Realization of high-fidelity CZ and ZZ-free iSWAP gates with a tunable coupler (2011.01261v3)

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 the existing framework for tunable couplers based on the dispersive approximation does not fully incorporate three-body multi-level 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 and $ZZ$-free iSWAP gates with two-qubit interaction fidelities of $99.76 \pm 0.07$% and $99.87 \pm 0.23$%, respectively, which are close to their $T_1$ limits.

High-Fidelity CZ and ZZ-free iSWAP Gates with a Tunable Coupler

This paper presents a significant advancement in quantum gate operations, specifically focusing on two-qubit gates using a tunable coupler. The authors introduce a systematic approach that extends beyond the dispersive approximation to exploit the engineered level structure of the coupler, aiming to optimize control and minimize errors like coherent leakage and parasitic longitudinal (ZZ) interactions. The paper discusses the experimental demonstration of Controlled-Z (CZ) and ZZ-free iSWAP gates with two-qubit interaction fidelities reaching 99.76% and 99.87%, respectively.

The primary challenge addressed in this research is enhancing the fidelity of coupler-mediated entangling gates. Superconducting qubits have evolved significantly, enabling fast and scalable two-qubit gates, but high-fidelity implementation remains elusive due to gate errors introduced by the incomplete theoretical models of tunable coupling. Specifically, existing models often overlook the nuanced three-body multi-level dynamics that become relevant as the qubit-coupler coupling moves beyond the weakly-dispersive regime during fast gate operations.

The authors propose to tackle these issues by optimizing the control segments and the level structure of the coupler, considering higher energy levels and dynamic interactions that are usually omitted in traditional perturbative approaches. By doing so, they aim to minimize coherent leakage to the coupler, which leads to several benefits, such as reducing non-adiabatic errors during fast gate operations and suppressing residual ZZ interactions for iSWAP gates.

The paper meticulously details the design and implementation aspects needed to inhibit these errors. For the CZ gate, they achieve a 60ns operation with leakage significantly reduced by optimized control. Meanwhile, the iSWAP gate, notorious for ZZ interactions due to the presence of higher qubit energy levels, demonstrates negligible residual ZZ interactions with a tunably optimized coupler. The result is a rapid 30ns iSWAP gate with high fidelity and a residual ZZ angle of virtually zero, marking it as ZZ-free.

A key innovation outlined is the use of a multi-level quantum system coupled via a capacitive mechanism, allowing for adjustable interactions through the manipulation of a central transmon coupler. This system is represented experimentally using integrated superconducting circuits, and the authors provide the technical specifics of this circuit designed using optimized energetic configurations. They further utilize numerical simulations to affirm the optimized control parameters and pulse shapes, which are validated experimentally through various benchmarking techniques.

The broadened theoretical approach coupled with empirical substantiation offers both practical and theoretical implications. Practically, the development of highly accurate and error-resilient quantum gates supports more efficient quantum algorithms applicable in noisy intermediate-scale quantum (NISQ) devices. Theoretically, the work advances the understanding of multi-level dynamics in tunable qubit systems, which challenges the current threshold-oriented discourse and opens pathways for new studies in quantum error correction schemes.

Moving forward, this thorough methodology for eliminating coherent leakage and minimizing ZZ interactions could be extended towards larger systems of qubits, contributing crucially towards scalable quantum processing units. Further investigations into complex multi-qubit interactions and error mechanisms could drive more significant strides in fault-tolerant quantum computing architectures, potentially transforming current quantum computational limitations.