Multimode superconducting circuits for realizing strongly coupled multiqubit processor units

Abstract

Interqubit coupling and qubit connectivity in a processor are crucial for achieving high-fidelity multiqubit gates and efficient implementation of quantum algorithms. Typical superconducting processors employ relatively weak transverse interqubit coupling which is activated via frequency tuning or microwave drives. Here, we propose a class of multimode superconducting circuits which realize multiple transmon qubits with all-to-all longitudinal coupling. These “artificial molecules” directly implement a multidimensional Hilbert space that can be easily manipulated due to the always-on longitudinal coupling. We describe the basic technique to analyze such circuits, compute the relevant properties, and discuss how to optimize them to create efficient small-scale quantum processors with universal programmability.