Fortifying the strong-coupling regime of quantum dot circuit QED using high-impedance superconducting resonators

In quantum dot cQED, the spin-photon coupling rate g_s is proportional to the charge-photon coupling rate g_c. To move deeper into the strong coupling regime, g_c can be enhanced through the use of high kinetic inductance films. We report direct current resistivity measurements and microwave investigations of niobium nitride (NbN) films of different thicknesses. Consistent with expectations, the kinetic inductance increases rapidly as the film thickness is decreased below 50 nm. For 15 nm NbN thin films, we measured a large kinetic inductance of L_K ∼ 41.2 pH/sq and sheet resistance of R_S ~ 274 Ohm/sq. We investigate the use of these films in the fabrication of high impedance half-wavelength microwave cavities.