The world’s most precise clock sits on a table in Jun Ye’s lab in Boulder, Colorado. A tangle of electronics, fiber optic cables, and laser beams, the clock is still a prototype, so no one actually uses it to tell time. Ye, who is a physicist at the research institute JILA, and his team have demonstrated that the clock can produce a second with precision in the parts per quintillion—that’s 10-19, some hundred billion times more precise than a quartz wristwatch.

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