You could have bought a Rolex, but now you learn about a new, matchbook-sized atomic clock. It’s portable, only about 1.5 inches on a side and less than a half-inch in depth and heck, it costs less, only about $1,500.
Created in a joint effort by the Massachusetts division of Symmetricom Inc., MIT’s Draper Lab, and researchers at Sandia’s MESA center, the new “Chip Scale Atomic Clock” (CSAC) is 100 times smaller than its commercial predecessors and requires a hundred times less power: instead of 10 watts, it uses only 100 millliwatts.
“It’s the difference between lugging around a device powered by a car battery and one powered by two AA batteries,” says Sandia lead investigator
Darwin Serkland (1742).
Despite common implications of the word “atomic,” the watch does not use radioactivity as an energy source. Instead, where an ordinary watch uses a spring-powered series of gears to tick off seconds, a CSAC counts the frequency of electromagnetic waves emitted by cesium atoms struck by a tiny laser beam to determine the passage of time. (There’s a more complete description of this process below.)
Still, given that the CSAC does not actually display the time of day — measured in millionths of a second, its passage would defy our ability to read it — why would anyone want this atomic clock?
Its uses are, indeed, specialized. Miners far underground or divers engaged in deep-sea explorations, blocked by natural barriers from GPS signals, could still plan precise operations with remotely placed comrades who also had atomic clocks, because their timing would deviate from each other less than one millionth of a second in a day.
Functions during GPS outages
If you were in the land of improvised explosive devices — IEDs — that could be detonated by a telephone signal, and your military deliberately set up electromagnetic interference to block those signals, even though GPS signals would also be blocked, your CSAC watch would still function.
If you were in charge of relay stations for cross-country phone and computer lines, which routinely break up messages into packets of information sent by a variety of routes but which must be reconstituted correctly at the end of their voyages, you might sleep better knowing that atomic clocks continue functioning during GPS outages.
The clock’s many uses, both military and commercial, are why the work was funded by the Defense Advanced Research Projects Agency (DARPA) from 2001 until its market arrival in January 2011.
“Because few DARPA technologies make it to full industrial commercialization for dual-use applications, this is a very big deal,” says Gil Herrera (1700), director of Sandia’s MESA center. “CSAC now has a data sheet and a price.”
Cesium atoms are housed in a thimble-sized container developed by Draper Lab. The cesium atoms are interrogated by a light beam from a laser called a VCSEL (vertical-cavity surface-emitting laser), contributed by Sandia. And Symmetricom, a leading atomic clock manufacturer, designed the electronic circuits and assembled the components into a complete functioning clock.
“The work between the three organizations was never ‘thrown over the wall,’” says Sandia manager Charles Sullivan (1742), using an expression that has come to mean complete separation of effort. “There was tight integration.”
A completely new architecture
Nevertheless, reduced power consumption was key to creating the smaller unit, says Darwin. That required, in addition to a completely new architecture, a VCSEL rather than the previous tool of choice, an atomic vapor lamp.
“It took a few watts to excite the rubidium lamp into a plasma-like state,” Darwin says. “Use of the VCSEL reduced that power consumption over a thousand times to 2 milliwatts.” (For obvious reasons, Darwin’s success in attaining this huge power reduction caused some in the clock business to refer to him as “the VCSEL wizard.”)
The way the clock keeps time may best be imagined by considering two tuning forks. If the forks vary only slightly in size, a series of regular beats are produced at the difference frequency when both forks vibrate. The same principle works in the new clock.
The VCSEL — in addition to being efficient, inexpensive, stable, and low-power — is able to produce a very fine, single-frequency beam. The beam, at 335 terahertz (894.6 nanometers), is midway between two hyperfine emission levels of the cesium atom, separated in terms of energy like the two differently sized tuning forks. One level is 4.6 gigahertz above and the other 4.6 gigahertz below the laser frequency. (Hyperfine lines are the energy signatures of atoms.) A tiny microwave generator sends an oscillating frequency that adds and then subtracts energy from the incoming laser carrier frequency. Thus, the laser’s single beam produces two waves at both hyperfine emission energies. The emitted waves, interacting, produce (like two tuning forks of different sizes) a series of ‘beats’ through a process known as interference.
One of three DARPA Phase IV projects
A photodiode monitors the slight increase in light transmission through the cesium vapor cell when the microwave oscillator is tuned to resonance. According to the international definition of the second (since 1967) the clock indicates that one second has elapsed after counting exactly 4,596,315,885 cycles (about 4.6 gigacycles) of the microwave oscillator signal.
Because magnetism has an influence on cesium atoms, atomic clocks are shielded from Earth’s magnetic field by a thin steel sheath.
While this sounds cumbersome, atomic clocks “beat” the difficulties that existed a century ago, when a pendulum clock in Paris was the source of the world’s exact time. Kept in a room that was temperature- and humidity-controlled, not only would a change of one degree affect the pendulum’s swing but the difficulty of bringing accurate time to the US was extreme: one synchronized a portable clock in Paris and then had to transport it across the ocean by ship, during which time the mechanical clock would inevitably drift from the frequency of the Paris pendulum.
The CSAC project is one of three DARPA Phase IV projects in the history of Sandia, says Gil. “The other two are the Micro Gas Analyzer (led by Sandia in Phase IV) and the Navigation Grade Integrated Microgyro led by Northrop Grumman with no present Sandia Phase IV participation.”
A follow-on technology MESA is working for DARPA is a trapped-ion-based clock. It will improve timing accuracy at similar size/weight/power to the CSAC. It was just approved for Phase II development, with the goal to produce the first compact prototype unit.
At this rapid rate of development, the sales outlook may darken for high-status watches that don’t evolve over time. — Neal Singer