  Besides striving to assemble more and more qubits in a single monolithic
quantum device, taking a modular design strategy may mitigate numerous
engineering challenges for achieving large-scalable quantum processors with
superconducting qubits. Nevertheless, a major challenge in the modular quantum
device is how to realize high-fidelity entanglement operations on qubits housed
in different modules while preserving the desired isolation between modules. In
this work, we propose a conceptual design of a modular quantum device, where
nearby modules are spatially separated by centimeters. In principle, each
module can contain tens of superconducting qubits, and can be separately
fabricated, characterized, packaged, and replaced. By introducing a bridge
module between nearby qubit modules and taking the coupling scheme utilizing a
tunable bus, tunable coupling of qubits that are housed in nearby qubit
modules, could be realized. Given physically reasonable assumptions, we expect
that sub-100-ns two-qubit gates for qubits housed in nearby modules which are
spatially separated by more than two centimeters could be obtained. In this
way, the inter-module gate operations are promising to be implemented with gate
performance comparable with that of intra-module gate operations. Moreover,
with help of through-silicon vias technologies, this long-range coupling scheme
may also allow one to implement inter-module couplers in a multi-chip stacked
processor. Thus, the tunable longer-range coupling scheme and the proposed
modular architecture may provide a promising foundation for solving challenges
toward large-scale quantum information processing with superconducting qubits.
