Superradiant laser could improve atomic clock accuracy

New technology might also potentially help in the search for gravitational waves.

14th October 2016
Superradiant laser could improve atomic clock accuracy © NIST

JILA's superradiant laser is expected to be more stable than ordinary lasers, making it a sharper tool for improving the performance of atomic clocks © NIST

Atomic clocks, as the name may suggest, are devices that register regular “ticks” in atoms to accurately keep the time. Time is considered one of the most useful measurements in the International System of Units, as it allow for nearly all others to be calculated. Although very precise and stable, atomic clocks are not without imperfections.

A team at the Joint Institute for Lab Astrophysics (JILA) have demonstrated a new “superradiant” laser technique that may not only provide superior atomic clocks, but also advance other scientific endeavours such as detecting gravitational waves.

The Mighty Atom

Back in 1955, the atoms of choice in atomic clocks were caesium, and it was at this point we move truly into atomic time keeping, with one caesium atom ‘transition’ equating to one second. Since then, many different elements and measuring techniques have been used in attempts to create the most accurate and most stable atomic clock.

JILA’s current world-leading clock relies on a strontium lattice to store and then emit information in the form of lasers. Strontium has an excellent memory of laser frequency and colour, and can hold the information for up to two and a half minutes, a huge advance compared to most atoms, which can only hold information for up to 100 billionths of a second.

“Strontium atoms have two little “magnets” inside of it, each attached to two electrons,” JILA/NIST scientist James Thompson told Science Focus in an email.  “To give up its energy/information, something has to flip one of the magnets without flipping the other magnet.  Because the electrons are separated by billionths of a metre, it is very hard for anything to flip one magnet without flipping the other magnet.  As a result, it takes a very long time before this happens.”

Typically lasers storing information are trapped between two mirrors, but vibrations in either mirror can lead to the laser being sent off course generating ‘laser noise’, which can lower the accuracy of readings.

“But here is the rub: The very long memory of the atoms is awesome, but it also makes it extremely difficult to get the atoms to emit any light, which provides the information for us to use,” explains Thompson. “But in this superradiant laser, for the first time, we have coaxed these atoms to emit their light 10,000 times faster than they would normally like to emit it”

You may fire when ready.

It might sound like science fiction, but unlike of the eight kyber crystals used in the Death Star’s superlaser, JILA’s superradiant laser uses 200,000 strontium atoms in layers of 5,000. The lattice is chilled to near absolute zero and held levitating between two mirrors by, you guessed it, more lasers.

The atoms in the lattice are excited by a weak amount of light, which bounces between the atoms and moves them into synchronised behaviour. When the atoms calm down, an energy emission is produced. The light bounces between the two mirrors nearly 30,000 times and the collective emission of the synchronised atoms is realised as a pulse of laser light that is 1,000 times more intense than that of the independent atoms.

Although the laser currently lasts only 50 hundredths of a second, researchers hope to create a continuous variant by constantly exciting the atoms.

This advance is not just useful for atomic clocks as the laser has massive potential as an absolute frequency reference and may also have applications in space science.

“Perhaps one day this system will realise a ruler that could extend over a distance of order the Earth to the Sun.” Says Thompson. “Such a ruler might allow us to precisely measure small changes in distance between objects, for instance to detect if a gravity wave were to pass by the ruler.  In addition, this new ruler would increase our ability to sense changes in the rate of flow of time that occurs in Einstein’s general relativity.”

Who knows, maybe the time has come for this super-stable strontium laser to give us an even more accurate measure of time…


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