  A challenge in building large-scale superconducting quantum processors is to
find the right balance between coherence, qubit addressability, qubit-qubit
coupling strength, circuit complexity and the number of required control lines.
Leading all-microwave approaches for coupling two qubits require comparatively
few control lines and benefit from high coherence but suffer from frequency
crowding and limited addressability in multi-qubit settings. Here, we overcome
these limitations by realizing an all-microwave controlled-phase gate between
two transversely coupled transmon qubits which are far detuned compared to the
qubit anharmonicity. The gate is activated by applying a single, strong
microwave tone to one of the qubits, inducing a coupling between the two-qubit
$|f,g\rangle$ and $|g,e\rangle$ states, with $|g\rangle$, $|e\rangle$, and
$|f\rangle$ denoting the lowest energy states of a transmon qubit. Interleaved
randomized benchmarking yields a gate fidelity of $97.5\pm 0.3 \%$ at a gate
duration of $126\,\rm{ns}$, with the dominant error source being decoherence.
We model the gate in presence of the strong drive field using Floquet theory
and find good agreement with our data. Our gate constitutes a promising
alternative to present two-qubit gates and could have hardware scaling
advantages in large-scale quantum processors as it neither requires additional
drive lines nor tunable couplers.
