  High fidelity two-qubit gates exhibiting low crosstalk are essential building
blocks for gate-based quantum information processing. In superconducting
circuits two-qubit gates are typically based either on RF-controlled
interactions or on the in-situ tunability of qubit frequencies. Here, we
present an alternative approach using a tunable cross-Kerr-type ZZ-interaction
between two qubits, which we realize by a flux-tunable coupler element. We
control the ZZ-coupling rate over three orders of magnitude to perform a rapid
(38 ns), high-contrast, low leakage (0.14 %) conditional-phase CZ gate with a
fidelity of 97.9 % without relying on the resonant interaction with a
non-computational state. Furthermore, by exploiting the direct nature of the
ZZ-coupling, we easily access the entire conditional-phase gate family by
adjusting only a single control parameter.
