Many proposals to scale quantum technology rely on modular or distributed designs wherein individual quantum processors, called nodes, are linked together to form one large multinode quantum computer (MNQC). One scalable method to construct an MNQC is using superconducting quantum systems with optical interconnects. However, internode gates in these systems may be two to three orders of magnitude noisier and slower than local operations. Surmounting the limitations of internode gates will require improvements in entanglement generation, use of entanglement distillation, and optimized software and compilers. Still, it remains unclear what performance is possible with current hardware and what performance algorithms require. In this article, we employ a systems analysis approach to quantify overall MNQC performance in terms of hardware models of internode links, entanglement distillation, and local architecture. We show how to navigate tradeoffs in entanglement generation and distillation in the context of algorithm performance, lay out how compilers and software should balance between local and internode gates, and discuss when noisy quantum internode links have an advantage over purely classical links. We find that a factor of 10–100× better link performance is required and introduce a research roadmap for the co-design of hardware and software towards the realization of early MNQCs. While we focus on superconducting devices with optical interconnects, our approach is general across MNQC implementations.

Measurements of internal printed circuit board (PCB) structures such as striplines and vias face the problem of launching clean test signals into the device under test (DUT). Traditionally, coaxial connectors or surface probing with high frequency microprobes are used to provide interfaces to test equipment. Both approaches have to be carefully optimized in order to give adequate results for the multi-GHz range. This paper discusses a different access technique, the recessed probe launch (RPL), which was previously used by the authors for measurements up to 40 GHz. Full-wave 3D electromagnetic modeling is applied to analyze the parasitics of the proposed launch technique and to find strategies for its optimization. Comparison to measurement shows that the models are able to predict the major physics of the launch but several details still need to be explored, e.g. accurate modeling of the microprobes, material parameters, and network analyzer calibration.