While chip-sized atomic clocks (CSACs) are commercially available, the researchers say these low-power devices — about the size of a matchbox - drift over time, and are less accurate than fountain clocks, the much larger atomic clocks that set the world's standard.

"We have a path towards making a compact, robust clock that's better than CSACs by a couple of orders of magnitude, and more stable over longer periods of time," said co-author Krish Kotru, a graduate student in Massachusetts Institute of Technology (MIT)'s Department of Aeronautics and Astronautics.

Kotru said such portable, stable atomic clocks could be useful in environments where GPS signals can get lost, such as underwater or indoors, as well as in militarily "hostile environments," where signal jamming can block traditional navigation systems.

The team came up with the new atomic timekeeping approach by making several "tweaks" to the standard method. The most accurate atomic clocks today use cesium atoms as a reference. Like all atoms, the cesium atom has a signature frequency, or resonance, at which it oscillates.

Since the 1960s, one second has been defined as 9,192,631,770 oscillations of a cesium atom between two energy levels. To measure this frequency, fountain clocks toss small clouds of slow-moving cesium atoms a few feet high, much like a pulsed fountain, and measure their oscillations as they pass up, and then down, through a microwave beam.

Instead of a microwave beam, the group chose to probe the atom's oscillations using laser beams, which are easier to control spatially and require less space - a quality that help in shrinking atomic clock apparatuses.

While some atomic clocks also employ laser beams, they often suffer from an effect called "AC Stark shift," in which exposure to an electric field, such as that produced by a laser, can shift an atom's resonant frequency. This shift can throw off the accuracy of atomic clocks.
    
To avoid this problem, most standard fountain clocks use microwave beams instead of lasers. However, Kotru and his team looked for ways to use laser beams while avoiding AC Stark shift.

In laser-based atomic clocks, the laser beam is delivered at a fixed frequency and intensity. Kotru's team instead tried a more varied approach, called Raman adiabatic rapid passage, applying laser pulses of changing intensity and frequency – a technique that is also used in nuclear magnetic resonance spectroscopy to probe features in individual molecules.
    
"For our approach, we turn on the laser pulse and modulate its intensity, gradually turning it on and then off, and we take the frequency of the laser and sweep it over a narrow range," Kotru said. The research was published in the journal Physical Review.