Thermal Fluctuations in Josephson Junctions and Superconducting Quantum Interference Devices

Igor S. Beloborodov, F. W. J. Hekking, F. Pistolesi

Ya Jia, Wen‐Rong Sun, Jiarong Li

A symmetric superconducting quantum interference device (SQUID) with two Josephson junctions in the presence of thermal fluctuations is theoretically studied through two-dimensional Fokker-Planck equations. An analytical formula of the SQUID transfer function (or responsitivity) is derived by using the potential condition in the case of the steady state. It is found that the relationship between the optimum bias current I opt and the thermal fluctuation parameter Γ is I opt /Γ>1. The optimal reduced dc SQUID inductance β opt is in the range of 0.80<β opt <1.25, which is remarkably in agreement with the result of numerical simulation β opt ≈1.

M. Naor, Claudia D. Tesche, M. B. Ketchen

Thermally induced vortex-to-vortex transitions between the zero-quantum vortex state and one- flux-quantum vortex state of a symmetric two-junction dc superconducting quantum interference device (SQUID) have been observed. The characterization of transitions of this type is an essential part of the modeling of Josephson junction logic and memory devices. The transition rates, measured as a function of bias current, applied flux, and temperature, are in excellent agreement with rates predicted by a thermal activation model for two-junction SQUID’s.

Matteo Cirillo, Susan R. Pace, S. Pagano +1 more

Aditya P. Devalapalli

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 ac-tivation on the superconducting properties of Josephson junctions. Specifically, ther-mal 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

Makoto R. Arai, Vinay Ambegaokar