Trapping and laser cooling of antihydrogen points to advances in antimatter experimentation

Although manufacturing and manipulating antimatter has been the subject of science fiction for decades in the form of matter-antimatter spaceship drives, true-life physicists also would like to make and capture antimatter—in this case, to observe its physical properties, and perhaps to learn something that might prompt new physical theories of matter. Now, the CERN-based Anti-hydrogen Laser Physics Apparatus (ALPHA) collaboration has achieved the world’s first laser-based manipulation of antimatter, taking advantage of a made-in-Canada laser system to cool a sample of antihydrogen down to near absolute zero.¹

Since its introduction 40 years ago, laser manipulation and cooling of ordinary atoms have revolutionized modern atomic physics and enabled several Nobel-winning experiments. The new results mark the first instance of scientists applying these techniques to antimatter. Because antimatter atoms annihilate upon contact with matter, they are exceptionally difficult to create and control and had never before been manipulated with a laser.

The ALPHA researchers first manufacture antihydrogen—a difficult-enough feat in its own right—and then magnetically trap the atoms so that they can be laser-Doppler cooled. Here, the magnetic trap aids the cooling; although the laser cools the atoms only in one dimension, the trap couples the motion of the atoms in all three axes, leading to 3D cooling. The antihydrogen atoms are cooled to submicroelectronvolt transverse kinetic energies, equivalent to speeds of at most a couple meters per second.

The antihydrogen atoms are made in a Penning trap (which has confining magnetic and electric fields) by mixing antiprotons from CERN’s antiproton decelerator with positrons from an accumulator, producing 10 to 30 atoms at a time; this cycle is repeated to accumulate up to 1000 atoms in a so-called “stacking” procedure.

Light at the Lyman-alpha line

Pulsed radiation at a 121.6 nm wavelength, which matches the Lyman-alpha line of hydrogen, was produced by frequency-doubling 724.9 nm laser light, then frequency-tripling the resulting light in a third-harmonic-generation (THG) cell containing krypton and argon gas (see figure). This light was used to slow down the antihydrogen atoms. In addition, a beam of light at 243.1 nm, boosted in amplitude by an optical cavity within the vacuum chamber, was used to provide photons that cancel the first-order Doppler shift and drive the atoms’ 1S-2S transition, enabling this transition and the effects of laser cooling on this transition to be observed. The scientists found that the resulting 1S-2S spectral line was about 4X narrower than that obtained without laser cooling.

“With this technique, we can address longstanding 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,” says Takamasa Momose, a researcher from the University of British Columbia (UBC; Vancouver, BC, Canada) with ALPHA’s Canadian team (ALPHA-Canada) who led the development of the cooling laser.

The laser cooling of antihydrogen opens the door to a variety of leading-edge physics advances. Momose and Makoto Fujiwara, who is ALPHA-Canada spokesperson, scientist at TRIUMF (Canada’s particle-accelerator center), and the original proponent of the laser cooling idea, are now leading a new Canadian project, dubbed HAICU, to develop new quantum techniques for antimatter studies. “My next dream is to make a ‘fountain’ of anti-atoms by tossing the laser-cooled antimatter into free space,” says Fujiwara. “If realized, it would enable an entirely new class of quantum measurements that were previously unthinkable.”

Momose adds, “Furthermore, we are one step closer to being able to manufacture the world’s first antimatter molecules by joining anti-atoms together using our laser manipulation technology.”

The results mark a major success for ALPHA-Canada, which makes up about one-third of the wider ALPHA collaboration, and contributors the University of Victoria (Victoria, BC, Canada) and the British Columbia Institute of Technology (Burnaby, BC, Canada). As lead institution for ALPHA-Canada, TRIUMF has spearheaded collaborative efforts on several key experimental technologies and analyses, including the upgraded ALPHA-2 cryostat that enabled the first laser spectroscopic measurements of antimatter and the design and fabrication of the detector apparatus for ALPHA-g, the experiment that will determine the effect of gravity on antimatter.

This is not the first of ALPHA’s achievements in antimatter physics: After creating and trapping antihydrogen for a world-record 1000 seconds in 2011, ALPHA provided a first glimpse of the antihydrogen spectrum in 2012, set guardrails confining the effect of gravity on antimatter in 2013, and demoed an antimatter counterpart to a key spectroscopic phenomenon in 2020.

REFERENCE

1. C. J. Baker et al., Nature (2021); https://doi.org/10.1038/s41586-021-03289-6.