Hamiltonian and materials engineering for superconducting qubit lifetime enhancement

Aug 28, 2023, 10:00 am11:30 am



Event Description

The potential for quantum computing to expand the number of solvable problems has driven 
researchers across academia and industry, in multiple disciplines, to develop a variety of 
different qubit platforms, algorithms, and scaling strategies. At its core, quantum com- putation 
relies on the robustness, or coherence, of its building blocks (“qubits”). In current small-scale 
superconducting qubit processors, the fidelity of operations is often limited by qubit coherence. 
The coherence time of a single qubit depends on its lifetime T₁ and pure dephasing time Tφ. In this 
thesis, we focus on the problem of improving T₁.

Strategies for improving lifetimes are informed by models for relaxation - specifically Fermi’s 
Golden Rule. Relaxation rates depend on noise properties of the environment and on properties of 
the qubit states. This dependence suggests two strategies for engineering longer lifetimes: 
environment engineering involves mitigating or filtering the noise that the qubit sees, and 
Hamiltonian engineering refers to optimizing the qubit circuit and its re- sulting eigenstates to 
optimize T₁. Significant enhancements of qubit lifetimes will require paradigm shifts in our 
approaches to both environment and Hamiltonian engineering.

First, I present a side-by-side study of transmon coherence and materials measurements of the 
constituent Nb films, including synchrotron x-ray spectroscopy and electron microscopy. We found 
correlations between qubit lifetimes and materials properties such as grain size, grain boundary 
quality, and surface suboxides. This study expands the scope of superconducting qubit research by 
presenting a broad set of materials analyses alongside device measurements.

Second, I will give an overview of Hamiltonian engineering, including the concepts be- hind 
intrinsic protection against relaxation and dephasing processes. I’ll describe the soft
0  π qubit, which is the first experimentally realized superconducting qubit to show sig- natures 
of simultaneous T₁ and T₂ protection. We improved coherence in the soft 0  π
through optimized fabrication processes. We have also characterized the effects of non- 
computational levels on gate fidelity, specifically AC Stark shifts and leakage.

From the results in this thesis, we have gained a deeper understanding of what limits qubit 
coherence, informing future directions on both the materials and Hamiltonian engineering fronts.

Adviser: Andrew Houck