Spectator Leakage Suppression via Invariant Subspace Engineering for cz Gates in Superconducting Quantum Circuits.

Spectator leakage poses a fundamental challenge to scalable quantum computing, particularly as frequency collisions become unavoidable in multiqubit processors. We introduce a leakage mitigation strategy based on dynamically reshaping the system Hamiltonian. Our technique utilizes a tunable coupler to enforce a block-diagonal structure on the effective Hamiltonian governing near-resonant spectator interactions, confining the gate dynamics to a two-dimensional invariant subspace and thus preventing leakage by construction. On a multiqubit superconducting processor, we experimentally demonstrate that this dynamic control scheme suppresses leakage rates to the order of 10^{-4}, across a wide near-resonant detuning range and with up to three simultaneous spectator qubits. These results demonstrate a robust and scalable method that resolves the critical trade-off between dense frequency packing and high-fidelity gate operation. Our Letter establishes dynamic Hamiltonian engineering as an essential technology for building fault-tolerant quantum computers.