Fault-tolerant interfaces for modular quantum computing on diverse qubit platforms

Modular architectures offer a scalable path toward fault-tolerant quantum computing by interconnecting smaller quantum processing units (QPUs) provided that high-rate, fault-tolerant interfaces can be realized across modules. We present a comprehensive analysis and comparison of known and new methods for establishing such interfaces, including lattice surgery, transversal gates, and novel grow-and-distil protocols based on code growing and logical distillation. Using the surface code, we identify optimal interface strategies across a wide range of hardware parameters, such as gate fidelities, entangling rates, and memory resources, and estimate the requirements to achieve logical error rates of 10^-6 and 10^-12. Our results establish when the interface become a bottleneck in the computation and provide guidance for experimental implementations with superconducting, atomic, and solid-state hardware.