seen more readily in Fig. \[F010206H1\](b), which shows the power spectral density (calculated as the absolute square of the discrete Fourier transform) of the escape rate data in Fig. \[F010206H1\](a) between $t = 1$ and $45\ {\ensuremath{{\ensuremath{\mathrm{ns}}}}}$. For this plot, each horizontal line has been normalized to its maximum value (black) in order to emphasize the location of the dominant frequency. Three distinct bands are visible.

For this data set, the level spacing ${\ensuremath{\omega_{0 1}}} / 2 \pi$ is equal to the microwave frequency ${\ensuremath{\omega_{r\!f}}}/ 2 \pi = 6.5\ {\ensuremath{{\ensuremath{\mathrm{GHz}}}}}$ at $I_b = 17.614\ {\ensuremath{\mu {\ensuremath{\mathrm{A}}}}}$. The band with the highest current in Fig. \[F010206H1\](b) is centered about $I_b = 17.624\ {\ensuremath{\mu {\ensuremath{\mathrm{A}}}}}$, suggesting that [$0 \rightarrow 1$]{} Rabi oscillations are the dominant process near this bias. For slightly higher or lower $I_b$, the oscillation frequency increases as ${\ensuremath{\Omega_{R,0 1}}} \approx \sqrt{\Omega_{01}^{\prime2} + {\ensuremath{\left( {\ensuremath{\omega_{r\!f}}}- {\ensuremath{\omega_{0 1}}} \right)}}^2}$, in agreement with simple two-level Rabi theory, leading to the curved band in the grayscale plot.

The other bands correspond to oscillations between the ground state and higher excited states. Due to the anharmonic level structure, ${\ensuremath{\omega_{0 2}}} / 4 \pi$ is 6.5 GHz at a smaller current bias $I_b = 17.594\ 