Applications for Better Atomic Clocks
History has shown that better clocks always find practical uses, often unpredictable ones.
Past advances in atomic clock performance enabled the development of technologies such as GPS positioning and navigation and made them commonplace – nearly everyone carrying a cell phone or smart phone is using GPS. NIST atomic clocks are used for many purposes, including guidance of deep space probes. The Financial Industry Regulatory Authority requires that all electronic transactions be stamped with a time traceable to NIST. NIST's Internet Time Service, which allows the public and other users to synchronize their computers' internal clocks with NIST time, received more than 12 billion hits per day as of 2013.
The record performance of optical atomic clocks has already contributed to scientific research. NIST clocks have provided record measurements of possible changes in the "fundamental constants" of nature, a line of inquiry that has huge implications for cosmology, and tested Einstein's theories of relativity.
NIST scientists also envision many practical applications. Optical atomic clocks also might be used to make new types of sensors measuring quantities that have tiny effects on ticking rates, including gravity, magnetic fields, force, motion and temperature. An example is precision geodesy, or mapping of gravity fields near the surface of the earth. A change in height of 10 centimeters (about four inches) near the Earth's surface corresponds to a change in atomic clock frequency (ticking rate) of about 1 x 10-17—the equivalent of about one second in 4 billion years. NIST experimental atomic clocks already exceed this performance level in the lab. If this performance can be realized in the field, precision measurements of gravity could be useful in geology or hydrology.
Use of optical atomic clocks for fast, precision measurements of magnetic and electrical fields, force, motion, temperature and other quantities could lead to technologies for new applications in advanced manufacturing, medical imaging and diagnosis, and many other areas.
Selection of a Future Time Standard
Time standards such as the SI second are chosen by consensus of the world scientific community and, more formally, by the General Conference for Weights and Measures. When cesium was selected as the international time standard in 1967, NIST and other national standards laboratories already had many years of experience with cesium clocks, leading to wide acceptance of the technology.
As many new types of optical atomic clocks are developed at NIST, JILA and across the world, and as these clocks continue to improve at an accelerating pace, the international scientific community may or may not re-define the SI second based on one new type of atomic clock. Instead, the scientific community has been discussing the possibility of selecting several different coordinated standards for the SI second. That approach would avoid some of the problems that stem from having one single standard (currently cesium) that is the sole source of "accuracy." NIST and timing laboratories throughout the world continue to discuss what might be the most practical and meaningful approaches for the future. (Some optical clocks are already recognized as "secondary representations of the SI second.")
Many experimental optical atomic clocks are operating around the world. The existence of many clocks of the same type provides ample opportunities for comparisons and a basis for future redefinitions of the SI second. About a dozen strontium lattice clocks and six ytterbium lattice clocks are operating in national metrology institutes and universities, with several more in the planning stages. There are also several different types of single-ion atomic clocks operating around the world. Currently only NIST operates single ion clocks based on mercury and aluminum, although aluminum ion clocks are under development in other laboratories.
As noted above, both accuracy and stability are important in timekeeping and atomic clock applications. For decades, the most accurate atomic clocks (cesium-based clocks) were not the most stable. That led to the current practice of using cesium clocks such as NIST-F1 to periodically calibrate collections of more stable atomic clocks such as hydrogen masers for practical applications. High performance optical lattice clocks are now simultaneously the most precise (or accurate in the terminology used for standards) and most stable clocks in the world. If the SI second were redefined to include ytterbium and strontium as standards, this coincidence of accuracy and stability could lead to many practical changes in atomic clock applications and the international system of timekeeping. In any case, it is likely that in the future even more types of atomic clocks will be in regular use for the ever-growing range of applications.