Fig. 2: Spectroscopy of two gatemon devices.
a One-tone spectroscopy of device A measuring the shift δfr of the cavity at 5.056 GHz. Outside a band gap of about 9 V, dense forests of positive shifts indicate the presence of the qubit mode at lower frequency. Larger δfr is caused by smaller qubit-cavity detuning due to higher qubit frequency, which stems from larger supercurrent in the carbon nanotube Josephson junction. b, c One-tone spectroscopy for device A (close up of (a)) and device B. d, e Two-tone spectroscopy of the gate-dependent qubit mode for the same gate voltage ranges. Shown is δfr compared to the vertical average (device A) or change in cavity transmission S21 compared to no second tone drive \({S}_{21}^{{{{{\rm{off}}}}}}\) (device B). Spectra of device A exhibit regularly spaced peaks owing to quantum phase transitions changing the charge parity (letters e/o for even/odd in (b)) of the quantum dot. Contrarily, the gate dependence for device B is smoother, the nanotube junction being in a regime of a superconducting quantum dot strongly coupled to the electrodes. Both devices show spurious modes, visible as horizontal lines. The qubit linewidth for frequencies below 2 GHz is up to ~1 GHz due to charge dispersion.
