Explainer: How lasers make ‘optical molasses’

Top-performing atomic clocks use lasers to bring their cesium atoms to a stop — and almost reach a temperature of absolute zero

By Janet Raloff, 16:43 PM April 15, 2014

Atomic clocks are some of the most accurate timepieces available. But they aren’t perfectly precise. So scientists compare their ticking rate to that of even better atomic clocks — the best in the world. In April 2014, NIST-F2 became the top clock in the United States. F2 currently is the premier “yardstick” for measuring a second of time.           

To measure F2’s ticking rate, physicists at the National Institute for Standards and Technology in Boulder, Colo., probe cesium atoms with microw...

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