Modular coupling approach using ancilla transmons with flux-tunable hybridization*

Recent demonstrations of small quantum processors (tens of qubits) based upon superconducting circuits have incorporated coupling elements to facilitate fast multi-qubit operation without sacrificing data storage fidelity. A recently proposed [Yan2018] and now state-of-the-art coupling approach leverages the interference of a static interaction between two data transmons and a separate virtual interaction through a non-computational flux tunable ancilla transmon to produce an effective tunable coupling. The resultant coupling is a non-trivial function of all three transmons’ frequencies and the static interactions between them. Consequently, adapting the coupler to new architectures and use cases can require considerable remodeling. To overcome this challenge, we introduce a new ancilla-based coupler that utilizes the hybridization of two ancillas to mediate the interaction of external circuitry across the coupler. Our proposed coupler is modular in design and simulation, has first order insensitivity to data-transmon parameters, and can mediate both degenerate and parametric interactions to implement a diverse set of entangling operations.

Reference: Yan F, et al. Tunable Coupling Scheme for Implementing High-Fidelity Two-Qubit Gates. Phys. Rev. Applied 10, 054062 (2018).