  Tunable coupling of superconducting qubits has been widely studied due to its
importance for isolated gate operations in scalable quantum processor
architectures. Here, we demonstrate a tunable qubit-qubit coupler based on a
floating transmon device which allows us to place qubits at least 2 mm apart
from each other while maintaining over 50 MHz coupling between the coupler and
the qubits. In the introduced tunable-coupler design, both the qubit-qubit and
the qubit-coupler couplings are mediated by two waveguides instead of relying
on direct capacitive couplings between the components, reducing the impact of
the qubit-qubit distance on the couplings. This leaves space for each qubit to
have an individual readout resonator and a Purcell filter needed for fast
high-fidelity readout. In addition, the large qubit-qubit distance reduces
unwanted non-nearest neighbor coupling and allows multiple control lines to
cross over the structure with minimal crosstalk. Using the proposed flexible
and scalable architecture, we demonstrate a controlled-$Z$ gate with $(99.81
\pm 0.02)\%$ fidelity.
