Explainer: How lasers make ‘optical molasses’

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 microwaves. Inside F2, those cesium atoms race around at some 1,600 kilometers (1,000 miles) per hour. Because studying atoms on the fly it too hard, F2 slows some of them down.

But there’s another reason for slowing those atoms down. The warmth that keeps those atoms moving also risks changing the ticking rate of this ultra-precise atomic timepiece, explains Thomas O’Brian. He runs the Time and Frequency Division at NIST.

To chill its cesium atoms, F2 gently bombards them with laser light. The process creates optical — or light-based — “molasses.”

Light can be thought of as tidbits of energy moving in individual packets, called photons. Each is a tiny particle. They’re like ping-pong balls being fired from a toy gun. Fire enough ping-pong balls “at a bowling ball rolling toward you — bang, bang, bang — and eventually it’s going to stop,” O’Brian says. F2’s lasers are those guns. If the lasers fire enough photons at a cesium atom, they’ll slow it down.

But if the gun didn’t stop firing, the ping-pong balls would cause the stopped bowling ball to start rolling back in the opposite direction. To prevent that, F2 fires a steady stream of photons at cesium atoms from many angles — up, down and four sides. Any flying cesium atom that happens to cross a laser beam’s path will get bombarded with those photons. And pushed a tiny bit.

Eventually, that atom may get pushed to the point at which all six laser beams cross. At this focal point, the lasers’ combined photon bullets will not just slow the atom but quickly stop it — and hold it almost completely still. And until those lasers are turned off, the atom won’t be able to leave the molasses-like conditions that all of those photons created.

An atom’s movement is a reflection of its temperature. The warmer it gets, the faster it moves. By freezing an atom in place, the lasers also turn it dead cold, hovering at just above absolute zero.

Held captive in F2, this atom — and many more — gets bombarded by microwaves. If the right frequency of microwaves hits those atoms, electrons orbiting in the outer shell of those atoms briefly absorb the radiation. Shortly afterward, the electrons release that energy. By measuring that microwave energy shed by electrons in the cesium atoms, F2 can ultra-precisely measure the length of a second.