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is of significant concern for reliable qubit operation. This is due to the fact that
for rotations around axes in the X/Y-plane of the Bloch sphere, the angle of the
rotation axis is determined by the phase of the signal.
The common approach is to synchronizing the clocks with the use of a 10 MHz
reference signal to which each channel locks a VCO (Voltage Controlled Oscillator)
circuit to generate the clock frequency it needs for its operation.
To achieve
best possible phase locking between the microwave control signals on different
channels, we currently use a single carrier signal that is split and distributed to
the different channels. But this approach is only feasible for a small number of
channels. Beyond that, the carrier signal generators will need to be chosen and
synchronized with phase jitter in mind.
9.1.2 Flux Bias Crosstalk
Unless the qubits’ integrated circuit is laid out perfectly, the magnetic field
created by the flux bias coil of one qubit will also be seen by all other qubits,
although to a much lesser degree. This leads to changes in the actual bias seen by
the qubits and thus to changes of the operating parameters (reset bias, resonance
frequency, etc.) found by calibrating the qubits independently from each other.
Therefore, it is necessary to repeat the single qubit bringup experiments that lead
to the parameters while putting all qubits through their motions as if the final
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