Atomic clocks are widely known to be the most accurate timepieces available, but not all atomic clocks are created equal. Physicists at the National Institute of Standards and Technology (NIST) are hard at work developing a new type of atomic clock utilizing cold atoms that could be smaller and more accurate than current versions.
The clock described in the new NIST research paper is about the size of a coffee mug right now — about 150 cubic centimeters. That sounds small without comparison, but it’s still 10 times larger than current chip-scale atomic clocks that rely on hot atoms. Both clocks are considerably smaller than laboratory-based cesium-beam clocks, but neither is as accurate. The NIST team hopes to combine the size of the chip-scale clocks with the precision of cesium-beam clocks with the new project.
The small hot-atom clocks work for basic timekeeping applications like GPS, but they tend to drift over the course of hours, eventually needing calibration from an outside source. This happens largely because the atoms used to keep time exist in a high-pressure gas cloud. As they move in space, their temperature varies and that changes the resonant frequency — the tick of an atomic clock. That frequency is generated by electrons as they change energy levels, and is measured as a microwave signal. A cold atom clock should be able to avoid this problem entirely.
NIST’s cold-atom clock consists of a small glass vacuum chamber containing about one million rubidium atoms. A magnetic field confines the atoms so they can be efficiently cooled with a pair of lasers to temperatures not far above absolute zero. A pair of near-infrared lasers are used to excite the rubidium atoms from above and below simultaneously, which avoids any troublesome Doppler shift effects. These lasers each produce two different frequencies which zero in on the resonant frequency of the atoms. Once that is found, the microwave output of the atoms becomes the ticking of the clock.
A second version of the cold clock is already in development partially using funds provided by DARPA. Researchers are reducing the size of the vacuum chamber and adding magnetic shielding to prevent interference from external power sources.
The team believes that with the right refinements, this system can be miniaturized to the size of current chip-scale hot atomic clocks while still being 1,000 times more precise. It might not need calibration at all — ever. That would put it in the realm of cesium-beam clocks, which could make scientific instrumentation considerably cheaper.
Research paper: doi: 10.1103 “Cold-atom double-Λ coherent population trapping clock”