Superinductance and fluctuating two-level systems: Loss and noise in disordered and non-disordered superconducting quantum devices
Doctoral thesis, 2020
For this purpose, we fabricated and characterised 20 nm thick, 40 nm wide niobium-nitride nanowires and determined the impedance to 6.795 kΩ. We demonstrate internal quality factors Qi = 2.5e4 in nanowire resonators at single photon excitation, which is significantly higher than values reported in devices with similar materials and geometries. Moreover, we show that the dominant dissipation in our nanowires is not an intrinsic property of the disordered films, but can instead be fully understood within the framework of two-level systems.
To further characterise these losses, we then explore the geometrical scaling, toward nanowire dimensions, of dielectric losses in superconducting microwave resonators fabricated with the same techniques and from the same NbN thin-film as the nanowire superinductors. For this purpose, we perform an experimental and numerical study of dielectric loss at low temperatures. Using 3D finite-element simulation of the Maxwell--London equations, we compute the geometric filling factors of the lossy regions in our resonator structures and fit the experimental data to determine the intrinsic loss tangents of its interfaces and dielectrics.
Finally, we study the effect of two-level systems on the performance of various superconducting quantum circuits. For this purpose, we measure coherence-time fluctuations in qubits and frequency fluctuations in resonators. In all devices, through statistical analysis, we identify the signature of individual Lorentzian fluctuators in the noise. We find that fluctuations in qubit relaxation are local to the qubit and are caused by instabilities of near-resonant two-level-systems. Furthermore, when examining the low-frequency noise of three different types of superconducting resonator - one NbN nanowire, one Al coplanar waveguide, and one Al 3D cavity - we observe a similar power-law dependence of the Lorentzian switching time and amplitude on the circulating power in the resonators, suggesting a common noise mechanism in the three different types of devices.
Chalmers, Microtechnology and Nanoscience (MC2), Quantum Technology
Geometric scaling of two-level-system loss in superconducting resonators
Superconductor Science and Technology,; Vol. 33(2020)
High Kinetic Inductance Nb N Nanowire Superinductors
Physical Review Applied,; Vol. 11(2019)
Decoherence benchmarking of superconducting qubits
npj Quantum Information,; Vol. 5(2019)
Noise and loss of superconducting aluminium resonators at single photon energies
Journal of Physics: Conference Series,; Vol. 969(2018)
Paper in proceeding
Stability of superconducting resonators: Motional narrowing and the role of Landau-Zener driving of two-level defects
Science advances,; Vol. 7(2021)
Unfortunately, quantum information is fragile: defects and time-varying properties in devices and materials can destroy it, a phenomenon known as decoherence. Recent years have witnessed tremendous progress in the preservation of quantum coherent states in superconducting quantum circuits. Nevertheless, current state-of-the-art devices are just barely good enough to start implementing any meaningful practical applications: much lower losses are desired in order make quantum computers useful.
In this thesis, we address this outstanding issue in two ways. First, we explore the means of protecting qubits - the fundamental building blocks of quantum computers - from these defects. For this purpose, we design, fabricate and characterise a novel type of superconducting circuit element, known as the nanowire superinductor. Superinductors have high reactive impedance at microwave frequencies, which suppresses charge fluctuations that can lead to qubit decoherence.
In a second part, we investigate the nature of these defects with the ultimate goal to learn how to eliminate them. To this end, we study fluctuations and noise in several different types of superconducting circuits using a statistical analysis technique common in the field of frequency metrology. This technique allows us to identify noise processes that are common among superconducting quantum devices and to understand their dynamics in greater detail than before.
The results presented in this thesis - the demonstration of a superinductor and a novel understanding of fluctuations - will benefit the development of novel devices for quantum technology.
Superinductors for quantum information science (Super JB)
Swedish Research Council (VR) (2013-4430), 2014-01-01 -- 2017-12-31.
Superinductance for very long-lived quantum coherence
Chalmers, 2013-10-01 -- 2014-06-30.
Areas of Advance
Nanoscience and Nanotechnology
Condensed Matter Physics
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4779
Kollektorn, MC2, Kemivägen 9, Chalmers.
Opponent: Prof. Martin Weides, University of Glasgow