Hitting the small time

With a volume of 16cm³, the Quantum Chip-Scale Atomic Clock (CSAC) from Symmetricom Inc., San Jose, Calif., is not only a third the size of its predecessors, it runs on just 1 percent of the power.

Until the CSAC, the smallest atomic clock was about the size of a 1990s-era cell phone, weighed a couple of pounds and required 10w of power to run. Portability wasn’t one of its strong points—unless you consider strapping a car battery to your back a sacrifice worth bearing in the name of mobility.

Though the Quantum CSAC requires only 115mW to run in its steady state—handled easily by two AA batteries—it isn’t simply a miniaturized version of larger atomic clocks, but a “reinvention,” according to a white paper released by Symmetricom. For example, rather than using a cesium tube as a resonance cell and a rubidium-based atomic vapor lamp for a light source, the Quantum CSAC employs a MEMS-based, hollowed-out silicon cube filled with cesium gas and a vertical-cavity, surface-emitting laser (VCSEL).

The CSAC tells time by using a microwave generator to oscillate the VCSEL’s single-laser-beam frequency to produce two waves intended to interfere with the two energy signatures of the cesium atom, according to a report by Sandia National Laboratories, Albuquerque, N.M. The waves produce a series of “beats,” or cycles, that are monitored by a photodiode, which detects increases in the light transmission through the cesium vapor cell. One second equals 4,596,315,885 cycles of the microwave oscillator signal.

By the way, the Quantum CSAC keeps track of time to within less than half of a microsecond per day. That’s well within the realm of atomic clock accuracy.

“This clock is a completely new architecture,” reported Dr. Darwin Serkland, the principal member of the technical staff at Sandia National Laboratories, who helped develop critical technology for CSAC. That change in architecture enabled “the huge drop in power consumption.”

The breakthrough that led to the first commercially available CSAC, however, owes a great deal to research conducted at the National Institute of Standards and Technology in the late 1990s, noted Serkland. NIST was the first to use a VCSEL instead of a lamp and “that basically gave some glimmer of hope in the laboratory setting that this type of clock would be possible to build.”

Soon afterward, the Defense Advanced Research Projects Agency (DARPA) sought to build on NIST’s work by initiating a CSAC development project that lasted from 2002 to 2008. During that time, multiple agencies, laboratories and private companies pursued a CSAC that would consume only 30mW of power in an overall package of just 1cm3. And, of course, it had to have the stability of, well, an atomic clock.

Of the three main challenges, the most difficult by Serkland’s measure was reducing power consumption from 10w to 30kW. That goal made switching from an optical lamp to a VCSEL a “relatively obvious choice,” and that helped lead the project to Serkland’s doorstep at Sandia in 2002, thanks to his extensive experience with VCSEL technology. (Well, either that or because he’s become known as the “VCSEL Wizard,” with credit for that moniker going to Sandia’s media relations staff.)

Toward the conclusion of the project in 2008, Serkland recalled, “there were three teams that came closest to meeting the goals of the program.” In addition to the Symmetricom team, which included Sandia, there were teams led by Honeywell International Inc. and by Teledyne Scientific & Imaging LLC.

Of the three, Symmetricom’s Quantum CSAC, which hit the market in January, remains the only commercially available CSAC.

The fact that any of the teams made it to market is a “very big deal,” said Gil Herrera, director of Sandia’s Microsystems and Engineering Sciences Application center, “because few DARPA technologies make it to full industrial commercialization for dual-use (military and civilian) applications. CSAC now is a product with a data sheet and a price.”

The Quantum CSAC, pictured along with a dime, is about the size of a small book of matches. Image courtesy Symmetricom.

By Serkland’s estimate, maybe 1 in 1,000 R&D projects involving fundamental science make it to the commercial market. For engineers working on a project involving a more mature science, the odds might come down to 1 in 100.

And yet, the advancement in fundamental science that the Quantum CSAC embodies may be more noteworthy. For your consideration:

• The VCSEL that the Symmetricom team used in the CSAC eliminated the need for about half of the 10w of power consumed by legacy atomic clocks.
• The MEMS-based silicon cesium cell reduced heating power needs from a few watts to less than 10mW.
• Finally, Symmetricom capitalized on the commercial success of cell phones that has driven integrated circuits to new lows in power consumption and new heights of stability, thanks to the phase-locked loop circuits required for frequency translation.

Taken together, these elements represent a design so far removed from its predecessors, some—well, me—might say the advancement is akin to the discovery of transistors. Serkland won’t say that, mind you. He likens the achievement to the elimination of vacuum tubes in televisions and radios.

“Before transistors,” he observed, “people used vacuum tubes for the same purpose.”

To be sure, even if researchers managed to shrink atomic clocks incorporating legacy technology, they would still have to overcome the need for 10w of power and the generation of significant ambient temperatures, according to Symmetricom’s white paper.

The Quantum CSAC, meanwhile, is said to be ideal for portable applications, such as for backpack-type military radios, enhanced military GPS receivers and tactical unmanned aerial vehicles. Other portable applications identified by Symmetricom include undersea seismic sensing for oil exploration and a signal-jamming device to prevent IEDs (improvised explosive devices) from detonating.

Anything that lightens the step of Allied troops on the ground in Afghanistan searching for IEDs is nothing short of a godsend in my book. µ