
NIST-F1 Cesium Fountain Atomic Clock - CaliforniaKarl
https://www.nist.gov/pml/time-and-frequency-division/primary-standard-nist-f1
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mola
I don't quite understand. Which element is measuring the second. The microwave
frequency or the atom ball time of travel?

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nullc
F1 is a source of precise frequency and only an intermittent one. It puts a
puff of cesium atoms into a state which is as controlled as is possible and
then finds the microwave frequency that causes the largest number of atoms to
change state. The second is currently defined as a multiple of the period of
the microwave signal that, under specified conditions, induces that particular
transition in cesium.

The up and down motion itself doesn't result in the time measurement.

The purpose of the 'thrown ball' in F1 is so that the atoms being measured are
in freefall during their measurement which eliminates uncertanties in the
measurement which would otherwise come from forces confining them and to keep
them in that unperturbed state for as long as possible. By using a very long
interaction (technically two interactions separated by a long time, a form of
interferometery) the peak response can be made very narrow which allows for a
more precise measurement.

F1's frequency is used to tune continuously running highly stable oscillators
(or, really, measure their offsets so that they can be corrected in post-
processing without being disturbed by frequent tuning).

The combination of a continuous stable oscillator and a counter gives you
something you'd recognize as a clock. So you can think of F1 as a calibration
process used to keep NIST's clocks on time.

This paper has a good technical overview of many of the considerations that go
into such a device:
[https://tf.nist.gov/general/pdf/1404.pdf](https://tf.nist.gov/general/pdf/1404.pdf)

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mwheeldown
Thanks. I also needed this explanation. The article annoyingly just ends with
"And of course, the improved frequency control leads to what is one of the
world's most accurate clocks."

