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9.3.1 2D-Spectroscopy
To couple a set of qubits via a resonator, one needs to find a way to bias
these to the same frequency as the resonator. Due to variations in the fabrication,
the resonance frequency of the resonator might not be known exactly and thus
needs to be measured. Since, in the simplest design, the resonator cannot be read
out directly, this is best done with the 2D-Spectroscopy experiment described
in Chapter 8.12. In the dataset, the resonator will show up as a splitting, just
like the two-level states. The size of the splitting will depend on the coupling
strength between the qubit and the resonator.
Since this coupling strength is
often comparable to the coupling strength between the qubit and a two-level
state, it might not be immediately obvious which splitting corresponds to the
resonator. In most cases, though, due to the random frequencies of the two-level
states, looking at the 2D-Spectroscopy of both qubits and finding a splitting at
the same frequency resolves this ambiguity.
9.3.2 Swapping a Photon into the Resonator
Since the resonator can accept many photons from an on-resonant microwave
drive, it is not possible to perform single qubit operations while the qubits are on
resonance with the resonator. Thus, the qubits need to be kept off resonance and
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