A cesium clock, accurate to .5 μSec/day, needed a whole lab. Now it fits on a small PCB.

The standard by which clocks in the United States are set is a cesium-based atomic clock in Boulder, Colorado, that loses less than a nanosecond per day. It’s the size of a small car and draws roughly a kilowatt of power.

Symmetricom, of San Jose, California, introduced the first commercial chip-scale atomic clock, the SA.45s. It measures 4 by 3.5 by 1.1 centimeters, weighs 35 grams, and draws a paltry 115 milliwatts. The tiny clock is accurate to within about less than half a microsecond per day.

A resonance cell containing cesium 133 and a buffer gas is heated until a moderately dense vapor of cesium is distributed throughout the cell. The vapor is illuminated with light from a semiconductor laser, which is modulated at a frequency near the 9.192-gigahertz natural oscillation frequency of the cesium atoms. Once the beam drives the atoms into an oscillating state, they absorb less of the light, and the photons transmitted through the cell can be used to determine whether the laser beam’s modulation frequency coincides with the resonant frequency of the atoms. A servomotor can then lock the modulation frequency of the laser to the atomic resonance, holding the clock's output stable.

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