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The ALPHA collaboration at CERN has succeeded in cooling down antihydrogen atoms – the simplest form of atomic antimatter – using laser light. The technique, known as laser cooling, was first demonstrated 40 years ago on normal matter and is a mainstay of many research fields. Its first application to antihydrogen by ALPHA, described in a paper recently published in Nature, opens the door to considerably more precise measurements of the internal structure of antihydrogen and of how it behaves under the influence of gravity.
Comparing such measurements with those of the well-studied hydrogen atom could reveal differences between matter and antimatter atoms. Such differences, if present, could shed light on why the universe is made up of matter only, an imbalance known as matter–antimatter asymmetry.
CI researchers at the universities of Manchester and Liverpool are carrying out studies into this exotic type of particle. Amongst others, CI has contributed the ALPHA-II catching trap to the experiment.
The ALPHA team makes antihydrogen atoms by binding antiprotons, taken from CERN’s Antiproton Decelerator, with positrons originating 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. Next, the team typically performs spectroscopic studies by measuring the anti-atoms’ response to electromagnetic radiation – laser light or microwaves. These studies have allowed the team to, for example, measure the 1S–2S electronic transition in antihydrogen with unprecedented precision. However, the precision of such spectroscopic measurements and of planned future measurements of the behaviour of antihydrogen in the Earth’s gravitational field in ongoing experiments is limited by the kinetic energy or, equivalently, the temperature, of the antiatoms.
By using the laser cooling technique, laser photons are absorbed by the atoms, causing them to reach a higher-energy state. The anti-atoms then emit the photons and spontaneously decay back to their initial state. Because the interaction depends on the atoms’ velocity and as the photons impart momentum, repeating this absorption–emission cycle many times leads to cooling of the atoms to a low temperature.
In their new study, the ALPHA researchers were able to laser-cool a sample of magnetically trapped antihydrogen atoms by repeatedly driving the anti-atoms from the atoms’ lowest-energy state (the 1S state) to a higher-energy state (2P) using pulsed laser light with a frequency slightly below that of the transition between the two states. After illuminating the trapped atoms for several hours, the researchers observed a more than tenfold decrease in the atoms’ median kinetic energy, with many of the anti-atoms attaining energies below a microeletronvolt (about 0.012 degrees above absolute zero in temperature equivalent).
Having successfully laser-cooled the anti-atoms, the researchers investigated how the laser cooling affected a spectroscopic measurement of the 1S–2S transition and found that the cooling resulted in a narrower spectral line for the transition – about four times narrower than that observed without laser cooling.
ALPHA’s demonstration of laser cooling of antihydrogen atoms and its application to 1S–2S spectroscopy represents the culmination of many years of antimatter research and developments at CERN’s Antiproton Decelerator. This new development is a game-changer for spectroscopic and gravitational measurements, which could lead to new perspectives in antimatter research.
CI researcher and coordinator of the antimatter network AVA, Prof Carsten P. Welsch, says: ‘These measurements mark a very important milestone towards studying antimatter in unprecedented detail. Whilst laser cooling is an established technique in atomic physics, it has been a significant challenge to adapt it for use with antihydrogen. Congratulations to the ALPHA collaboration for this excellent result!’
Baker, C.J., Bertsche, W., Capra, A. et al. Laser cooling of antihydrogen atoms. Nature 592, 35–42 (2021). https://doi.org/10.1038/s41586-021-03289-6