Suppression of charge dispersion by resonant tunneling in a single-channel transmon qubit*

Motivated by the importance of understanding the underlying charge physics in superconducting qubits, we investigate the charge dispersion of a gate-controlled nanowire-based transmon. When approaching the pinch-off regime of the nanowire junction, we observe resonant behavior of the plasma frequency, which we attribute to the formation of a quantum dot in the junction. By measuring the charge dispersion while crossing a resonance, we observe that it is suppressed far below the range expected for a conventional transmon at comparable values of the Josephson and charging energies. The enhanced suppression can be explained and quantitatively modeled by the presence, at resonance, of a single transport channel with near-unity transmission. Our results establish an experimental validation of the theory of Coulomb oscillations in Josephson junctions in a previously unexplored regime. In addition, these results show that charge dispersion can be suppressed without the necessity of large E_J/E_C ratios, potentially allowing a very large qubit anharmonicity.