Thermal activation of superconducting Josephson junctions

Abstract

Superconducting quantum circuits (SQCs) are being explored as model systems for scalable quantum computing architectures. Josephson junctions are extensively used in superconducting quantum interference devices (SQUIDs) and in persistent-current qubit systems. Noise excitations, however, have a critical influence on their dynamics. Thus, the primary focus of this research was to investigate the effects of thermal activation on the superconducting properties of Josephson junctions. Specifically, thermal noise tends to result in a range of switching currents, values less than the critical current at which a junction switches from the superconducting to the normal state. First, a general review of superconductivity concepts is given, including a treatment of the Josephson phenomena. Next, I describe some of my work on characterizing the current-voltage traces of Josephson junctions tested at 4 K with a Multi-Chip Probe (MCP). Then, I describe thermal activation theory and examine the equations useful for modeling switching current distributions. The Josephson junctions of a SQUID with a ramped bias current were tested for numerous temperatures T =/< 4.5 K (and with various magnetic flux frustrations). Fit parameters of critical current, capacitance, resistance, and temperature were determined from modeling the escape rates and switching current probability distributions. The thermal activation model succeeded in fitting the results to good agreement, where parameters C = 2.000 ± 0.002 pF and T = 1.86 ± 0.06 K were obtained for 1.8 K data. For significantly lower temperatures, the model tends to predict higher than expected temperatures; further analysis would need to include the quantum mechanical tunneling model better in the fitting scheme.