Editing Antihydrogen (section)

===1s–2s transition measurement===
In 2016, the [[Antiproton Decelerator#ALPHA|ALPHA]] experiment measured the [[atomic electron transition]] between the two lowest [[energy level]]s of antihydrogen, 1s–2s. The results, which are identical to that of hydrogen within the experimental resolution, support the idea of matter–antimatter symmetry and [[CPT symmetry]].<ref>{{cite journal|url=http://www.nature.com/news/ephemeral-antimatter-atoms-pinned-down-in-milestone-laser-test-1.21193|title=Ephemeral antimatter atoms pinned down in milestone laser test|journal=Nature|date=19 December 2016|access-date=20 December 2016|author=Castelvecchi, Davide |doi=10.1038/nature.2016.21193|s2cid=125464517}}</ref>

In the presence of a magnetic field the 1s–2s transition splits into two [[Hyperfine structure|hyperfine]] transitions with slightly different frequencies. The team calculated the transition frequencies for normal hydrogen under the magnetic field in the confinement volume as:

:f<sub>dd</sub> = {{val|2466061103064|(2)|u=kHz}} 
:f<sub>cc</sub> = {{val|2466061707104|(2)|u=kHz}}

A single-photon transition between s states is prohibited by quantum [[selection rule]]s, so to elevate ground state positrons to the 2s level, the confinement space was illuminated by a laser tuned to half the calculated transition frequencies, stimulating allowed [[two photon absorption]].

Antihydrogen atoms excited to the 2s state can then evolve in one of several ways:

*They can emit two photons and return directly to the ground state as they were
*They can absorb another photon, which ionizes the atom
*They can emit a single photon and return to the ground state via the 2p state—in this case the positron spin can flip or remain the same.

Both the ionization and spin-flip outcomes cause the atom to escape confinement. The team calculated that, assuming antihydrogen behaves like normal hydrogen, roughly half the antihydrogen atoms would be lost during the resonant frequency exposure, as compared to the no-laser case. With the laser source tuned 200&nbsp;kHz below half the transition frequencies, the calculated loss was essentially the same as for the no-laser case.

The ALPHA team made batches of antihydrogen, held them for 600 seconds and then tapered down the confinement field over 1.5 seconds while counting how many antihydrogen atoms were annihilated. They did this under three different experimental conditions:

*Resonance: exposing the confined antihydrogen atoms to a laser source tuned to exactly half the transition frequency for 300 seconds for each of the two transitions,
*Off-resonance: exposing the confined antihydrogen atoms to a laser source tuned 200 kilohertz below the two resonance frequencies for 300 seconds each,
*No-laser: confining the antihydrogen atoms without any laser illumination.

The two controls, off-resonance and no-laser, were needed to ensure that the laser illumination itself was not causing annihilations, perhaps by liberating normal atoms from the confinement vessel surface that could then combine with the antihydrogen.

The team conducted 11 runs of the three cases and found no [[statistically significant|significant]] difference between the off-resonance and no laser runs, but a 58% drop in the number of events detected after the resonance runs. They were also able to count annihilation events during the runs and found a higher level during the resonance runs, again with no significant difference between the off-resonance and no laser runs. The results were in good agreement with predictions based on normal hydrogen and can be "interpreted as a test of CPT symmetry at a precision of 200&nbsp;ppt."<ref>{{cite journal |last=Ahmadi|first=M|display-authors=et al|date=19 December 2016|title=Observation of the 1S–2S transition in trapped antihydrogen|journal=Nature|volume=541|issue=7638|pages=506–510|doi=10.1038/nature21040|pmid=28005057|bibcode = 2017Natur.541..506A |s2cid=3195564|url=http://discovery.ucl.ac.uk/1537231/1/Guiterrez_nature21040.pdf|doi-access=free}}</ref>