Spectator Leakage Elimination in CZ Gates via Tunable Coupler Interference on a Superconducting Quantum Processor

TL;DR

This paper presents a method using a tunable coupler to dynamically reshape the Hamiltonian in superconducting quantum processors, effectively eliminating spectator-induced leakage during CZ gates and enhancing scalability.

Contribution

The authors introduce a novel dynamic Hamiltonian engineering technique that suppresses spectator leakage in multi-qubit superconducting processors, enabling high-fidelity gates in dense frequency environments.

Findings

Leakage rates reduced to ~10^{-4} across detuning range

Method scales with number of spectators, maintaining low leakage

Total leakage below surface code error correction threshold

Abstract

Spectator-induced leakage poses a fundamental challenge to scalable quantum computing, particularly as frequency collisions become unavoidable in multi-qubit 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 multi-qubit 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. The method also scales effectively with the number of spectators. With three simultaneous spectators, the total leakage remains below the threshold relevant for surface code error correction. This approach eases the tension between dense frequency packing and high-fidelity gate operation, establishing dynamic Hamiltonian engineering as an essential tool for advancing fault-tolerant quantum computing.