Will Take a Little Time
New chip-scale atomic clocks
Historians continue to wrestle over an exact date for the start of the Industrial Revolution. Generally placed in the late 1700s, it is often correlated with the widespread use of the steam engine and coal as an energy source. As a technophile, I like to attribute it the invention of tools having an intrinsic awareness of time. Hammers, chisels and
 click to enlarge Stylized Chip-Scale atomic clock. Frequency-modulated laser sends near-infrared light through the atomic gas cell and onto the photodiode detector. All components are created using micro-electro-mechanical systems (MEMS) fabrication technology. Image courtesy of DARPA CSAC Program. |
saws have been in use for centuries, and complicated musical instruments arrived with the Renaissance, but each requires a human being for their operation. The intricate gears, valves and cogs of a steam engine and the pulleys and cables of a weaving machine grant them autonomy and, more importantly, the ability to work faster than their human counterparts. An increase in speed requires a precise knowledge of the system time so that all components work in concert. A faulty timing belt in a mechanical system often leads to catastrophic failure, as parts are in the wrong position at the wrong time. Modern electronics commonly rely on the vibration of a quartz crystal in an oscillation circuit as a “timing belt.” While not resulting in physical damage, an inaccurate or defective quartz oscillator can cause malfunction of the electronic device and the systems it controls.
The most precise oscillation circuit is based on the frequency of microwave radiation absorbed by a specific ground state of cesium (Cs). Since 1967, the International System of Units (SI) has based the value of one second on the Cs absorption of 9,192,631,770 Hz (ca. 9.2 GHz) radiation. The U.S. National Institute of Standards and Technology (NIST) in Boulder, CO, operates the NIST-F1 Cs atomic clock that serves as the primary time and frequency standard for the U.S., and contributes to the international group of atomic clocks that define coordinated universal time (UTC), the timing belt for the planet earth. While most electronic devices hum along happily using their quartz oscillators, the increasing globalization of network communication and global positioning systems (GPS) for military and civilian applications can benefit from the improved precision of an atomic clock, which is over five million times more precise than quartz.
The increased precision comes at the expense of size. NIST-F1 occupies 3.7 m3 and consumes 500 W of power. Smaller, less accurate atomic clocks are used by satellites, network providers and broadcasters, but the U.S. Defense Advanced Research Projects Agency (DARPA) would like to incorporate chip-scale atomic clocks (CSACs) into portable spread-spectrum communication, radar and GPS devices. Over the past few years, DARPA, NIST, universities and companies including Honeywell and Rockwell Collins, have been working on the CSAC project to develop a 1-cm3 atomic clock that consumes less than 100 mW.A major technical obstacle is the physics of 9.2 GHz radiation. It has a wavelength of 3.2 cm — much too large to fit into a 1-cm3 box filled with Cs gas. NIST scientists lead by Dr. John Kitching have solved this problem by using a recently discovered spectroscopic technique known as coherent population trapping (CPT). While Cs atoms can hop between low-energy levels in their ground state using microwave energy, they require near-infrared energy at 852 nm to reach an excited state — a wavelength small enough to fit inside a cubic-centimeter cavity. Although the mathematics are a bit involved, when 852-nm light is modulated at 4.6 GHz (exactly one-half of 9.2 GHz), upper and lower sidebands spaced at the correct 9.2 GHz are placed on the 852-nm carrier. When the modulation of the CPT circuit matches the exact spacing of the Cs ground state, the sidebands are absorbed and a greater amount of the 852-nm carrier intensity reaches the photodiode detector. The CPT technique achieves the same microwave absorption used by NIST-F1 using much shorter near-infrared light from a diode laser. Recent versions of the CSAC utilize more stable rubidium (Rb) atoms and have demonstrated a 1000x increase in precision over the use of quartz in a total package of 9.5 mm3 requiring 75 mW.
CSACs also may help to reduce the bandwidth congestion of consumer cell phones and WiFi devices. The frequencies allocated for communication using a specific technology are divided into separate channels that can be used simultaneously. The frequency width and number of channels is limited by the ability of each device to accurately transmit and receive within its assigned bandwidth. As timing precision increases, channels can be narrowed to make room for additional ones. When the DARPA CSAC program concludes this fall, the race will be on to commercialize the technology. I hope I’m not too late to pre-order my wristwatch version.
Bill Weaver is an assistant professor in the Integrated Science, Business and Technology Program at La Salle University. He may be contacted at [email protected]
