4 ms·

I don't quite understand. Which element is measuring the second. The microwave frequency or the atom ball time of travel?

by mola 7y ago

nullc 7y ago
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
- mwheeldown 7y ago
  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."

I don't quite understand. Which element is measuring the second. The microwave frequency or the atom ball time of travel?

by mola 7y ago

I don't quite understand. Which element is measuring the second. The microwave frequency or the atom ball time of travel?

nullc 7y ago

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

mwheeldown 7y ago
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."