A project of the Theoretical Chemical and Quantum Physics Group


Mr. Tim DuBois, Mr. Martin Cyster, Dr. Nicolas Vogt, A/Prof. Jared Cole, Prof. Salvy Russo


J. Lisenfeld, M. Marthaler, A. Ustinov: Karlsruhe Institute of Technology

C. Müller: University of Queensland

Brief Project Outline

Although superconducting devices are based on the dissipation-less properties of metals below the superconducting transition, they suffer from a fundamental drawback. The quantum effects on which these devices rely stem from Josephson junctions, which are ultra-small, ultra-thin insulating barriers. At present the best method for fabricating these junctions is to take advantage of the native amorphous oxide, which forms on many superconducting metals when exposed to oxygen. This oxide layer in turn contains defects, due to its amorphous nature, which provides a new and dominant dissipation channel. Although much is known about the effects of these defects, very little is known about their precise microscopic origins.

The TCQP group is working on this problem from several directions:

  • Developing microscopic models of Josephson junctions themselves to better understand the formation of the junction and their amorphous nature.
  • Developing effective models of two-level defects with the aim of comparing to microscopic parameters.
  • Working with our experimental collaborators to test theoretical models and design new experiments to more accurately measure junction defects.

Two-state defects are a generic feature of disorder crystals. An amorphous crystal can be thought of as a conventional periodic crystal where there are small random perturbations to the bond lengths and angles. This tends to disrupt the long range periodicity, resulting in the amorphous nature of the solid. At the same time, this localised disorder in the bonds allows for degenerate atomic positions, ie. one or more atoms can take two equivalent positions which are very close in energy. The formation of these two-state defects profoundly modifies the thermodynamic and electrical properties of the material.

Anatomy of phase qubit spectroscopy

Anatomy of a phase-qubit/two-level fluctuator spectrum. A phase qubit circuit is a very sensitive probe of coherent two-level systems embedded in the circuit itself (typically within the Josephson junction). The physics of the two-state system can be probed via dynamical spectroscopy experiments where typically such experiments have a number of different "free' parameters which must be precisely measured in order to characterise the system precisely. Using a series of measurements, each of the parameters can be independently determined, providing more information on the nature of the two-level fluctuator.
Cole et al., Appl. Phys. Lett. 97, 252501 (2010)

Oxide deposition simulation

A Josephson junction is formed using a controlled oxidation of aluminium metal. The oxidation is performed at relatively low temperatures and pressures which result in an amorphous aluminium oxide layer. This movie shows a detailed simulation of the growth of aluminium oxide produced by simulating the impact and equilibration of each oxygen atom individually. For particular sets of pressure, temperature and thermostat configurations, we see formation of either amorphous or crystalline layers which can be then compared to experimental measurements of example junctions.

Recent Publications

T. C. DuBois, S. P. Russo and J. H. Cole, Atomic delocalization as a microscopic origin of two-level defects in Josephson junctions, New Journal of Physics 17 23017 (2015)

J. Lisenfeld, G. Grabovskij, C. Muller, J. H. Cole, G. Weiss and A. V. Ustinov, Observation of directly interacting coherent two-level systems in an amorphous material, Nature Communications 6 6182 (2015)

T. C. DuBois, M. Per, S. P. Russo and J. H. Cole, Delocalized Oxygen as the Origin of Two-Level Defects in Josephson Junctions, Physical Review Letters 110 077002 (2013)

J. H. Cole, C. Müller, P. Bushev, G. J. Grabovskij, J. Lisenfeld, A. Lukashenko, A. V. Ustinov and A. Shnirman, Quantitative evaluation of defect-models in superconducting phase qubits, Appl. Phys. Lett. 97, 252501 (2010)

For more information about this project, please contact Jared Cole.