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simultaneous application of such measure pulses would yield all possible outcomes
with equal probability. For a two-qubit system, this would mean that the readout
would yield the states | 00 〉, | 01 〉, | 10 〉, and | 11 〉 all with 25% probability. In a
situation with crosstalk where qubit 1 (2) was measured significantly before qubit
2 (1) the probability of measuring | 10 〉 (| 01 〉) would be reduced and | 11 〉 would
be increased. Thus, if the probabilities are measured as a function of the delay
t Offset between the measure pulses, one obtains a plot like Figure 9.2.
The point on this plot where the probability of | 11 〉 is minimized will then
correspond to the point where the qubits are measured at the same time. The
difference at this point between 25% and the measured probability of | 11 〉 captures
the amount of residual measurement crosstalk.
It is possible to minimize this
residual crosstalk by carefully shaping the measure pulses in a way that maximizes
each qubit’s visibility while minimizing crosstalk. A saw-tooth shape seems to
work well for this.
9.2.2 Spectroscopy
In the same way that the frequency of the microwave bias needs to be matched
to the qubits resonance frequency in order to drive transitions, to get several qubits
to couple via a simple capacitor, they need to be biased such that their resonance
frequencies are the same.
To achieve this, the most useful scan is the simple
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