Noise spectroscopy using fluxonium as a sensor. (Part 1)

The equilibrium populations of superconducting qubits are measured to be higher than what is expected from the Boltzmann distribution derived from the fridge temperature. While this affects all types of qubits, different designs show varying degrees of susceptibility to this phenomenon. Active cooling schemes have been shown to be effective, but incur the cost of increased circuit complexity, particularly for coupler elements, whose spurious excitations can lead to gate errors. Passive strategies, such as filtering, can reduce the excess population to some extent, but are limited by as of yet not understood sources of noise. In this work, we leverage the rich Hamiltonian of the fluxonium to investigate the properties of the qubit's environment. In particular, we utilize complementary measurement techniques in different parameter regimes of fluxonium to characterize the noise exciting the qubit. Our findings demonstrate the enhanced capabilities of using these complementary detection techniques together and could pave the way for improved mitigation strategies. Part 1 of 2.