LONDON Researchers at the US Government's National Institute of Standards and Technology (NIST) are claiming a tiny atomic clock they have demonstrated could form the basis of significantly more precise timekeeping than achievable now for mobile phones and other portable, battery-powered devices.
They suggest the breakthrough could also lead to more secure wireless communications, more precise navigation and other applications.
The clock's inner workings measure just 1.5 millimeters on a side and 4 millimeters high, consume less than 75 thousandths of a watt, and are stable to one part in 10 billion, equivalent to gaining or losing just one second every 300 years.
The chip-scale clock comprises a laser, a lens, an optical attenuator to reduce the laser power, a waveplate that changes the polarization of the light, a cell containing a vapor of cesium atoms, and a photodiode to detect the laser light transmitted through the cell. Tiny gold wires provide electrical connections to the electronics for the clock.
Significantly, the researchers say the device could be manufactured and assembled on semiconductor wafers using existing techniques for making micro-electro-mechanical systems (MEMS).
This opens the possibility for low-cost, mass production of an atomic clock about the size of an IC that could be easyly integrated with other electronics. In the long term, NIST says, the package would be integrated with an external oscillator and control circuitry into a finished clock measuring about 1 cubic centimetre.
"The real power of our technique is that we're able to run the clock on so little electrical power that it could be battery operated and that it's small enough to be easily incorporated into a cell phone or some other kind of handheld device," says physicist John Kitching, principal investigator for the project. "And nothing else like it even comes close as far as being mass producible."
The chip-scale clock is comparable in size and long-term stability to temperature-compensated quartz crystal oscillators, currently used in portable devices.
The NIST researchers say they expect to be able to improve the clock's long-term stability and reduce its power consumption to the point where the device could substantially improve the performance of many commercial and military systems that require precision time keeping.
The device is based on the same idea as other atomic clocks such as the NIST-F1 fountain clock measuring time by the natural vibrations of cesium atoms, at 9.2 billion 'ticks' per second but uses a different design.
Here, cesium vapor is confined in a sealed cell and probed with light from an equally small infrared laser, which generates two electromagnetic fields. The difference in frequency of these two fields is tuned until it equals the difference between two energy levels of the atoms. The atoms then enter a 'dark state' in which they stop absorbing and emitting light this point defines the natural resonance frequency of cesium.
An external oscillator can then be stabilized against this standard.
The chip-scale clock is less accurate than larger atomic clocks such as fountain clocks. However, NIST says compared to quartz crystal oscillators the most precise time and frequency references of equivalent size and power the latest device could potentially offer a 1,000-fold improvement in long-term timing precision.
In wireless communications devices, these clocks could improve network synchronization and channel selection to enhance security and anti-jamming capabilities. In Global Positioning System (GPS) receivers, small clocks could improve the precision of satellite-based navigation systems such as those used in commercial and military vehicles and emergency response networks.