Technically, no clock can be more accurate than cesium standards such as NIST-F1 – the cesium fountain atomic clock that serves as the United States' primary time and frequency standard. But researchers have managed to develop an experimental atomic clock based on ytterbium atoms that boasts precision comparable to that of NIST-F1. The humble second was chosen as the International System of Units' (SI) base unit of time since it is based on the properties of the cesium atom (one second is the duration of 9,192,631,770 cycles of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom).
The scientists from the National Institute of Standards and Technology (NIST) used NIST-F1 as a 'ruler' for comparison in evaluating the clock by measuring the natural frequency of ytterbium, carefully accounting for all possible deviations, such as those caused by collisions between atoms. The results were good enough to indicate that the ytterbium clock is competitive in some respects with NIST-F1, which has been improving steadily and now keeps time to within one second in about 100 million years.
The NIST ytterbium clock is based on about 30,000 heavy metal atoms that are cooled to 15 microkelvins (close to absolute zero) and trapped in a column of several hundred pancake-shaped wells - an 'optical lattice' - made of laser light. A laser that 'ticks' 518 trillion times per second induces a transition between two energy levels in the atoms. The clock is about four times more accurate than it was several years ago thanks to improvements in the apparatus and a switch to a different form of ytterbium that is less susceptible to key errors than the form of ytterbium used previously.
NIST scientists are developing five versions of next-generation atomic clocks, each using a different atom and offering different advantages. The experimental clocks all operate at optical (visible light) frequencies, which are higher than the microwave frequencies used in NIST-F1, and thus can divide time into smaller units, thereby yielding more stable clocks. Additionally, optical clocks could one day lead to time standards up to 100 times more accurate than today's microwave clocks.
Anyone thinking that the NIST is a little on the anal retentive side with its obsession with time precision should realize that advances in atomic clock performance don't just let them know exactly how late someone is for a meeting. They also support the development of technologies such as high data rate telecommunications and the GPS.
Optical clocks are already providing record measurements of possible changes in the fundamental 'constants' of nature, a line of inquiry that has huge implications for cosmology and tests of the laws of physics, such as Einstein's theories of special and general relativity.
Next-generation clocks might lead to new types of gravity sensors for exploring underground natural resources and fundamental studies of the Earth. Other possible applications may include ultra-precise autonomous navigation, such as landing planes by GPS.
