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Mercury Atomic Clock Keeps Time with Record Accuracy

NIST physicist Jim Bergquist with the mercury ion clock

NIST physicist Jim Bergquist holds a portable keyboard used to set up the world's most accurate clock. The silver cylinder in the foreground is a magnetic shield that surrounds a cryogenic vacuum system, which in turn holds the heart of the clock, a single mercury ion (electrically charged atom). The ion is brought to rest by laser-cooling it to near absolute zero. The optical oscillations of the essentially motionless ion are used to produce the ticks or heartbeat of the world's most stable and accurate clock.

Credit: @ Geoffrey Wheeler

BOULDER, COLO.—An experimental atomic clock based on a single mercury atom is now at least five times more precise than the national standard clock based on a "fountain" of cesium atoms, according to a paper by physicists at the Commerce Department's National Institute of Standards and Technology (NIST) in the July 14 issue of Physical Review Letters. 

The experimental clock, which measures the oscillations of a mercury ion (an electrically charged atom) held in an ultra-cold electromagnetic trap, produces "ticks" at optical frequencies. Optical frequencies are much higher than the microwave frequencies measured in cesium atoms in NIST-F1, the national standard and one of the world's most accurate clocks. Higher frequencies allow time to be divided into smaller units, which increases precision.

A prototype mercury optical clock was originally demonstrated at NIST in 2000. Over the last five years its absolute frequency has been measured repeatedly with respect to NIST-F1. The improved version of the mercury clock is the most accurate to date of any atomic clock, including a variety of experimental optical clocks using different atoms and designs.

The current version of NIST-F1—if it were operated continuously—would neither gain nor lose a second in about 70 million years. The latest version of the mercury clock would neither gain nor lose a second in about 400 million years.

"We finally have addressed the issue of systemic perturbations in the mercury clock. They can be controlled, and we know their uncertainties," says NIST physicist Jim Bergquist, the principal investigator. "By measuring its frequency with respect to the primary standard, NIST-F1, we have been able to realize the most accurate absolute measurement of an optical frequency to date. And in the latest measurement, we have also established that the accuracy of the mercury-ion system is at a level superior to that of the best cesium clocks."

Improved time and frequency standards have many applications. For instance, ultra-precise clocks can be used to improve synchronization in navigation and positioning systems, telecommunications networks, and wireless and deep-space communications. Better frequency standards can be used to improve probes of magnetic and gravitational fields for security and medical applications, and to measure whether "fundamental constants" used in scientific research might be varying over time—a question that has enormous implications for understanding the origins and ultimate fate of the universe.

Scientists have long recognized that optical atomic clocks could be more stable and accurate than cesium microwave clocks, which have kept world time for more than 50 years. Even with the latest results at NIST, however, optical clocks based on mercury, strontium or other atoms remain a long way from being accepted as standards. Research groups around the world would first need to agree on an atom and clock design to be used internationally.

In addition, a system of additional optical clocks would be needed to continuously keep time, because primary standard clocks—such as the mercury ion or other future optical standard—are generally operated only periodically for calibrations. NIST-F1, for instance, is operated several times a year for periods of about one month to calibrate the frequencies of several NIST microwave atomic clocks that continuously track current time. These clocks contribute to an international group of atomic clocks that define the official world time.

Funding for the research was provided by NIST and the Office of Naval Research.

As a non-regulatory agency of the Commerce Department's Technology Administration, NIST promotes U.S. innovation and industrial competitiveness by advancing measurement science, standards and technology in ways that enhance economic security and improve our quality of life.

W.H. Oskay, S.A. Diddams, E.A. Donley, T.M. Fortier, T.P. Heavner, L. Hollberg, W.M. Itano, S.R. Jefferts, M.J. Jensen, K. Kim, F. Levi, T.E. Parker and J.C. Bergquist. 2006. A single-atom optical clock with high accuracy. Physical Review Letters. July 14.

Background: How the Mercury Clock Works

NIST-F1 uses a tiny, cigar-shaped cloud of cesium atoms that is tossed in the air to determine the length of a second. Atoms in different parts of the cloud are subject to slight variations in electric and magnetic fields that affect clock accuracy. By contrast, single ions such as mercury can be isolated at a point in space for better control. The ion is created inside an electromagnetic trap, which holds the ion in one place with electric fields oscillating at radio frequencies. Once trapped, the ion is cooled with lasers until it is nearly motionless. The ticks are generated by the oscillations of the ultraviolet light used to induce the ion's transition between two energy levels. These oscillations are about 100,000 times faster than the corresponding oscillations in NIST-F1, which also improves the precision of the mercury ion clock.

An optical atomic clock has three components: an atom that switches from one energy level to another when probed by a laser at a well-defined optical frequency; the laser used to induce this transition; and a counter that faithfully records each oscillation per unit time (the ticks) of the probe laser.

The latest version of the NIST mercury clock incorporates several improvements over the original design. Most importantly, it compensates for the irregular shape of mercury's electron cloud, which causes a frequency shift that was not previously measured or corrected. NIST developed a method for nullifying the shift by applying a magnetic field on three different axes of the trap, measuring the frequencies at each axis, and calculating the average. The researchers also reduced the frequency shift caused by magnetic fields by reducing the strength of the field used to hold the ion in the trap.

The NIST mercury clock is unusual in part because of the method for stabilizing the ultraviolet laser used to probe the clock transition. The laser light is locked to an ultra-stable reference cavity that sits on an isolation platform suspended from the ceiling by latex tubing, essentially a set of huge rubber bands. The suspended platform offers passive isolation against vibration to very low frequency, according to Bergquist, the system designer. Better vibration isolation improves laser frequency stability and that leads to higher resolution when probing the laser's clock transition.

"You only want the atom to respond at precisely one frequency," Bergquist says. "An acoustical analogy is the resonant response of a tuning fork to an exquisitely pure musical note, either sung or played."

NIST also built the optical "frequency comb" used to convert the optical ticks to microwave frequencies so they can be counted. (No electronics exist that are fast enough to directly count optical frequencies.) A frequency comb is a very precise tool for measuring different frequencies, or colors, of light (see Optical Frequency Combs).The comb was used to compare the mercury and cesium clocks, serving as the gears, or clockworks, for converting the stable and accurate optical frequency of the mercury clock transition to a stable microwave frequency. The accuracy of the optical frequency measurement is made possible largely by the high accuracy of the NIST-F1 standard.  


Released July 14, 2006, Updated February 2, 2023