# caesium standard

> primary frequency standard in which the photon absorption by transitions between the two hyperfine ground states of caesium-133 atoms is used to control the output frequency

**Wikidata**: [Q5017052](https://www.wikidata.org/wiki/Q5017052)  
**Wikipedia**: [English](https://en.wikipedia.org/wiki/Caesium_standard)  
**Source**: https://4ort.xyz/entity/caesium-standard


## References

1. [Source](https://www.bipm.org/en/history-si/second)
2. [Source](https://www.ptb.de/cms/ptb/fachabteilungen/abt4/fb-44/ag-441/realisierung-der-si-sekunde/funktionsweise-und-typische-technische-realisierungen-von-atomuhren.html)