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The second qubit is not driven and should thus remain in the | 0 〉 state.
Measurement crosstalk causes the second qubit to be sometimes measured as
| 1 〉 conditionally on the first qubit being measured as | 1 〉. Therefore, measurement
crosstalk should introduce a small oscillation of P | x1 〉 at the same phase
and frequency of the oscillation of P | 1x 〉 caused by the Rabi drive. Fourier transforming
the measurements of both qubits exposes this oscillation as a peak at the
same frequency as the peak on the driven qubit. The amplitude ratio of the two
peaks then gives a number for the strength of the measurement crosstalk. Here,
the crosstalk is less than 1% in either direction.
As mentioned above, measurement crosstalk causes a correction to the limits
on S that are achievable by a hidden variable theory. According to Equation 11.4
the crosstalk measured here yields the following new limit on S:
S ≤ 2 + 2 ∣ p
a
c − p b ∣
c = 2 + 2 |0.0059 − 0.0031| = 2.0056 (11.9)
This new limit lowers the violations quoted in Table 11.12 by about 10%. For
example, the S-value and standard error of section 19 imply a violation in the
presence of measurement crosstalk of 55.5σ rather than the quoted 59.5σ. This
correction is sufficiently small to not challenge the underlying claim of a violation.
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