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9.3 Resonator Based Coupling
The measurement crosstalk described in Section 9.2.1 can be extremely detrimental
to the quality of the final data for a coupled qubit experiment.
Especially
for an experiment that attempts to violate Bell’s inequality, measurement
crosstalk is unacceptable as it actively introduces correlations into the dataset,
the very thing that the experiment is trying to quantify [Kofman and Korotkov,
2008b]. Therefore, it is necessary to develop coupling schemes that prevent measurement
crosstalk as much as possible. One such scheme is based on placing a
resonator between the qubits as shown in Figure 2.5b. This resonator will effectively
act as a band-pass filter for the coupling between the qubits. Thus, during
the decay of the tunneled state, only photons at the frequency of the resonator
can couple to the other qubit. Since this frequency will not be on resonance with
that other qubit, there will be no unwanted excitations, effectively eliminating the
problem of measurement crosstalk.
For the purpose of understanding the following sections, and even for the
implementation of the Bell experiment described in Chapter 11, the resonator can
be understood simply as a third qubit that is capacitively coupled to each of the
two real qubits. This works here, since there is always only one photon present
in the circuit during coupling operations, which prevents the resonator from ever
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