  Trapped-ion applications, such as in quantum information, precision
measurements, optical clocks, and mass spectrometry, rely on specialized
high-performance ion traps. The latter applications typically employ
traditional machining to customize macroscopic 3D Paul traps, while quantum
information processing experiments usually rely on photo-lithographic
techniques to miniaturize the traps and meet scalability requirements. Using
photolithography, however, it is challenging to fabricate the complex
three-dimensional electrode structures required for optimal confinement. Here
we address these limitations by adopting a high-resolution 3D printing
technology based on two-photon polymerization supporting fabrication of large
arrays of high-performance miniaturized 3D traps. We show that 3D-printed ion
traps combine the advantages of traditionally machined 3D traps with the
miniaturization provided by photolithography by confining single calcium ions
in a small 3D-printed ion trap with radial trap frequencies ranging from 2 MHz
to 24 MHz. The tight confinement eases ion cooling requirements and allows us
to demonstrate high-fidelity coherent operations on an optical qubit after only
Doppler cooling. With 3D printing technology, the design freedom is drastically
expanded without sacrificing scalability and precision so that ion trap
geometries can be optimized for higher performance and better functionality.
