Physicists from the ALPHA Collaboration at CERN have detected the Lyman-alpha transition — when the hydrogen electron transitions between the lowest-energy (1S) level and the higher-energy (2P) level, emitting or absorbing ultraviolet light of 121.6 nm wavelength — in the antihydrogen atom, the antimatter counterpart of hydrogen.

The Lyman-alpha transition is one of several in the Lyman series of electronic transitions that were discovered in atomic hydrogen in 1906 by American physicist Theodore Lyman.

The transition occurs when an electron jumps from 1S to 2P level and then falls back to 1S by emitting a photon at a wavelength of 121.6 nm.

“When an excited atom relaxes, it emits light of a characteristic color, the yellow color of sodium street lights is an everyday example of this,” said Swansea University’s Professor Mike Charlton, member of the ALPHA Collaboration.

“When the atom is hydrogen, which is a single electron and a single proton, and the excited electron decays to the lowest energy state from a higher one, the discrete series of ultraviolet light emitted forms the Lyman series, which is named after Theodore Lyman.”

“The presence of these discrete lines helped to establish the theory of quantum mechanics which governs the world at an atomic level and is one of the corner stones of modern physics.”

“The Lyman-alpha line is of fundamental importance in physics and astronomy.”

“For example, observations in astronomy on how the line from distant emitters is shifted to longer wavelengths (known as the redshift), gives us information on how the Universe evolves, and allows testing models which predict its future.”

The ALPHA experiment makes antihydrogen atoms by taking antiprotons from the Antiproton Decelerator at CERN and binding them with positrons from a sodium-22 source.

It then confines the resulting antihydrogen atoms in a magnetic trap, which prevents them from coming into contact with matter and annihilating. Laser light is then shone onto the trapped atoms to measure their spectral response. The measurement involves using a range of laser frequencies and counting the number of atoms that drop out of the trap as a result of interactions between the laser and the trapped atoms.

The ALPHA team has previously employed this technique to measure the so-called 1S-2S transition.

Using the same approach and a series of laser wavelengths around 121.6 nm, the physicists have now detected the Lyman-alpha transition in antihydrogen and measured its frequency with a precision of a few parts in a hundred million, obtaining good agreement with the equivalent transition in hydrogen.

This precision is not as high as that achieved in hydrogen, but the finding represents a pivotal technological step towards using the Lyman-alpha transition to chill large samples of antihydrogen using a technique known as laser cooling.

“This represents another landmark advance in atomic physics, which should open the way to manipulation of the kinetic energies of the trapped anti-atoms,” Professor Charlton said.

“We are really excited about this result,” said Professor Jeffrey Hangst, spokesperson for the ALPHA Collaboration and a physicist at Aarhus University.

“The Lyman-alpha transition is notoriously difficult to probe — even in ‘normal’ hydrogen. But by exploiting our ability to trap and hold large numbers of antihydrogen atoms for several hours, and using a pulsed source of Lyman-alpha laser light, we were able to observe this transition.”

“Next up is laser cooling, which will be a game-changer for precision spectroscopy and gravitational measurements.”

The results are published in the journal Nature.

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M. Ahmadi et al. Observation of the 1S-2P Lyman-α transition in antihydrogen. Nature, published online August 22, 2018; doi: 10.1038/s41586-018-0435-1