Phase Qubit and Resonators
Superconducting Quantum Computing


Superconducting qubits display decoherence mechanisms related to phenomena on the superconductor surfaces or within nearby dielectrics. The defects attributable to the phenomena are often described by two quantum-mechanical states, but there are also some very interesting exceptions. We have found that low-loss silicon nitride displays clear evidence for interactions between two-level systems. We also have developed a new device for measuring the defects within a Josephson junction barrier. This device shows clear evidence for strong coupling between the defects and the photon field stored in a frequency-tunable resonator.

Quantum Dissipation and Noise in Dielectrics

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 and Qubit Readout using Josephson Junction Resonators

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.

To contact us:

Laboratory for Physical Sciences
8050 Greenmead Dr.
College Park, MD  20740
Lab Phone: 301-935-6415
Reception: 301-935-6400
Facsimile: 301-935-6723


Images from Quasilumped versus waveguide resonator study
arXiv:1008.2929), and L versus
C coupling study

Two-Level Systems in Amorphous Dielectrics

Resonator Spectroscopy of Quantum Defects

Josephson Bifurcation

Parallel plate LC, coplanar, and tunable Josephson junction resonators
(from top to bottom)