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picking the “operating branch”, i.e. the potential minimum in which the qubit
state will reside during operations. Here, for example, we can choose the branch
that extends from V Bias ≈ −2.4 V to V Bias ≈ −0.3 V. The qubit is reset into
this branch by biasing it to a point V Reset at which the operating branch is the
only minimum of the potential, e.g. V Reset = −1.35 V. If the qubit is held at this
bias for a time t Reset ≫ T 1 , its state will decay into the operating branch with
certainty.
To maximize the non-linearity during operation, i.e. the difference in energy
spacing of the lowest levels in the operating minimum, the qubit is biased close to
the “end” of the operating branch, i.e. to a point where the operating minimum
becomes very shallow, for example around V Operate ≈ −0.3 V. This is necessary
so that the lowest two energy levels in the minimum can be addressed exclusively
as described in Chapter 3.3.
To measure the qubit, the excited state (| 1 〉) will be selectively tunneled out
of the operating branch as described in Section 8.5 into the neighboring branch –
here, the branch that extends from V Bias ≈ −1.2 V to V Bias ≈ −1.0 V. The qubit
is then biased to the point V Readout where the depth of the two potential minima
is maximized, e.g. V Readout = −0.75 V. This maximizes the barrier height between
the two minima to reduce the chance of the qubit state tunneling between the
minima by accident.
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