Engineering Light-Matter Interactions with Metastable States in a Heavy Fluxonium

In the past few years there has been an increasing amount of media coverage on quantum computing, largely due to the substantial investment in the superconducting circuit quantum electrodynamics (cQED) technologies by several governments and technology and finance companies throughout the world. Since their inception, superconducting qubits have realized several orders of magnitude improvement in information lifetimes, but individual qubit lifetimes must be further improved to realize a fault-tolerant quantum computer. A critical component of these circuits, which allows quantum states to be addressed, is the non-linear inductive element known as the Josephson junction. In this thesis, we will present experimental studies showing how to design and fabricate the Josephson element to leverage a recently developed `superinductor' as a way to engineer new qubit regimes, such as the heavy fluxonium. We create the heavy fluxonium with a chain of large Josephson junctions and a large shunting capacitor. This shunting capacitor increases the effective mass of a fictitious particle inside the wells of the heavy fluxonium potential energy landscape, giving rise to two characteristically different transitions: intra-well transmon-like plasmon transitions and inter-well fluxon transitions. The fluxon transitions are heavily suppressed by a reduced dipole matrix element realized from the increased effective mass, and result in metastable fluxon transitions, with $T_1$'s as large as 8ms. This qubit design offers the possibility to study qubit interaction with the thermal bath, the implementation of atomic state preparation techniques such as Raman gates, optical pumping and fluorescent style readout, as well as the possibility to make substantial improvements in qubit lifetimes and create protected qubits in the cQED platform.