Impact of time-ordered measurements of the two states in a niobium superconducting qubit structure

Measurements of thermal activation are made in a superconducting, niobium persistent-current qubit structure, which has two stable classical states of equal and opposite circulating current. The magnetization signal is read out by ramping the bias current of a dc superconducting quantum interference device. This ramping causes time-ordered measurements of the two states, where measurement of one state occurs before the other. This time ordering results in effective measurement time, which can be used to probe the thermal activation rate between the two states. Fitting the magnetization signal as a function of temperature and ramp time allows one to estimate a quality factor of 3×10⁵ for our devices, a value favorable for the observation of long quantum coherence times at lower temperatures.