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where the potential has two stable local minima, e.g. for −1.2 V ≤ V Bias ≤ −0.3 V,
and regions where the potential only has one stable minimum, e.g. for −0.2 V ≤
V Bias ≤ 0.0 V.
As the bias is increased, i.e. the potential is tilted more and more, the plot
shows local minima appearing on the right (bigger δ, longer t Switch ), e.g. at V Bias ≈
−1.2 V, growing to their maximum depth, e.g. at V Bias ≈ −0.1 V, and becoming
shallower again until they disappear on the left (smaller δ, shorter t Switch ), e.g.
at V Bias ≈ 1.0 V. We call all points in the scatter plot that correspond to the
same local minimum a “branch” and the maximum number of simultaneous local
minima the “overlaps”. The qubit here shows three full branches (and two partial
ones) and has two overlaps.
For a qubit to be usable, it must have at least two overlaps so it can be
measured (to hold the | 0 〉-state and the | 1 〉-state after tunneling) and it must
show at least one full branch so it can be reset. Figure 8.3 shows a collection
of non-optimal squid steps resulting from bad squid bias ramps, which should be
correctable by readjusting the ramp parameters. But qubits that show no bias
dependence of the scatter plot at all or have no regions of two or more overlaps
usually must be pronounced dead.
The squid steps dataset (Figure 8.2b) now tells us how to reset and read
out the qubit.
The first step in desigining the operation sequence consists of
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