Laser-cooled antihydrogen takes ALPHA into new realm

3 May 2021
Antihydrogen’s 1S–2S spectral line

After many years of research and development, the ALPHA collaboration has succeeded in laser-cooling antihydrogen – opening the door to considerably more precise measurements of antihydrogen’s internal structure and gravitational interactions. The seminal result, reported on 31 March in Nature, could also lead to the creation of antimatter molecules and the development of antiatom interferometry, explains ALPHA spokesperson Jeffrey Hangst. “This is by far the most difficult experiment we have ever done,” he says. “We’re over the moon. About a decade ago, laser cooling of antimatter was in the realm of science fiction.” 

The ALPHA collaboration synthesises antihydrogen from cryogenic plasmas of antiprotons and positrons at CERN’s Antiproton Decelerator (AD), storing the antiatoms in a magnetic trap. Lasers with particular frequencies are then used to measure the antiatoms’ spectral response. Finding any slight difference between spectral transitions in antimatter and matter would challenge charge–parity–time symmetry, and perhaps cast light on the cosmological imbalance of matter and antimatter.

Historically, researchers have struggled to laser-cool normal hydrogen, so this has been a bit of a crazy dream for us for many years.

Makoto Fujiwara

Following the first antihydrogen spectroscopy by ALPHA in 2012, in 2017 the collaboration measured the spectral structure of the antihydrogen 1S–2S transition with an outstanding precision of 2 × 10–12 – marking a milestone in the AD’s scientific programme. The following year, the team determined antihydrogen’s 1S–2P “Lyman–alpha” transition with a precision of a few parts in a hundred million, showing that it agrees with the prediction for the equivalent transition hydrogen to a precision of 5 × 10–8. However, to push the precision of spectroscopic measurements further, and to allow future measurements of the behaviour of antihydrogen in Earth’s gravitational field, the kinetic energy of the antiatoms must be lowered.

In their new study, the ALPHA researchers were able to laser-cool a sample of magnetically trapped antihydrogen atoms by repeatedly driving the antiatoms from the 1S to the 2P state using a pulsed laser with a frequency slightly below that of the transition between them. After illuminating the trapped antiatoms for several hours, the researchers observed a more than 10-fold decrease in their median kinetic energy, with many of the antiatoms attaining energies below 1 μeV. Subsequent spectroscopic measurements of the 1S–2S transition revealed that the cooling resulted in a spectral line about four times narrower than that observed without laser cooling – a proof-of-principle of the laser-cooling technique, with further statistics needed to improve the precision of the previous 1S–2S measurement (see figure).

“Historically, researchers have struggled to laser-cool normal hydrogen, so this has been a bit of a crazy dream for us for many years,” says Makoto Fujiwara, who proposed the use of a pulsed laser to cool trapped antihydrogen in ALPHA. “Now, we can dream of even crazier things with antimatter.”

Further reading

The ALPHA Collaboration 2021 Nature 592 35.

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