Pure kinetic inductance coupling between generalized flux qubits and their readout

We develop a qubit-readout circuit coupled through the kinetic inductance of superconducting granular aluminum (grAl). Utilizing the material properties of grAl to implement the dispersive shift removes the need for electromagnetic coupling. This enables a localized tuning knob to engineer the readout independent of the capacitance matrix. If the capacitance matrix is designed to be symmetric, the qubit-readout coupling is entirely mediated by the grAl kinetic inductance. We validate the pure kinetic coupling concept and demonstrate various generalized flux qubit regimes from plasmon to fluxon, with dispersive shifts ranging from 30 kHz to 7 MHz at the half-flux quantum sweet spot. Using purely kinetic coupling, we achieve readout performance comparable to standard electromagnetic coupling, with quantum state preparation fidelity of 99.7 % and 92.7 % for the ground and excited states, respectively, and below 0.1 % leakage to non-computational states. The excited state fidelity is limited by qubit relaxation to the ground state with quantum demolishing effects below 1%.