at the Josephson junction, where pair-breaking radiation is rectified into QPs by the junction, as opposed to tunneling of QPs residing in the electrodes. To understand the difference in the measured parity rates, I simulated the absorption efficiency of pair-breaking radiation for my two designs and observed a similar difference in absorption rates when a single-mode blackbody radiation with an effective temperature T* ~ 300 mK was assumed to be illuminating the devices. This large T*, which is significantly higher than the device's nominal temperature of 20 mK, is consistent with values reported in the literature. However, its physical origin remains not fully understood. For the other part of my dissertation, I developed a new simplified process for making superconducting air bridges. Air bridges are used to suppress unwanted extraneous high frequency chip modes parasitic to superconducting quantum circuits that can couple to qubits and cause decoherence. Traditional ways of making air bridges utilize thermal reflow to create arched photoresist scaffolds that are limited in heights. This results in insufficient mechanical strength of the bridges which tend to collapse when spanning to larger lengths. My simplified fabrication method that utilizes two-photon lithography, a technique that enables creation of three-dimensional micron-scale structures, relaxes this limitation and is capable of making mechanically stable bridges with lengths up to 100 μm. The resulting Al air bridges, having a superconducting transition temperature of Tc = 1.08 mK and a dc residual resistivity ratio of 3.85, were confirmed to form a good electrical contact with the base metal layers and exhibit a small measurable amount of rf loss when placed over coplanar waveguide resonators, on par with other air bridge fabrication methods.