Second: The Future

Next-generation atomic clocks promise to chop up the second into much smaller parts, allowing unprecedented accuracy and precision that will usher in a host of additional applications.

NIST-F1 and NIST-F2 are microwave clocks, based on the microwaves emitted by cesium atoms that have been excited with microwave energy. The cesium atoms shed their energy by giving off their own microwaves. These microwaves hit a detector, which reads their frequency as “ticks” marking fractions of a second. The F2 is accurate to within 1/300,000,000th of a second per year.

In recent times, NIST scientists have been experimenting with “optical clocks,” based on atoms such as strontium and ytterbium and ions such as mercury and aluminum that can split up the second into chunks up to a million times smaller.

Optical atomic clocks are excited at frequencies of light that are hundreds of thousands of times higher than the microwave frequencies emitted by cesium atoms. Like a ruler with finer tick marks, optical clocks divide time into smaller units and could ultimately be 1,000 times more accurate and stable than the cesium clocks we have today.

One key advance that made optical atomic clocks possible was the frequency comb, the development of which earned NIST Fellow Jan Hall a share of the 2005 Nobel Prize in Physics. Frequency combs produce very finely separated colors of light, which can be used not only to precisely energize the atoms in an optical clock but also to translate the ultra-high (trillions of cycles per second, or terahertz) optical frequencies to lower frequencies. Those can be meshed with microwave standards and counted to measure the duration of a second.

NIST's first all-optical atomic clock, built in 2006, was based on a single mercury ion. Its performance was then surpassed in 2010 by NIST's quantum logic clock, based on a single aluminum ion. 

These clocks were so stable that they would not gain or lose a second in a billion years. Even as astonishing as the precision of these clocks is, they have been surpassed by clocks powered by ytterbium and strontium. Experimental strontium optical clocks would not have gained or lost a second if they had begun running at the moment of the Big Bang, estimated at 13.8 billion years ago.

These new clocks are approaching the accuracy and precision necessary to go beyond merely measuring time. They may also have powerful applications in geology. According to Einstein’s theory of general relativity, time moves more slowly for objects moving at high speed. Under this scheme, gravity is equivalent to accelerated motion, so clocks closer to the Earth’s surface should tick more slowly than clocks further away from it. This has been noted at high altitudes, but clocks had not been sensitive enough to see, for example, how time ticks more slowly on the ground than it does on the second floor of a building.

Optical clocks, however, are so precise that they can show a difference between two clocks differing in elevation by as little as one centimeter. This is not only interesting from the standpoint of showing that Einstein was right yet again, but these clocks open new possibilities in earth science. Future devices could detect subtle changes in the elevation of the surface of the Earth, which could warn us of volcanic eruptions and possibly earthquakes.

In general, the ticking rate of these next-generation atomic clocks can still be altered by outside influences such as magnetic and electric fields, force, motion, temperature and gravity. But this can be a benefit instead of a bug. Their sensitivity to outside disturbances makes optical clocks prime candidates for a host of sensors.

Time will tell if these advances come to pass, but it seems certain that our measurement of the second will only get more precise in the future. As it continues to improve, we have developed new measurement capabilities that go far beyond getting to our appointments on time.