Superconducting qubits have achieved steady improvements over the last decade. In this group we have studied fundamental physics related to qubit improvement. Qubits defect phenomena and the prospect of creating coherent qubits in a small footprint is of particular interest. New materials and qubit designs are being studied, sometimes with other nationally-recognized research institutions.
Studies of two-level systems in amorphous films and other quantum computing structures:
The low-temperature rf properties of amorphous dielectrics is traditionally explained by the resonant excitation of electric-dipole-type two-level systems. These systems become saturated at high-powers as expected, and the loss tangent is proportional to the defect density. In this group we are studying dielectrics films with different growth conditions, in order to optimize the conditions and discover the microscopic defect(s) responsible for their loss. In the course of this research, we have demonstrated an improvement in the unsaturated loss tangent of silicon nitride by over an order of magnitude. This project has expanded to include the use of new materials, growth techniques, and noise measurements.
Defect Spectroscopy with the Josephson Junction Defect Spectrometer:
A Josephson junction is the nonlinear circuit element in superconducting circuits. In a phase qubit the nonlinearity allows one to isolate two states for manipulation, but in a resonator the nonlinearity is used to create quasi-harmonic levels for various purposes. We are currently studying individual quantum defects, large-ensemble noise sources, and bifurcation. We have observed individual quantum defects in the Josephson junction barrier using this technique, and it is being used in an effort to identify the atomic structure and reduce these defects. These resonators are also sensitive to large-ensemble noise sources, such as flux noise or correlated two-level systems, which are directly relevant to qubit coherence. With these resonators we have also observed Josephson bifurcation, which is being pursued to read out superconducting qubits.
Reversible Computing with Flux Solitons:
Ballistic flux solitons may allow researchers to develop record-breaking efficient computing using physically-reversible gates. The structures are intended to dissipate less than the temperature in energy units (or the temperature times Boltzmann’s constant), per gate operation. New simulations of ballistic flux solitons in computing structures have been performed.