High-fidelity two-qubit quantum logic gates in a trapped-ion chain using axial motional modes

Trapped-ion systems are one of the leading platforms for quantum information processing, where a key challenge is to scale up system size while maintaining high-fidelity two-qubit operations. A promising approach is to build high-performance modules interconnected via strong coupling. In particular, axial motional modes offer a practical mechanism to couple the ions in a chain, enabling the preparation of Greenberger-Horne-Zeilinger states with up to 24 ions using global operations, as well as high-fidelity two-qubit gates (96.6-98.0%) in fully connected five-ion chains. Here, we demonstrate two-qubit quantum logic gates in a 5-ion ⁴⁰Ca⁺ chain using axial modes, achieving fidelities exceeding 99% for adjacent pairs and over 98% for arbitrary pairs by carefully tackling dominant error sources. Our results are beneficial to the development of scalable ion-trap quantum processors, quantum simulation and quantum-enhanced metrology.