Coloquios del Departamento de Física – FCEyN – UBA

Recent progress with microfabricated quantum devices has revealed that an ubiquitous source of noise originates from parasitic two-level systems (TLSs). For superconducting qubits, TLSs residing on surfaces or in tunnel junctions account for a major part of dephasing and thus pose a roadblock to the realization of solid-state quantum processors. Recent experiments of Lisenfeld et al. [1] demonstrated that it is possible to utilize a superconducting qubit to explore the quantum state evolution of such high-frequency TLSs. We found that a major source of dephasing of the probed high-frequency TLSs is their interaction with thermal, low-frequency TLSs [2]. The so-called standard model of TLS  is sufficient to quantitatively explain the Ramsey-type experiments. The effect of low-frequency TLSs is efficiently reduced by spin-echo techniques. The remaining dephasing observed in the echo protocol is due to further noise sources with a more white spectrum.