Approaching ultrastrong coupling in transmon circuit QED using a high-impedance resonator
Sal J. Bosman, Mario F. Gely, Vibhor Singh, Daniel Bothner, Andrés Castellanos-Gómez, Gary A. Steele · Physical review. B./Physical review. B · 2017
In this experiment, we couple a superconducting transmon qubit to a high-impedance $645\phantom{\rule{0.28em}{0ex}}\mathrm{\ensuremath{\Omega}}$ microwave resonator. Doing so leads to a large qubit-resonator coupling rate $g$, measured through a large vacuum Rabi splitting of $2g\ensuremath{\simeq}910$ MHz. The coupling is a significant fraction of the qubit and resonator oscillation frequencies $\ensuremath{\omega}$, placing our system close to the ultrastrong coupling regime ($\overline{g}=g/\ensuremath{\omega}=0.071$ on resonance). Combining this setup with a vacuum-gap transmon architecture shows the potential of reaching deep into the ultrastrong coupling $\overline{g}\ensuremath{\sim}0.45$ with transmon qubits.