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8.6 Spectroscopy
Using this preliminarily calibrated measure pulse, it is possible to investigate
the level structure in the operating minimum in more detail.
Specifically, the
resonance frequency of the transition from the ground to the first excited energy
level is of particular interest as it is needed to perform logic operations on the
qubit. This resonance frequency can be found by irradiating the qubit with long
(≫ T 1 ) microwave pulses of varying frequency and then measuring the occupation
of the excited levels in the qubit operating minimum using the measure pulse from
above. Depending on the power of the drive, the data should resemble Figure 8.7b
or 8.7c. If multiple peaks are visible in the data, the power can be reduced until
only one peak remains, which should correspond to the qubit’s transition frequency
from the ground-state to the first excited state. The width of the peak gives a
first hint at the relevant quality measures of the qubit, like the dephasing time T ϕ .
But since there are more sensitive measures for these, the only calibration value
that needs to be taken away from this dataset is the resonance frequency ω 01 .
8.7 Rabi Oscillation
The next step is to investigate the temporal response of the qubit to a microwave
pulse like the one used above. For this, the qubit is driven with a micr-
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