  All-microwave control of fixed-frequency superconducting quantum computing
circuits is advantageous for minimizing the noise channels and wiring costs.
Here we introduce a swap interaction between two data transmons assisted by the
third-order nonlinearity of a coupler transmon under a microwave drive. We
model the interaction analytically and numerically and use it to implement an
all-microwave controlled-Z gate. The gate based on the coupler-assisted swap
transition maintains high drive efficiency and small residual interaction over
a wide range of detuning between the data transmons.
