The US National Institute of Standards and Technology (NIST) has improved its prototype ytterbium atomic clock, giving it a precision comparable to that of the NIST-F1 caesium fountain clock - the nation's civilian time standard.
"It is about four times more accurate than it was several years ago," said NIST, adding that NIST-F1 currently keeps time to within 1 second in about 100 million years.
The ytterbium clock is based on about 30,000 metal atoms cooled to 15µK and trapped in a column of several hundred pancake-shaped wells made of laser light.
Light at 518THz induces a transition between two energy levels in the atoms. Recent precision improvements are due to changes in the apparatus and a switch to a different form of ytterbium whose nucleus is slightly magnetic due its 'spin-one half' angular momentum. "This atom is less susceptible to key errors than the spin-zero form of ytterbium used previously," said NIST.
The Institute is developing five versions of next-generation atomic clocks, each using a different atom and offering different advantages.
All operate at visible light frequencies, which are higher than the microwave frequencies used in NIST-F1.
NIST scientists believe that optical clocks could one day lead to time standards up to 100 times more accurate than today's microwave clocks.
"The best optical clocks are currently based on single ions, such as the NIST 'logic clock' using an aluminium ion. but lattice clocks have the potential for higher stability because they simultaneously average signals from tens of thousands of atoms," said NIST.
There are also ongoing comparisons of the ytterbium clock to the strontium lattice clock located at the US Joint Institute for Laboratory Astrophysics [JILA].
"At this point it is far from clear which atom and clock design will be selected by research groups around the world as a future time and frequency standard," said NIST.
Other possible application of next-generation atomic clocks is ultra-precise autonomous navigation - GPS is referenced to atomic clocks - allowing planes to be landed by a future GPS.
Around 1 million ytterbium atoms held in a lattice made of intersecting laser beams.