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energy landscape (see Chapter 2.2.3) as a function of the applied flux bias DC bias .
For this, the qubit is held at a given bias point for a time much longer than T 1 so
that the qubit state has time to decay into the lowest energy state of one of the
local minima of the potential. The δ-coordinate of the minimum then corresponds
to a certain magnetic flux in the qubit loop and thus to a certain critical current
of the readout squid. This critical current is then probed via the described squid
bias current ramp and the switching time t Switch is recorded. This procedure is
repeated many times each for various bias voltages V Bias to generate a scatter plot
of the observed qubit states as a function of qubit bias as shown in Figure 8.2b,
which we call “Squid Steps”.
Since the choice of the local minimum into which the qubit state settles depends
hysteretically on the biasing history, it is useful to take this data in two steps.
First, the qubit is biased at the maximum negative bias V min before bringing it
to the bias point of interest V Bias . Since the bias does not change instantaneously
from V min to V Bias , this leads to the qubit state preferentially settling into the
left-most local minimum at V Bias . To map out the right-most local minima as
well, the qubit is then biased at the maximum positive bias V max before bringing
it to V Bias . This sequence is shown in Figure 8.2a.
The resulting scatter plot shows the location of the qubit potential’s minima
as a function of the qubit bias V Bias . The qubit examined here shows bias regions
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