Flux-Noise Spectroscopy on a Superconducting Transmon Qubit

Superconducting qubits can act as noise sensors because their decoherence times under certain pulse sequences can be linked to the system's noise power spectral density (PSD). These sequences also set the observable frequency range of the PSD. The PSD during free evolution of the qubit can be obtained with a dynamical decoupling (CPMG-) pulse sequence, which renders the qubit only susceptible to noise within a narrow frequency band. The PSD during driven evolution is measured with a so-called spin-locking sequence, during which only noise at the Rabi frequency can affect the qubit. Both protocols were successfully tested on a flux qubit by Bylander $[$1$]$ and Yan $[$2$]$. In this work, we apply these two protocols to a tunable concentric transmon $[$3$]$, overcome the challenge of the transmon's low anharmonicity by using DRAG pulses and compare the PSDs of free and driven evolution. Our results show a clear 1/f dependence in both cases, as well as a white spectrum at frequencies above 1 MHz during free evolution. The successful application of these protocols to a transmon permits magnetic noise sensing with such a qubit in future experiments.