  Modular architectures are a promising approach to scale quantum devices to
the point of fault tolerance and utility. Modularity is particularly appealing
for superconducting qubits, as monolithically manufactured devices are limited
in both system size and quality. Constructing complex quantum systems as
networks of interchangeable modules can overcome this challenge through
`Lego-like' assembly, reconfiguration, and expansion, in a spirit similar to
modern classical computers. First prototypical superconducting quantum device
networks have been demonstrated. Interfaces that simultaneously permit
interchangeability and high-fidelity operations remain a crucial challenge,
however. Here, we demonstrate a high-efficiency interconnect based on a
detachable cable between superconducting qubit devices. We overcome the
inevitable loss in a detachable connection through a fast pump scheme, enabling
inter-module SWAP efficiencies at the 99%-level in less than 100 ns. We use
this scheme to generate high-fidelity entanglement and operate a distributed
logical dual-rail qubit. At the observed ~1% error rate, operations through the
interconnect are at the threshold for fault-tolerance. These results introduce
a modular architecture for scaling quantum processors with reconfigurable and
expandable networks.
