  Assembling future large-scale quantum computers out of smaller, specialized
modules promises to simplify a number of formidable science and engineering
challenges. One of the primary challenges in developing a modular architecture
is in engineering high fidelity, low-latency quantum interconnects between
modules. Here we demonstrate a modular solid state architecture with
deterministic inter-module coupling between four physically separate,
interchangeable superconducting qubit integrated circuits, achieving two-qubit
gate fidelities as high as 99.1$\pm0.5$\% and 98.3$\pm$0.3\% for iSWAP and CZ
entangling gates, respectively. The quality of the inter-module entanglement is
further confirmed by a demonstration of Bell-inequality violation for disjoint
pairs of entangled qubits across the four separate silicon dies. Having proven
out the fundamental building blocks, this work provides the technological
foundations for a modular quantum processor: technology which will accelerate
near-term experimental efforts and open up new paths to the fault-tolerant era
for solid state qubit architectures.
