On a frigid day last January in northern Sweden, a German-led team of physicists loaded a curious machine onto an unmanned rocket. The payload, about as tall as a single-story apartment, was essentially a custom-made freezer—a vacuum chamber, with a small chip and lasers within, that could cool single atoms near absolute zero. They launched the rocket about 90 miles past the boundary of outer space, monitoring a livestream from a heated building nearby.