Advances in nanotechnology lay the ground for a thriving variety of novel devices suitable to explore new realms of quantum phenomena on the mesoscopic scale. Superconducting quantum bits, as an example, have reached very long coherence times by designs that avoid or weaken their coupling to undesired degrees of freedom. An important remaining problem are parasitic Two-level systems (TLS) which emerge from tunneling defects in the device material. These TLS act as a source of noise and decoherence which is detrimental to a large variety of microfabricated quantum devices. The flip side of this problem is that the strong coupling to even individual TLS makes superconducting qubits ideal tools for the study of material defects in the coherent regime.
Here, we show how a phase qubit is used to directly manipulate and readout the quantum states of individual TLS. We tune TLS properties by the applied mechanical strain and perform high-resolution defect spectroscopy by which we obtain a novel view onto the TLS distribution and their mutual interactions. Moreover, by analyzing their coherent dynamics, we utilize single defects as quantum spectrum analyzers in order to shed new light on their environment. I will present studies of TLS coherence in dependence of strain, temperature, and the density of in-situ generated quasi-particles. Moreover, I will briefly discuss our recently initiated project to create superconducting quantum interfaces for the study of TLS in arbitrary materials.
