  We have proposed and experimentally verified a tunable inter-qubit coupling
scheme for large-scale integration of superconducting qubits. The key feature
of the scheme is the insertion of connecting pads between qubit and tunable
coupling element. In such a way, the distance between two qubits can be
increased considerably to a few millimeters, leaving enough space for arranging
control lines, readout resonators and other necessary structures. The increased
inter-qubit distance provides more wiring space for flip-chip process and
reduces crosstalk between qubits and from control lines to qubits. We use the
term Tunable Coupler with Capacitively Connecting Pad (TCCP) to name the
tunable coupling part that consists of a transmon coupler and capacitively
connecting pads. With the different placement of connecting pads, different
TCCP architectures can be realized. We have designed and fabricated a few
multi-qubit devices in which TCCP is used for coupling. The measured results
show that the performance of the qubits coupled by the TCCP, such as $T_1$ and
$T_2$, was similar to that of the traditional transmon qubits without TCCP.
Meanwhile, our TCCP also exhibited a wide tunable range of the effective
coupling strength and a low residual ZZ interaction between the qubits by
properly tuning the parameters on the design. Finally, we successfully
implemented an adiabatic CZ gate with TCCP. Furthermore, by introducing TCCP,
we also discuss the realization of the flip-chip process and tunable coupling
qubits between different chips.
