Editing Second (section)

== Future redefinition ==
In 2022, the best realisation of the second is done with caesium primary standard clocks such as IT-CsF2, NIST-F2, NPL-CsF2, PTB-CSF2, SU–CsFO2 or SYRTE-FO2. These clocks work by laser-cooling a cloud of Cs atoms to a microkelvin in a magneto-optic trap. These cold atoms are then launched vertically by laser light. The atoms then undergo Ramsey excitation in a microwave cavity. The fraction of excited atoms is then detected by laser beams. These clocks have {{val|5|e=-16}} systematic uncertainty, which is equivalent to 50 picoseconds per day. A system of several fountains worldwide contribute to International Atomic Time. These caesium clocks also underpin optical frequency measurements.

[[Optical clock]]s are based on forbidden optical transitions in ions or atoms. They have frequencies around {{val|u=Hz|e=15}}, with a natural linewidth <math>\Delta f</math> of typically 1&nbsp;Hz, so the [[Q-factor]] is about {{val|e=15}}, or even higher. They have better stabilities than microwave clocks, which means that they can facilitate evaluation of lower uncertainties. They also have better time resolution, which means the clock "ticks" faster.<ref>{{Cite journal |last=National Physical Laboratory |date=2011 |title=When should we change the definition of the second? |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=369 |issue=1953 |pages=4109–4130 |bibcode=2011RSPTA.369.4109G |doi=10.1098/rsta.2011.0237 |pmid=21930568 |s2cid=6896025}}</ref> Optical clocks use either a single ion, or an [[optical lattice]] with {{val|e=4}}–{{val|e=6}} atoms.

=== Rydberg constant ===
A definition based on the [[Rydberg constant]] would involve fixing the value to a certain value: <math>R_{\infty}=\frac{m_\text{e} e^4}{8 \varepsilon_0^2 h^3 c}=\frac{m_\text{e} c \alpha^2}{2h}</math>. The Rydberg constant describes the energy levels in a hydrogen atom with the nonrelativistic approximation <math>E_n \approx -\frac{R_{\infty} c h}{n^2}</math>.

The only viable way to fix the Rydberg constant involves trapping and cooling hydrogen. This is difficult because it is very light and the atoms move very fast, causing Doppler shifts. The radiation needed to cool the hydrogen – {{val|121.5|u=nm}} – is also difficult. Another hurdle involves improving the uncertainty in QED calculations, specifically the [[Lamb shift]] in the 1s-2s transition of the hydrogen atom.<ref name=":10">{{Cite journal |last=Gill |first=Patrick |date=2011-10-28 |title=When should we change the definition of the second? |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=369 |issue=1953 |pages=4109–4130 |bibcode=2011RSPTA.369.4109G |doi=10.1098/rsta.2011.0237 |pmid=21930568 |s2cid=6896025|doi-access=free }}</ref>

=== Requirements ===
A redefinition must include improved optical clock reliability. TAI must be contributed to by optical clocks before the BIPM affirms a redefinition. A consistent method of sending signals must be developed before the second is redefined, such as [[Optical fiber|fiber-optics.]]<ref name=":10" />