Enabling full localization of qubits and gates with a multi-mode coupler

Abstract: Tunable couplers are a key building block of superconducting quantum processors, enabling high on-off ratios for two-qubit entangling interactions. While crosstalk can be mitigated in idle mode, conventional single-mode couplers lack independent control over interactions in the one- and two-excitation manifolds, leading to unitary errors such as leakage during gate operations. Moreover, even at the nominal decoupled point, residual wavefunction delocalization persists, causing unintended qubit-qubit coupling. Here we propose a multi-mode tunable coupler that enables nonlinear control of interactions across excitation manifolds, achieving a high on-off ratio in the one-excitation manifold while suppressing coupling in the two-excitation manifold. The proposed design also realizes complete localization between qubits, providing perfect isolation at the decoupled point and opening new possibilities for scalable, high-fidelity quantum gates.