A single atom that acts like a mirror won’t do much for your personal hygiene. But it could lead to transistors for computers that use light instead of electricity to perform computations. Researchers in Austria trapped a single barium ion, cooled it with a laser and used it to replace one of the mirrors in a Fabry-Pérot interferometer — a device that filters or amplifies light by bouncing it between a pair of mirrors. When energized by light of the proper wavelength the ion became reflective, the researchers report in an upcoming Physical Review Letters. —Devin Powell
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