In a paper published this week in Nature, the ALPHA collaboration reports the most precise direct measurement of antimatter ever made, revealing the spectral structure of the antihydrogen atom in unprecedented accuracy. The result marks a new era of high precision tests between matter and antimatter and is the outcome of three decades of research and development at CERN. 

Antihydrogen, the bound state of an antiproton and a positron, holds the promise of some of the most precise tests of fundamental symmetries between matter and antimatter, including the CPT theorem of particle physics. The properties of antihydrogen are expected to be identical to those of hydrogen, and any differences would constitute a profound challenge to the fundamental theories of physics. While P (parity) and CP (charge-parity) are known to be violated, CPT (charge-parity-time reversal) are believed to be conserved by virtue of the CPT theorem according to which the fundamental laws are invariant under a combined reversal of charge, parity and time. Several accurate tests of CPT invariance have been performed on leptons and hadrons, and in exotic atoms, yet given its fundamental importance, CPT should be tested in all particle sectors

There are a number of challenges to be faced to reach the precision of the measurement on hydrogen, the foremost being the very low number of available antihydrogen atoms and their relatively high temperature. Further complications stem primarily from the fact that the antihydrogen atoms must be made in the laboratory as they do not occur in nature, thus inducing a number of geometric constraints. 

ALPHA researchers created and captured hundreds of antihydrogen atoms in a magnetic bottle for a first time in 2010 and probed their internal states by bathing them in microwave radiation that flipped the spins of the positrons, causing the immediate ejection of the atoms from the magnetic trap and their annihilation on the trap wall. Key to anti-atomic spectroscopy, as developed so far, is to illuminate a sample of trapped antihydrogen atoms with electromagnetic radiation (microwaves or laser photons) that causes atoms to be lost from the trap if the radiation is on resonance with the transition of interest. 

The ALPHA team used this approach in 2016, to measure the frequency of the electronic transition between the lowest-energy state and the first excited state (the so-called 1S to 2S transition) of antihydrogen, holding the record of being measured to a precision of 4.2 × 10⁻¹⁵. The result is in good agreement with the equivalent transition in hydrogen. The measurement involved using two laser frequencies — one matching the frequency of the 1S–2S transition in hydrogen and another “detuned” from it — and counting the number of atoms that dropped out of the trap as a result of interactions between the laser and the trapped atoms.

The latest result from ALPHA takes antihydrogen spectroscopy to the next level, using not just one but several detuned laser frequencies, with slightly lower and higher frequencies than the 1S–2S transition frequency in hydrogen. This allowed the team to measure the spectral shape, or spread in colours, of the 1S–2S antihydrogen transition and get a more precise measurement of its frequency. The shape of the spectral line agrees very well with that expected for hydrogen and that the resonance frequency agrees with that in hydrogen to about 5 kHz out of 2.5 × 10¹⁵ Hz. This is consistent with charge–parity–time invariance at a relative precision of 2 × 10⁻¹²   a factor of 100 better than the previous measurement.

With this new result, the ALPHA collaboration has clearly demonstrated the maturity of its techniques for probing the properties of antimatter atoms. Although the precision still falls short of that for ordinary hydrogen, the rapid progress made by ALPHA suggests hydrogen-like precision in antihydrogen — and thus unprecedented tests of CPT symmetry — are now within reach