"The real power of our technique is that we are able to run the clock on so little electrical power that it could be battery operated and that it is small enough to be easily incorporated into a cell phone or some other kind of handheld device," says John Kitching of NIST. "And nothing else like it even comes close as far as being mass producible."

For more than 50 years, atomic clocks have set the gold standard for time and frequency measurement but their applications have been limited by their complexity, size and expense. The scale and ease with which the NIST design could be made potentially opens the door to low-cost mass-production of miniature atomic clocks that can easily be integrated with electronics.

The chip-scale clock contains a vertical-cavity surface-emitting laser (VCSEL), a lens, an optical attenuator, a polarizing waveplate, a cell containing cesium vapour and a photodiode. The VCSEL emits two light signals that are separated by just a few gigahertz. These are focused onto the cesium atoms and tuned until they exactly match the hyperfine D2 transition in cesium. This gives an incredibly precise measure of frequency and thus time.

The clock is stable to one part in 10 billion, equivalent to 1 second in 300 years -- a long-term stability which is several orders of magnitude better than competing portable units such as temperature-compensated quartz crystal oscillators. However, this precision is still a long way from that achieved by large atomic clocks. For example, NIST's F1 clock boasts a stability of 1 part in 1015, which is equivalent to 1 second in 30 million years.