Efficiently trapping new qubits in chip-based ion traps

What is it about?

Certain atomic species have useful properties for quantum computing applications (for example, states that have long decay lifetimes or that are insensitive to magnetic fields, both of which can improve quantum memory) but can be difficult to trap safely and reliably in practical laboratory systems. One example is barium-133, which is radioactive and can only be safely handled in small quantities. Laser ablation is useful for loading these species from small quantities: when a sample is hit by a high energy pulsed laser, a small volume of vaporized material is released from the sample surface. The ejected atoms can then be photo-ionized and trapped, in a process called “loading” an ion trap. This requires much smaller source samples than other common loading methods. However, the ejected atoms can be highly energetic after ablation, which can make them difficult to trap. To better understand the loading process and improve our ability to load rare ion species, we developed a geometry- and source-dependent theoretical model for the probability of loading ion traps. We validated this model experimentally by measuring the probability of loading both barium and strontium ions in two ion traps of different sizes under varying experimental conditions. We find that loading rates are not just dominated by the usual parameter of the potential energy at the trapping bounder (the “trap depth”) but by the physical size of the trap itself. Furthermore, in the small, micrometer-scale traps produced using modern microfabrication techniques, we find an optimal trap depth after which increasing the trap depth causes a decrease in trapping efficiency.

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Photo by Zoltan Tasi on Unsplash

Photo by Zoltan Tasi on Unsplash

Why is it important?

Using modern microfabrication techniques, chip-based ion traps are likely to be a critical tool for scaling up trapped-ion quantum computers to increasingly large system sizes. These microfabrication techniques are used to print metal electrodes on a single plane that can be used to trap ions tens of micrometers above the trap surface. Further, these processes can be used to integrate new technologies like integrated optics or active electronic devices into the ion trapping chip itself, enabling the miniaturization of the ion trapping laboratory. However, these small chips generally load ions less efficiently than three-dimensional ion traps. The improved understanding of the ion trap loading process that we develop in this work is critical to loading rare species (including radioactive barium or more exotic ion species like molecular ions) into modern, scalable microfabricated ion traps, and particularly for the future design of ion traps.