Realization of High-Fidelity CZ and ZZ-Free iSWAP Gates with a Tunable Coupler

Abstract

High-fidelity two-qubit gates at scale are a key requirement to realize the full promise of quantum computation and simulation. The advent and use of coupler elements to tunably control two-qubit interactions has improved operational fidelity in many-qubit systems by reducing parasitic coupling and frequency crowding issues. Nonetheless, two-qubit gate errors still limit the capability of near-term quantum applications. The reason, in part, is that the existing framework for tunable couplers based on the dispersive approximation does not fully incorporate three-body multilevel dynamics, which is essential for addressing coherent leakage to the coupler and parasitic longitudinal (ZZ) interactions during two-qubit gates. Here, we present a systematic approach that goes beyond the dispersive approximation to exploit the engineered level structure of the coupler and optimize its control. Using this approach, we experimentally demonstrate CZ andZZ-free iSWAP gates with two-qubit interaction fidelities of99.76±0.07%and99.87±0.23%, respectively, which are close to theirT1limits.

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Plain Language Summary

The full promise of quantum computation depends on the ability to perform computations with very low error rates. Despite tremendous progress toward this goal with superconducting quantum bits (qubits), a major bottleneck remains with errors in two-qubit gates, one of the fundamental building blocks of quantum computers. Here, we demonstrate a way to sharply reduce errors in two-qubit gates, demonstrating fidelities that are close to their coherence limits.

Operational fidelity in two-qubit gates has vastly improved in recent years thanks to the introduction of tunable couplers, an architectural add-on that can control interactions among neighboring qubits. However, improvements to tunable couplers have run into a roadblock—standard control and design techniques do not account for all the elements that introduce errors through parasitic interactions among qubits or leakage to the coupler itself.

We present a systematic approach to coupler control and design that goes beyond previous methods, allowing us to turn on and off the qubit-qubit coupling that we desire while eliminating unwanted interactions. Using this approach, we demonstrate two-qubit interaction fidelities of nearly 99.8%.

Leakage errors are detrimental to the implementation of quantum error-correcting codes, and gates free of parasitic interactions enable efficient circuit compilation with improved algorithmic accuracy. Taken together, the principles and demonstrations we present will help resolve major challenges facing quantum computing hardware for contemporary applications.