CERN Physicists Cool Antimatter Using Laser Light for First Time
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Physicists from the ALPHA (Antihydrogen Laser Physics Apparatus) Collaboration at CERN have successfully demonstrated laser cooling of antihydrogen, the antimatter atom consisting of an antiproton and a positron.

Trapped antihydrogen atoms being cooled. Image credit: Chukman So.

Antimatter is a necessity in the most successful quantum mechanical models of particle physics. It became available in labs nearly a century ago with the discovery of the positively charged positron.

When matter and antimatter come together annihilation occurs; a striking effect wherein the original particles disappear.

Annihilation can be observed in the laboratory and is even used in medical diagnostic techniques such as positron emission tomography scans.

However, antimatter presents a conundrum. An equal amount of antimatter and matter formed in the Big Bang, but this symmetry is not preserved today as antimatter seems to be virtually absent from the visible Universe.

“The ability to laser-cool antihydrogen atoms is a game-changer for spectroscopic and gravitational measurements, and it could lead to new perspectives in antimatter research, such as the creation of antimatter molecules and the development of antihydrogen atom interferometry,” said Dr. Jeffrey Hangst, spokesperson of the ALPHA Collaboration.

“We’re over the Moon. About a decade ago, laser cooling of antimatter was in the realm of science fiction.”

The ALPHA physicists produce antihydrogen atoms by taking antiprotons from CERN’s Antiproton Decelerator and binding them 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 researchers typically perform spectroscopic studies, that is, it measures the antimatter atoms’ response to electromagnetic radiation — laser light or microwaves.

These studies have allowed them 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 antimatter atoms.

This is where laser cooling comes in. In this technique, laser photons are absorbed by the atoms, causing them to reach a higher-energy state.

The antimatter 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 the new study, the ALPHA scientists were able to laser-cool a sample of magnetically trapped antihydrogen atoms by repeatedly driving the antihydrogen 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, they observed a more than tenfold decrease in the atoms’ median kinetic energy, with many of the antihydrogen atoms attaining energies below a microeletronvolt (about 0.012 degrees above absolute zero in temperature equivalent).

Having successfully laser-cooled the antihydrogen atoms, the ALPHA team 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.

“Our 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,” Dr. Hangst said.

“This is by far the most difficult experiment we have ever done.”

“Historically, researchers have struggled to laser-cool normal hydrogen, so this has been a bit of a crazy dream for us for many years,” said Dr. Makoto Fujiwara, a physicist at TRIUMF, Canada’s particle accelerator centre, who proposed to use a pulsed laser to cool trapped antihydrogen.

“Now, we can dream of even crazier things with antimatter.”

“With this technique, we can address long-standing mysteries like: How does antimatter respond to gravity? Can antimatter help us understand symmetries in physics?. These answers may fundamentally alter our understanding of our Universe,” said Dr. Takamasa Momose, a physicist at the University of British Columbia.

“Since there is no antihydrogen around, we have to make it in the lab at CERN. It’s a remarkable feat that we can now also laser-cool antihydrogen and make a very precise spectroscopic measurement, all in less than a single day,” said Professor Niels Madsen, a physicist at Swansea University.

“Only two years ago, the spectroscopy alone would take ten weeks. Our goal is to investigate if the properties of our antihydrogen match those of ordinary hydrogen as expected by symmetry.”

“A difference, however small, could help explain the some of the deep questions surrounding antimatter.”

“This spectacular result takes antihydrogen research to the next level, as the improved precision that laser cooling brings puts us in contention with experiments on normal matter,” said Professor Stefan Eriksson, a physicist at Swansea University.

“This is a tall order since the spectrum of hydrogen that we compare with has been measured with a staggering precision of fifteen digits. We are already upgrading our experiment to meet the challenge!”

The team’s paper was published in the journal Nature.

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C.J. Baker et al. 2021. Laser cooling of antihydrogen atoms. Nature 592, 35-42; doi: 10.1038/s41586-021-03289-6

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