  We experimentally demonstrate a parametric iSWAP gate in a superconducting
circuit based on a tunable coupler for achieving a continuous tunability to
eliminate unwanted qubit interactions. We implement the twoqubit iSWAP gate by
applying a fast-flux bias modulation pulse on the coupler to turn on parametric
exchange interaction between computational qubits. The controllable interaction
can provide an extra degree of freedom to verify the optimal condition for
constructing the parametric gate. Aiming to fully investigate error sources of
the two-qubit gates, we perform quantum process tomography measurements and
numerical simulations as varying static ZZ coupling strength. We quantitatively
calculate the dynamic ZZ coupling parasitizing in two-qubit gate operation, and
extract the particular gate error from the decoherence, dynamic ZZ coupling and
high-order oscillation terms. Our results reveal that the main gate error comes
from the decoherence, while the increase in the dynamic ZZ coupling and
high-order oscillation error degrades the parametric gate performance. This
approach, which has not yet been previously explored, provides a guiding
principle to improve gate fidelity of parametric iSWAP gate by suppression of
the unwanted qubit interactions. This controllable interaction, together with
the parametric modulation technique, is desirable for crosstalk free multiqubit
quantum circuits and quantum simulation applications.
