A New Era for Atomic Clocks
February 4, 2014
A revolution is under way in timekeeping. Precision timekeeping based on atomic clocks already underpins much of our modern technology—telecommunications, computer networks and satellite-based positioning systems like GPS that are used by billions of people every day. But now research at NIST, JILA* and other organizations throughout the world is improving atomic clock performance so quickly that many new applications are expected in the near future. At both NIST and JILA, NIST scientists are leading research on new atomic clocks in collaboration with students, postdoctoral fellows and scientists from across the world.
Atomic clocks keep time based on the natural oscillations (or frequencies) of atoms, which are much more stable and accurate than any mechanical devices. The atomic clock was invented at NIST in 1949 (then the National Bureau of Standards), and atomic clocks quickly became more accurate than any other timekeeping technologies.
Atomic clocks began to have notable impacts on other technologies about 20 years later. Atomic time standards have been in use since 1967, when one of the cesium atom’s natural frequencies was selected by an international committee as the basis for the international unit of time, the “SI second”. The second is defined as exactly 9,192,631,770 oscillations or cycles of this cesium atom resonant frequency. NIST operates two U.S. civilian time and frequency standards based on cesium: NIST-F1 keeps time to within 1 second in about 100 million years as of 2013, and NIST-F2, introduced in early 2014, is about three times better at plus or minus 1 second in about 300 million years.
Meanwhile, NIST scientists have been developing next-generation atomic clocks based on a variety of different atoms, including mercury, aluminum, ytterbium, strontium and calcium. These experimental clocks have been improving rapidly, and each offers different advantages. These clocks may enable new technologies, and one or more of them might become future time standards.
Improved atomic clocks obviously will benefit widely used technologies that have long relied on precision timekeeping, such as communications and GPS positioning. But the new atomic clocks are becoming so extraordinarily precise that they are likely also to be used as extremely sensitive detectors for many things besides time.
For example, the frequency (“ticking rate”) of atomic clocks is altered slightly by gravity, magnetic fields, electrical fields, force, motion, temperature and other phenomena. In today’s conventional atomic clocks, those frequency changes are errors to be tightly controlled. In next-generation atomic clocks, the frequency changes are measured to such a fine degree that the clocks could become world-class instruments for measuring gravity, magnetic and electrical fields, force, motion, temperature and many other quantities.
Today’s precision timekeeping technologies rely on several different types of atomic clocks, but in the future, an even wider range of clocks might be used, each optimized for different applications. NIST invests in a number of atomic clock technologies because the results of scientific research are unpredictable, and because different clocks are suited for different applications.
The unpredictability of research outcomes can be seen in recent history. Several years ago, an experimental NIST clock based on a single ion (electrically charged atom) of mercury was the world record holder for precision (see definitions below). Then a NIST clock based on a single aluminum ion and quantum computing technologies surged ahead. Now NIST clocks based on cold neutral atoms in optical lattices have taken the lead after lagging substantially behind for many years. All these clocks are likely to continue improving, and leadership in performance may continue to shift back and forth in the future. Meanwhile, the research has already produced spinoff innovations, such as the world’s most stable lasers, and more such discoveries can be expected. Additional new types of atomic clocks not yet envisioned might also be developed.
The suitability of various clocks for different applications can also be seen in recent history. For example, the aluminum ion logic clock’s world-record timekeeping performance was due in part to this clock’s insensitivity to changes in magnetic fields, electrical fields, and temperature. This insensitivity is highly desirable for the best timekeeping results. But it also means the aluminum ion clock is not a good candidate for measuring magnetic and electrical fields or temperature, whereas other NIST atomic clocks have greater sensitivity to those quantities.
*JILA is the joint research and training institute of NIST and the University of Colorado Boulder.