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this leads to a modification of the inequality that raises the bound on the value
of S that can be achieved with a locally realistic hidden variable theory [Kofman
et al., 2007]:
−2 + 4 min{p a c, p b c} ≤ S ≤ 2 + 2 ∣ ∣p a c − p b ∣
c
(11.4)
Here, p a c (p b c) is the classical probability that a tunneling of qubit A (B) causes a
tunneling of qubit B (A), i.e. the probability that the state | 10 〉 (| 01 〉) is measured
as | 11 〉 for reasons other than the non-ideal measurement fidelity of qubit B (A).
This effect on the inequality itself makes measurement crosstalk very undesirable
as it significantly complicates the justification of a claimed violation. To
address this issue, a different coupling scheme can be used that allows the qubits to
be decoupled during the measurement. Since controllable coupling was not available
at the time of the implementation of this experiment, a temporary solution
can be found by coupling the qubits through a coplanar resonator.
11.4.2 Resonator Coupling
Resonant buses are frequently used in other qubit designs (like the charge
qubit) to enable coupling despite the qubits’ high impedance [Majer et al., 2007].
Here, the purpose of the resonator instead is to act as a band-pass filter for
the coupling. Since the qubits are far off resonance from the pass-band during
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