Tunnelling two-level systems in superconducting quantum circuits

Superconducting circuits comprising qubits and resonators have emerged as a viable platform for quantum computers. A major issue facing the implementation of complex superconducting quantum circuits is their interaction with incidental two-level system (TLS). These notoriously ubiquitous defects resulting from atomic-scale materials imperfections are the source of noise, decoherence, and parameter fluctuations. Improvements upon the device design have yielded most of the gain in coherence times over the last decades. This resource looks all but exhausted by now. Eliminating TLS on the other hand requires major advances in materials analysis performed at the qubit operation temperatures in the millikelvin range and relevant energies corresponding to some gigahertz.
To this end we use planar superconducting resonators as a probe for the TLS physics. In this talk I will overview recent progress in identification of a variety of these detrimental defects using spectroscopic techniques. Consequently, we are now able to formulate some recommendations for improved qubit materials. I will also present recent work on immersion cooling of superconducting resonators 3He which results in reduced noise. This development suggests that quantum engineering may benefit from access to a wider arsenal of quantum condensed matter.