The strange world of qubit types and quantum computing approaches keeps growing. A team of researchers from Center for Quantum Nanoscience, Institute for Basic Science (IBS), Korea, report this Month in Science performing quantum calculations by dragging the tip of a scanning tunneling microscope over titanium atoms on a surface. While no one expects the approach to soon join the crowd of techniques and qubit modalities already vying for sway in quantum computing, it is fascinating.

Here’s the paper’s abstract:

“Individual electron spins in solids are promising candidates for quantum science and technology, where bottom-up assembly of a quantum device with atomically precise couplings has long been envisioned. Here, we realized atom-by-atom construction, coherent operations, and readout of coupled electron-spin qubits using a scanning tunneling microscope. To enable the coherent control of “remote” qubits that are outside of the tunnel junction, we complemented each electron spin with a local magnetic field gradient from a nearby single-atom magnet.

“Readout was achieved by using a sensor qubit in the tunnel junction and implementing pulsed double electron spin resonance. Fast single-, two-, and three-qubit operations were thereby demonstrated in an all-electrical fashion. Our angstrom-scale qubit platform may enable quantum functionalities using electron spin arrays built atom by atom on a surface.”

There’s a nice summary description of the work in Nature written by Davide Castelvecchi: “The researchers started by scattering titanium atoms on a perfectly flat surface made of magnesium oxide. They then mapped the atoms’ positions using the STM, which has atomic resolution. They used the tip of the STM probe to move the titanium atoms around, arranging three of them into a triangle.

“Using microwave signals emitted from the STM tip, the researchers were able to control the spin of a single electron in one of the titanium atoms. By tuning the frequencies of the microwaves appropriately, they could also make its spin interact with the spins in the other two titanium atoms, similarly to how multiple compass needles can influence each other through their magnetic fields. By doing this, the team was able to set up a simple two-qubit quantum operation, and also to read out its results. The operation took just nanoseconds — faster than is possible with most other types of qubit.”

Nature reports that Andreas Heinrich, who led the research, says it will be fairly straightforward to extend the technique to perhaps 100 qubits, possibly by manipulating spins in a combination of individual atoms and molecules.

Link to Nature article, https://www.nature.com/articles/d41586-023-03141-z?utm_source=Nature+Briefing&utm_campaign=92a97c1689-briefing-dy-20231009&utm_medium=email&utm_term=0_c9dfd39373-92a97c1689-46067094#ref-CR1

Link to Science paper, https://www.science.org/doi/full/10.1126/science.ade5050