Direct Implementation of High-Fidelity Three-Qubit Gates for Superconducting Processor with Tunable Couplers

TL;DR

This paper demonstrates a high-fidelity, direct three-qubit CCZ gate implementation in superconducting processors, reducing circuit depth and improving quantum algorithm performance, exemplified by a successful Grover search.

Contribution

The work introduces a novel direct three-qubit gate implementation using tunable couplers, achieving higher fidelity and lower leakage than decomposed methods.

Findings

Achieved 97.94% state fidelity for the CCZ gate.

Demonstrated lower leakage with direct implementation.

Successfully used the CCZ gate in Grover search algorithm.

Abstract

Three-qubit gates can be constructed using combinations of single-qubit and two-qubit gates, making their independent realization unnecessary. However, direct implementation of three-qubit gates reduces the depth of quantum circuits, streamlines quantum programming, and facilitates efficient circuit optimization, thereby enhancing overall performance in quantum computation. In this work, we propose and experimentally demonstrate a high-fidelity scheme for implementing a three-qubit controlled-controlled-Z (CCZ) gate in a flip-chip superconducting quantum processor with tunable couplers. This direct CCZ gate is implemented by simultaneously leveraging two tunable couplers interspersed between three qubits to enable three-qubit interactions, achieving an average final state fidelity of $97.94\%$ and a process fidelity of $93.54\%$. This high fidelity cannot be achieved through a simple combination of single- and two-qubit gate sequences from processors with similar performance levels. Our experiments also verify that multilayer direct implementation of the CCZ gate exhibits lower leakage compared to decomposed gate approaches. As a showcase, we utilize the CCZ gate as an oracle to implement the Grover search algorithm on three qubits, demonstrating high performance with the target probability amplitude significantly enhanced after two iterations. These results highlight the advantage of our approach, and facilitate the implementation of complex quantum circuits.