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Figure 9.2: Measure Pulse Timing – a) Sequence: Both qubits are measured with
a pulse that tunnels the | 0 〉 state 50% of the time. The measure pulse on the
second qubit is shifted in time by t Offset relative to the first qubit. b) Crosstalk:
When the measure pulses reach the qubits at the same time (dashed line), the
measurement crosstalk is minimized leading to a dip in P | 11 〉 .
happens before the measurement of these qubits is over, these states will tunnel
as well. For a two-qubit system, this leads to a | 01 〉-state or a | 10 〉-state to be
misidentified as a | 11 〉-state. The | 00 〉-state is not affected by this problem.
This crosstalk does offer an opportunity, though, in that it provides a way to
synchronize the timing of the measurement channels between the different qubits.
The method is based on the fact that the crosstalk can only affect qubits until the
point when their measurement is complete. This temporal asymmetry can be used
to adjust the relative timing of the different channels. For this, an S-Curve experiment
is used to independently calibrate a measurement pulse for each qubit that
yields a | 0 〉-state tunneling probability of about 50%. As this tunneling probability
is a purely classical probability, in a system without measurement crosstalk, a
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