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singlet state due to the extra factor of i, it nevertheless shows the same degree of
entanglement. Since the initial state of the qubit pair prepared by the source is
not a condition used in the derivation of the Bell inequality, it is not of fundamental
importance that the state created by the “particle source” yields qubit pairs
in the Bell singlet state. In fact, depending on the specifics of the experiment’s
imperfections, a different initial state might yield a higher S-value.
Simulations and experiments show that imperfections in the state preparation
caused by sweeping the qubits on or off resonance are more detrimental to the
value of S than imperfections caused by an impaired π-pulse due to the qubits
being coupled during the pulse. Thus, it turns out to be beneficial for the outcome
and the simplicity of the experiment to begin the sequence with both qubits
immediately on resonance and applying the π-pulse while the coupling is on.
This works due to the fact that the π-pulse is shorter than the time-scales of the
coupling operation. The final entangling sequence for the qubit pair source using
always-on capacitive coupling is shown in Figure 11.1a.
11.1.3 Entangling the Qubits – Resonator Coupling
The pair preparation using a resonator based coupling element requires a sequence
that is slightly different from the simple capacitive coupling. This is due to
the fact that the resonator can itself store excitations which impede the entangling
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