Bubbles of positronium in liquid helium could make a gamma-ray laser possible

Dec. 7, 2019
Theoretical calculations show that positronium can be stable inside microscopic bubbles of liquid helium, resulting in the prerequisite conditions for a gamma-ray laser.

Five years ago, we covered a "pie-in-the-sky" theoretical conjecture by physicists at the Joint Quantum Institute (JQI), which is based at the University of Maryland (College Park, MD) -- that of a potential gamma-ray laser using positronium as the laser gain material (such a laser could be useful in medical imaging or cancer treatment). For the technique to work, the positronium -- a short-lived "atom" comprising an electron and a positron -- has to exist as a Bose-Einstein condensate (BEC), which is a collection of identical matter particles laser-cooled to near zero Kelvin so that most of the particles are in the lowest quantum state.

As we put it back then, "Creating such a BEC will likely not be feasible for years to come, and if and when it is done, the resulting gamma-ray device would surely fill a room at least."

Now, Allen Mills, a physicist at the University of California, Riverside, has performed calculations showing that hollow spherical bubbles filled with a gas of positronium atoms are stable in liquid helium, potentially easing the path to a gamma-ray laser.1

"My calculations show that a bubble in liquid helium containing a million atoms of positronium would have a number density six times that of ordinary air and would exist as a matter-antimatter Bose-Einstein condensate," says Mills.

Helium, the second-most abundant element in the universe, exists in liquid form only at extremely low temperatures. Mills explains that helium has a negative affinity for positronium; bubbles form in liquid helium because helium repels positronium. Positronium's long lifetime in liquid helium was first reported in 1957.

When an electron meets a positron, their mutual annihilation could be one outcome, accompanied by the production of gamma radiation with a 511-keV energy. A second outcome is the formation of positronium.

Mills, who directs the Positron Laboratory at UC Riverside, said the lab is configuring an antimatter beam in a quest to produce the exotic bubbles in liquid helium that Mills’ calculations predict. Such bubbles could serve as a source of positronium Bose-Einstein condensates.

Nearer-term possibility: a positronium-atom laser
"Near-term results of our experiments could be the observation of positronium tunneling through a graphene sheet, which is impervious to all ordinary matter atoms, including helium, as well as the formation of a positronium atom laser beam with possible quantum-computing applications," Mills says.

Source: https://news.ucr.edu/articles/2019/12/05/gamma-ray-laser-moves-step-closer-reality


1. A.P Mills, Jr., Physical Review A (2019); https://link.aps.org/doi/10.1103/PhysRevA.100.063615.

Got optics- and photonics-related news to share with us? Contact John Wallace, Senior Editor, Laser Focus World

Get more like this delivered right to your inbox

About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.

Voice your opinion!

To join the conversation, and become an exclusive member of Laser Focus World, create an account today!