Laser Cools Antimatter to Near Absolute Zero

JOEL WILLIAMS, ASSOCIATE EDITOR
joel.williams@photonics.com

A custom-built laser has cooled antimatter to near absolute zero. The work, carried out by researchers with the CERN-based ALPHA collaboration, holds significant implications for the study of antimatter and its properties, and of foundational theories of the universe.

Antimatter is nearly identical to matter in terms of its behavior and characteristics, but it possesses the opposite charge. When matter and antimatter come into contact, they annihilate one another. Experimentation remains difficult for this reason.

The movement of an antihydrogen atom in the ALPHA magnetic trap, before (gray) and after (blue) laser cooling. Courtesy of Chukman So, TRIUMF.

“The antimatter atoms must be held in an extremely high vacuum in order to prevent them from annihilating with residual gases,” said Makoto Fujiwara, TRIUMF scientist and original proponent of the laser cooling idea. “We manipulate antimatter without destroying it by exerting force from laser light onto the anti-atoms.”

To cool the anti-atoms, the researchers had to design a laser system themselves.

“Since we cannot use any conventional dye laser, we had to design the laser system from scratch using nonlinear crystals (Ti:sapphire),” Takamasa Momose, University of British Columbia researcher and designer of the laser, told Photonics Media.

Among the challenges of building the laser that Momose noted were frequency purity and high power requirements.

“The precise control of the detuning frequency from the resonant frequency is key for precise manipulation of anti-atoms,” Momose said. In the case of the antihydrogen atoms, the frequency purity, or laser linewidth, had to be less than 50 MHz at 121.6 nm.

To keep the linewidth narrow, the team employed third harmonic generation with one color to generate 121.6 nm, rather than using four-wave mixing of two colors, which usually provides about 100× greater intensity but with a much broader linewidth.

Aligning the laser also proved challenging, Momose said. The researchers constructed an optical transport system consisting of several mirrors and windows to introduce the laser pulses, which had to be roughly 1 nJ per pulse, into the magnetic trap within the cryogenic chamber.

The cooling of antimatter enables a variety of precision tests to further investigate the characteristics of 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,” Momose said.

To that end, Momose and Fujiwara are now leading a new project dubbed HAICU, which aims to develop new quantum techniques for antimatter studies.

“My next dream is to create an ‘anti-atomic fountain’ by tossing the anti-atoms into free space, which would in turn allow antimatter-wave interferometry,” Fujiwara told Photonics Media. “These techniques would permit extremely precise measurements on antimatter properties by taking advantage of emerging quantum technology.”

“Since we can increase the density of antihydrogen using this cooling technique, we now have a chance to create a first antimatter molecule (like H₂⁺),” Momose said. “Making a first antimatter molecule is my primary interest.”

Momose additionally expressed interest in the spectroscopy of antimolecules. That, he said, may be more sensitive than spectroscopy of anti-atoms in the investigation of fundamental physics matters such as violations of charge, parity, and time symmetry.

The research was published in Nature (www.doi.org/10.1038/s41586-021-03289-6).