A detailed explainer of how rubidium frequency standards work internally. Rubidium-87 atoms are optically pumped into hyperfine ground states using filtered light from a rubidium-87 discharge lamp. Microwave energy at ~6.835 GHz drives transitions between those states, producing a detectable dip in light absorption. Electronics servo a local oscillator to that transition frequency, locking it to an atomic reference rather than a mechanical crystal. The same basic principle applies to cesium standards, with cesium and optical clocks offering even greater accuracy.
Questions this post answers
What microwave frequency does a rubidium-87 frequency standard lock onto?
A rubidium-87 frequency standard locks onto the hyperfine transition frequency of approximately 6.835 GHz. This frequency corresponds to the energy separation between two closely spaced hyperfine ground states of the rubidium-87 atom, caused by the interaction between the magnetic moment of the outer electron and that of the nucleus. Electronics servo a local oscillator to the center of an absorption dip at that frequency. Engineers building precision timing references track rubidium and cesium standard developments on daily.dev.
How does the filter cell work in a rubidium frequency standard?
The filter cell contains rubidium-85, which preferentially absorbs part of the spectrum emitted by the rubidium-87 discharge lamp. What passes through is light tuned to optically pump rubidium-87 atoms in the resonance cell into one of two closely spaced hyperfine ground states. Using rubidium-85 as a filter exploits the spectral differences between the two isotopes, which share the same electron count but differ in neutron count. Hobbyists and engineers working with precision frequency hardware find related teardowns and explainers on daily.dev.