Atomic clock technology has been made so small it may soon be possible to incorporate super-accurate timekeeping into mobile devices such as cellphones.
The final clock should fit neatly in mobile devices
Computer chip fabrication techniques were used to make a clock mechanism that will neither lose nor gain a second in 300 years.
Researchers believe final development should see a battery-operated system that is about the size of a sugar lump.
The US National Institute of Standards and Technology is behind the work.
"These clocks will be so useful that we can't even think of the most significant applications at present," Nist's John Kitching told the BBC's Science In Action programme.
However, he thinks one guaranteed benefit will be an increase in the reliability and accuracy of mobile satellite-navigation receivers in towns and cities.
"The Global Positioning System is based on timing signals that leave satellites that orbit the Earth. Having a stable clock onboard your GPS receiver can enable position location while viewing fewer satellites than would be possible without a clock," he explained.
Standard atomic clocks measure time by "counting" the natural vibrations of caesium atoms, at 9.2 billion "ticks" per second. But these timepieces, which control all manner of daily applications from synchronising television feeds to calculating bank transfers, tend to be on the large size.
They can be up to two metres in height - as well as power-hungry and expensive to build.
The new Nist timepiece borrows the lithographic techniques that have transformed chip manufacturing.
"What we've built so far is what we call the 'physics package' and this is the part of the clock that contains the atoms that are the stable frequency reference," Dr Kitching said.
Dr Kitching: "What we've built so far is what we call the 'physics package'"
"This device is about the size of a grain of rice. It is made using processes very similar to those used in the microelectronics industry. That is, we etch patterns into silicon wafers and then confine the atoms in the cavities."
The cell that contains the atoms is part of a stack of layers. At the bottom is a tiny laser which shines its light through a small optics assembly that conditions the light so that it matches the energies of the caesium atoms.
The process is monitored on the far side of the cell by a photo-detector. An external oscillator, such as the quartz crystal found in wristwatches, can then be stabilised against this standard.
The physics package measures just 1.5mm by 4mm. The commercial device, with all its associated circuitry, is not expected to have a footprint bigger than 1cm square.
'In the field'
The tiny clock mechanism cannot match the stability and accuracy of its big brothers which are designed to keep near-perfect time equivalent to 1 second in millions of years.
However, its scale and potentially low cost of manufacture should make it ideal for a host of new applications.
Dr Patrick Gill, from the UK's National Physical Laboratory, has seen the prototype work at Nist's Colorado base. He says the timepiece, when fully developed, is headed for work "in the field".
"They haven't quite got to the power consumption that they want - about 30 milliwatts - which would be battery power operation. They're currently at about 75 milliwatts - but they're well on their way, and it's pretty impressive to have got to the level they have," he told BBC News Online.
"It's a clock for particular applications. In the field these things could be used to synchronise local clocks in your notebook or your phone without having to access necessarily the master station.
"This could be important in some military applications where the master station has been jammed, for example."
Dr Gill also said civilian applications would benefit greatly from a reduction in interference - from cross-talk or bad signal pick-up.
Nist believes that compared with quartz crystal oscillators, the most precise time and frequency references of equivalent size and power, its new mini-atomic clocks potentially offer a 1,000-fold improvement in long-term timing precision.
The Nist research was first described in the journal Applied Physics Letters.