It’s ten seconds to midnight on New Year’s Eve, but by whose watch? Like all standards, measurements of time are arbitrary, and only as good as the precision of each ‘tick.’ As no clock is perfect, each will eventually speed up or slow down, making that stroke of midnight a bit fuzzy. However, advances in physics and engineering over the past fifty years have decreased that uncertainty with the development of the gold standard of timekeeping: atomic clocks.
Tuning into Atomic Frequencies
While atomic clocks are technologically more complicated than the average timepiece, their operating principle is more or less the same – time is kept by precisely measuring the frequency of a signal. Frequency expresses how often a periodic signal repeats itself. In a grandfather clock, the frequency refers to the number of times per second a pendulum swings back and forth as it mechanically drives the clock’s hands; for a modern quartz watch, the frequency of interest is the number of electrically driven vibrations per second in a crystal, a signal that is converted into electronic pulses that are displayed digitally.
In contrast with conventional clocks, atomic clocks tune their ticks to the oscillations of atoms as they absorb particular frequencies of light. A fundamental principle of quantum mechanics dictates that atoms can only absorb and emit light at a discrete set of frequencies. As with much of the quantum world, this should seem strange – imagine if you could only hear a handful of acoustic frequencies – could only hear changing sounds in “steps”, not in a smooth range! However, the fact that atoms only respond to discrete frequencies of light allows a single frequency to be selected and used as a reference for keeping time.
Caesium Clocks and the Advent of International Time
Although the universe provides ready-made atoms with their own characteristic frequencies, building an atomic clock requires a way to produce electromagnetic radiation at a single frequency, or ‘coherent radiation.’ The most commonly used standard is the oscillation of a soft, metallic element called caesium (or cesium) as it is exposed to microwaves – the same kind of radiation we use to heat our food.
The development of early caesium clocks was made possible by the invention of coherent microwave sources called masers (the predecessors of lasers) in the 1950s . When the microwave source is tuned to the right frequency, oscillations of the caesium atoms can be measured by a magnetic field or laser. This measurement provides feedback and allows the microwave source to be locked into that precise frequency, making the perfect signal for a time reference.
Caesium clocks are so accurate that only a decade after their development, they had become the international time standard. In 1967, the second was re-defined as 9,192,631,770 repeats of the caesium microwave frequency, a number chosen to match the previous definition based on the solar year . Technological improvements, including the laser cooling techniques recognized by the 1997 Nobel Prize in Physics, further improved the precision of caesium clocks. Today, scientists around the world use a collection of about 300 atomic clocks to maintain Coordinated Universal Time (UTC), the scientific version of Greenwich Mean Time . Many of these clocks are precise to one part in a million billions – akin to knowing the distance between the Sun and the Earth down to a hair’s width!
Modern Uses of Atomic Clocks and the Future of Timekeeping
The high precision time keeping made possible by atomic clocks has quietly revolutionized global commerce and everyday life. Electrical grids and telecommunication systems both rely on them for time synchronization. The networks that deliver this web content to your Internet-enabled device, for example, require synchronization better than a millionth of a second . Global Positioning System (GPS) satellites, whether used directly by GPS navigation systems or indirectly via online maps, require even more precise time synchronization to triangulate a position.
Atomic clocks have also been used in research labs to test the fundamental laws of nature. Earlier in 2010, scientists at the National Institute of Standards and Technology (NIST) used a pair of atomic clocks to verify a prediction of Einstein’s theory of relativity: clocks will tick more slowly in a stronger gravitational field. Although this is hardly the first such test, the scientists at NIST vindicated Einstein by simply elevating one clock above the other by about a foot. Since the elevated clock was a bit further from the Earth’s center, it experienced the tiniest change in gravity, and ticked faster . Although the change was extraordinarily small, it was still detectable by the high-precision atomic clocks.
Researchers are continuing to improve the accuracy of atomic clocks. For example, advances in laser technologies are predicted to lead to the replacement of microwave-based systems, such as the caesium clock, with light-based systems. These more accurate “optical atomic” clocks use atomic oscillations at the frequencies of visible light . In addition, a team of researchers in Germany recently used a phenomenon called “entanglement” to dampen interactions among a group of super-cooled atoms of an element similar to caesium, rubidium, trapped on a chip. Using this phenomenon, the researchers were able to surpass a previous limit on the measurement of atomic oscillation, improving the precision of the clock. Although this research will not lead to the development of an ‘atomic wristwatch’ anytime soon, the researchers predict that the chip-based approach will both increase the accuracy and portability of atomic clocks .
Advancements in atomic clocks may not affect your New Year’s countdown anytime soon, but they will continue to re-define how we view and measure time. Let’s raise a glass and drink to precision!
James Kath is a graduate student at Harvard Medical School
 Diddams et al. Standards of Time and Frequency at the Outset of the 21st century. Science (2004) vol. 306, pp. 1318-1324
 A Walk through Time. National Institute of Standards and Technology (5 October 2010). http://physics.nist.gov/time
 World’s Most Precise Clocks Test Relativity. Talk of the Nation: Science Friday (24 September 2010). http://www.npr.org/templates/story/story.php?storyId=130104039
 Merali, Zeeya. Atomic clocks use quantum timekeeping. Nature News (31 March 2010). http://www.nature.com/news/2010/100331/full/news.2010.163.html