Developing superconducting qubits into universal material defect analyzers

• Applicant: Dr. Jürgen Lisenfeld
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2015 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 276467699

The objective of this project is to employ superconducting quantum bits (qubits) for the experimental investigation of individual microscopic tunneling systems (TLS) in various materials. TLS emerge from structural material defects that occur for example in amorphous metal oxides. Recently, TLS received much attention because they are blamed to generate noise in a large variety of microfabricated devices, ranging from nanomechanical resonators to superconducting electronics. In our previous work, we have demonstrated that qubits can be utilized as powerful tools for the study of material defects because they provide full access to the coherent quantum states of individual TLS. In this project's first funding period, all required preliminary work was accomplished, which includes installation of a new experimental setup, the sample design, and establishment of a recipe for sample fabrication. Moreover, we developed a new method to characterize individual TLS using microwave resonators, and accomplished first strain-spectroscopic investigations of the TLS distribution in transmon-type qubits. The continuation of the project has the goal to complete pending milestones from the first funding period. This concerns the coupling of custom-made qubits to TLS in different materials and alkali halide crystals. Further goals are to study the interaction of qubits and resonators with non-stationary TLS ensembles, the implementation of electric TLS tuning in combination with strain-tuning, and a study of the emergence and avoidance of surface TLS. We expect that our project will continue to deliver valuable insights concerning the fundamental properties of TLS, and in particular help to clarify how parasitic TLS emerge in microstructured quantum devices.

• DFG Programme: Research Grants