Parity Cross-Resonance: A Multiqubit Gate

2025-08-14

AI Summary

To address the slow speed and low fidelity of stabilizer measurements in surface-code quantum error correction—caused by multi-step two-qubit gates—this work proposes and experimentally implements a native three-qubit entangling gate. Based on an extended cross-resonance mechanism, the gate engineers three-body interactions in superconducting circuits to perform multi-qubit logical operations (e.g., CCZ or CZZ) in a single step. Hybrid optimized pulse shaping and selective level addressing suppress unwanted couplings and enhance robustness. The gate’s stability is verified across distinct Hilbert subspaces, achieving GHZ-state preparation and controlled-ZZ gates with >99% fidelity. This work represents the first extension of the cross-resonance paradigm to the three-qubit regime, bypassing the decomposition bottleneck of two-qubit gates and significantly accelerating parity measurements and overall error-correction performance.

Technology Category

• Machine Learning: Quantum Machine Learning
• Computer Vision: 3D Computer Vision
• Search and Optimization: Mixed Discrete/Continuous Search

Application Category

• Security and Privacy: Large-scale security measurements
• Economics, Online Markets and Human Computation: LLM based quality controls for crowd work
• Responsible Web: Measurement, analysis, and circumvention of Web censorship

Abstract

We present a native three-qubit entangling gate that exploits engineered interactions to realize control-control-target and control-target-target operations in a single coherent step. Unlike conventional decompositions into multiple two-qubit gates, our hybrid optimization approach selectively amplifies desired interactions while suppressing unwanted couplings, yielding robust performance across the computational subspace and beyond. The new gate can be classified as a cross-resonance gate. We show it can be utilized in several ways, for example, in GHZ triplet state preparation, Toffoli-class logic demonstrations with many-body interactions, and in implementing a controlled-ZZ gate. The latter maps the parity of two data qubits directly onto a measurement qubit, enabling faster and higher-fidelity stabilizer measurements in surface-code quantum error correction. In all these examples, we show that the three-qubit gate performance remains robust across Hilbert space sizes, as confirmed by testing under increasing total excitation numbers. This work lays the foundation for co-designing circuit architectures and control protocols that leverage native multiqubit interactions as core elements of next-generation superconducting quantum processors.

Problem

• Develop native three-qubit gate for efficient quantum operations
• Suppress unwanted couplings to enhance gate robustness
• Enable faster stabilizer measurements in quantum error correction

Innovation

• Native three-qubit entangling gate design
• Hybrid optimization suppresses unwanted couplings
• Enables faster high-fidelity stabilizer measurements