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

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

REFERENCE:

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

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