Improving Quantum Hardware: Building New Superconducting Qubits and Couplers

Abstract

Over the past 10 years, improvements to the fundamental components in superconducting qubits and the realization of novel circuit topologies have increased the lifetimes of qubits and catapulted this architecture to become one of the leading hardware platforms for universal quantum computation. Despite the progress that has been made in increasing the lifetime of the charge qubit by almost six orders of magnitude, further improvements must be made to climb over the threshold for fault tolerant quantum computation. Two complementary approaches towards achieving this goal are investigating and improving upon existing qubit designs, and looking for new types of superconducting qubits which would offer some intrinsic improvements over existing designs. This thesis will explore both of these directions through a detailed study of new materials, circuit designs, and coupling schemes for superconducting qubits. In the first experiment, we explore the use of disordered superconducting films, specifically Niobium Titanium Nitride, as the inductive element in a fluxonium qubit and measure the loss mechanisms limiting the qubit lifetime. In the second experiment, we work towards the experimental realization of the $0-\pi$ qubit architecture, which offers the promise of intrinsic protection in lifetime and decoherence compared to existing superconducting qubits. In the final experiment, we design and measure a two qubit device where the static $\sigma_z \otimes \sigma_z$ crosstalk between the two qubits is eliminated via destructive interference. The use of multiple coupling elements removes the $\sigma_z \otimes \sigma_z$ crosstalk while maintaining the large $\sigma_z \otimes \sigma_x$ interaction needed to perform two qubit gates.