Nicholas Masluk

Fluxonium is a highly anharmonic artificial atom, which makes use of an array of large Josephson junctions to shunt the junction of a Cooper-pair box for protection from charge noise. At microwave frequencies the array forms a “superinductance”, a superconducting inductance whose impedance exceeds the resistance quantum h/(2e)^2 = 6.5 kOhm. The first excited state transition frequency is widely tunable with flux, covering more than five octaves, yet the second excited state remains well within one octave. This unique spectrum permits a dispersive readout over the entire flux tunable range, in contrast to the flux qubit.

By measuring the energy relaxation time of the qubit over the full range of flux dependent transition energies, it is possible to determine the dominant loss mechanisms, and therefore implement design changes to reduce their contribution. The losses in several fluxonium samples is explored, with progressive improvements made towards reducing capacitive loss, the dominant loss mechanism. Additionally, the detailed characterization of Josephson junction array superinductances is examined.