  Large scale quantum computing motivates the invention of two-qubit gate
schemes that not only maximize the gate fidelity but also draw minimal
resources. In the case of superconducting qubits, the weak anharmonicity of
transmons imposes profound constraints on the gate design, leading to increased
complexity of devices and control protocols. Here we demonstrate a
resource-efficient control over the interaction of strongly-anharmonic
fluxonium qubits. Namely, applying an off-resonant drive to non-computational
transitions in a pair of capacitively-coupled fluxoniums induces a
$\textrm{ZZ}$-interaction due to unequal ac-Stark shifts of the computational
levels. With a continuous choice of frequency and amplitude, the drive can
either cancel the static $\textrm{ZZ}$-term or increase it by an order of
magnitude to enable a controlled-phase (CP) gate with an arbitrary programmed
phase shift. The cross-entropy benchmarking of these non-Clifford operations
yields a sub $1\%$ error, limited solely by incoherent processes. Our result
demonstrates the advantages of strongly-anharmonic circuits over transmons in
designing the next generation of quantum processors.
