NIST’s fountain clocks measure the response of cesium atoms as they rise and fall. That response is used to tune a frequency and realize an accurate standard second.
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Atoms rise and fall inside a clock: a fountain made of cesium rather than water. The apparatus has no obvious hands or pendulum, yet cesium fountain clocks at the US National Institute of Standards and Technology help realize the second precisely.
First, the atoms must move very slowly. Lasers from several directions cool an atomic cloud before it is launched upward. Gravity brings the atoms back down, and they interact with microwaves on both parts of the journey. The fountain path creates time between those interactions.
The instrument then reads the atoms’ energy states. The result shows how closely the microwave frequency matches the cesium reference and allows adjustment. It does not simply label the upward-and-downward travel time as one second. The journey enables a more sensitive examination of the atoms’ response.
An ordinary clock displays time; this apparatus helps establish the reference against which other clocks are checked. It need not show changing digits like a household clock. It contributes to a system that maintains accurate time through comparisons with stable signals.
The photograph shows pipes and wiring instead of a column of water. Inside, cesium atoms encounter microwaves twice as they rise and fall. Adjusting the frequency from the measured response helps establish a precise reference against which other clocks can be compared.
Sources and editorial notes
NIST · Cesium fountain atomic clocks
Image use terms · US government work
Editorial note: Facts were checked against the linked material. Interpretations are MUADAILY’s editorial perspective. Image: R. Eskalis/NIST · US government work · NIST-F4 cesium fountain clock.
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