Tuning Josephson Junctions

David Schuster, Andrew Oriani, Larry Chen

Technologies for tuning a resistance of tunnel junctions such as Josephson junctions are disclosed. In the illustrative embodiment, a Josephson junction is heated to 85 Celsius, and an electric field is applied to the Josephson junction. The heat and the electric field cause the resistance of the Josephson junction to increase. Monitoring the Josephson junction during the application of the electric field allows for the resistance of the Josephson junction to be adjusted to a particular value.

David Reginald Tilley

Oscar W. Kennedy, Jared H. Cole, Connor D. Shelly

Superconducting quantum computers leverage superconducting qubits, systems with well-defined energy levels, the spacing of which is determined by the properties of embedded Josephson junctions. Indeterminacy of Josephson junction manufacturing limits large-scale superconducting circuits; however, junction-by-junction tuning affords the possibility of an order of magnitude improvement of determinacy-although the origin of this tuning effect is uncertain. Understanding how tuning techniques may physically modify the tunnel barrier of a Josephson junction is important and will enable these techniques to be optimized. We develop a model of junction tuning based on depinning theory to interpret a phase diagram of the tuning rate. We extract the dependence on temperature, time-varying voltages, and oscillation frequency. Comparing the effect of junction tuning on room-temperature I-V's, electrical breakdown, and frequency of higher-energy levels of transmon qubits indicates that the tuning process leaves the barrier thickness unchanged but modifies its composition.

J.T. Chen

B. Liu, Michael J. Wengler

Progress in understanding the mechanism of coherent oscillation in twodimensional (2-D) Josephson junction arrays has been made through analysis of the coupling circuit between two Josephson junctions. For a 2-D array to oscillate in phase, capacitive coupling is required for adjacent junctions in the same row and inductive coupling for adjacent junctions in the same column of the array. Computer simulation has been developed for two junctions coupled through a general circuit to find the optimum coupling between two junctions. The junctions in the array are also integrated with inductive tuning elements to tune out the junctions' parasitic capacitance. We have built 2-D Josephson junction arrays but no coherent oscillation has yet been observed.