Fluxonium as a Tunable Spectrometer of Quantum Noise

Superconducting qubits and resonators are measured to have equilibrium populations higher than what is expected from the Boltzmann distribution at the temperature of the refrigerator, even when the control lines are heavily filtered. This presents both a practical challenge, and an interesting fundamental question. Practically, this impedes the usage of thermal equilibrium as a reset of the device. Although active cooling schemes exist, they require increased complexity in the form of additional drives, especially for coupler elements, excitations in which can lead to gate errors. Fundamentally, this indicates that the qubit is coupled to noisy baths which are not thermalised to the refrigerator. In this talk, we use a fluxonium qubit to measure the spectral noise density of this bath as a function of frequency, taking advantage of the tunability of the qubit.