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Novel laser makes atomic clock '100 times sharper'

14 Oct 2016

JILA team has now built the super-radiant design based on strontium atoms that it first proposed four years ago.

A team of researchers at a laboratory affiliated with the US National Institute of Standards and Technology (NIST) in Colorado has built a new laser for atomic clocks that they believe will make the high-precision timepieces more than 100 times sharper.

The Joint Institute for Laboratory Astrophysics (JILA) group working on the project first proposed the so-called “super-radiant” laser back in 2012 with a prototype based around rubidium atoms.

It produced stimulated emission in a different way to most lasers, and although very dim it delivered light within an extremely narrow range of frequencies - while lead scientist James Thompson believed that the brightness could be scaled up by using more atoms, without compromising the laser linewidth.

JILA animation explaining super-radiant strontium laser mechanism:

Strontium switch
Since then, Thompson and colleagues have switched to strontium atoms – the same element that is used in today's best-performing optical lattice clocks, and said to offer much greater stability and much-reduced environmental sensitivity.

They claim in a paper just published in the journal Science Advances that the new laser appears stable enough to improve atomic clock performance by two orders of magnitude, and is much less sensitive to environmental “noise” such as mirror vibrations. It could even be used as a clock itself – perhaps aiding the search for gravitational waves in planned orbiting observatories.

Thompson explains that although it is the extremely long “memory” of strontium atoms, which can store frequency information for a remarkably long time compared with most other atoms, that provides the better stability, the same phenomenon makes it extremely difficult to get them to emit the amount of light needed for the atomic clock to work effectively.

“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,” Thomson said of the research, which was partly funded by the US Defense Advanced Research Projects Agency (DARPA) and Army Research Office.

Excited states
In the new design, 200,000 strontium atoms were stacked in layers of 5000, trapped in a cavity, cooled to near absolute zero and levitated in a vacuum by an optical lattice - itself created by intersecting laser beams from a different source.

Thompson added: “The super-radiant laser light is still billions of times weaker than typical lasers, but the key point is that the frequency of the light should be very stable.”

At present, the strontium source can only be used for a limited time, because the system effectively “runs out” of atoms as they are drained from the excited state to emit light. But some of the future work planned includes creating a continuous super-radiant beam by continuously returning atoms to the excited state.

If that kind of laser can be built, it ought to be just as stable as the atoms used in today’s most advanced atomic clocks, whose precision is limited partly by laser noise - and could even be used as a clock all by itself.

• Another NIST team has found a way to synchronize optical clocks over a long distance, through turbulent air that usually distorts optical signals. Laura Sinclair and colleagues say that by sending pulses from a frequency comb between two sites they were able to retain femtosecond-level synchronization.

"The 12 km of turbulent air results in massive distortions of the laser beams yet the two clocks agree in time to 20 digits," Sinclair pointed out, adding that synchronization over an even larger distance should be possible, provided that nothing blocks the beam.

The work was published in the latest issue of the journal Applied Physics Letters.

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