Fig. 5: Resonant features of a quantum multi-level structure of the CSFQ.
a Persistent current readout giving the right-well probability as a function of the initial tilt bias, for a linear anneal in both φx and φz, as illustrated by the slanted arrows in Fig. 3. Tilt bias anneal amplitude is amp = 0.326π, i.e., φz(t) = φz(0) + amp × (t/tf), where tf = 60 ns is the anneal time. The black solid line shows the experimental result, and the dashed blue line is the AME result for the CSFQ circuit with parameters extracted from spectroscopy (see text). Experimental error bars show standard deviation and are calculated as in Fig. 4. b Population of CSFQ circuit eigenstates (indicated by the numbers in the legend, with 0 being the ground state) at the end of each annealing, along with the experimental result (exp) also shown in (a). Only avoided level crossings lead to observable features in the persistent-current readout, indicated by the green circles in (a). c An example of the CSFQ spectrum vs normalized anneal time s = t/tf, for an initial tilt bias corresponding to the gray vertical dashed line in panels (a) and (b). The blue arrow marks the avoided level crossing between levels 5 and 6 at the end of the anneal, which corresponds to the population exchange between these two levels in panel (b), and the corresponding experimental resonant feature. Cascaded level crossings that transfer the population are visible throughout the anneal.
