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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

9:30am, April 16, 2014

As cesium atoms inside the atomic clock known as NIST-F2 pass through the combined focal point of six lasers, they instantly get stopped, cooled and quickly collected into a ball.  


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.

Power Words

absolute zero  The coldest possible temperature, also known as 0 kelvin. It is equal to minus 273.15 degrees Celsius (minus 459.67 degrees Fahrenheit).

atom  The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

atomic clock   A timekeeping device that relies on the frequency of microwave emissions from excited cesium atoms. That frequency is 9,192,631,770 hertz (or cycles/oscillations per second). Many common devices including cell phones, computers and GPS-satellite receivers rely on the high accuracy of atomic clocks to regularly reset their time (known as synchronization).

cesium    A metallic chemical element with the atomic number 55. Among its many uses, cesium serves as the basis of today’s atomic clocks and is used in many photo-electric cells.

electron    A negatively charged particle; the carrier of electricity within solids. Atoms will have one or more of these orbiting around their nucleus.

frequency  The number of times a specified periodic phenomenon occurs within a specified time interval. (In physics) The number of wavelengths that occurs over a particular interval of time.

laser  A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, and in surgery.

microwaves  An electromagnetic wave with a wavelength shorter than that of normal radio waves but longer than those of infrared radiation (heat) and of visible light.

NIST   (short for National Institute of Standards and Technology) Created in 1901 as the National Bureau of Standards, this government agency is now part of the U.S. Department of Commerce. It is charged with figuring out the best or most accurate way to measure things and with developing new and more accurate ways to do that measuring. To develop and test those measurement technologies, this agency has maintained one of the oldest physical science laboratories in the United States. It actually maintains several laboratories, although its headquarters is in Gaithersburg, Md.

radiation  Energy, emitted by a source, that travels through space in waves or as moving subatomic particles. Examples include visible light, infrared energy and microwaves.

vacuum   (in science) Space with little or no matter in it. 

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