We can use just about any part of the electromagnetic spectrum to make a laser - from long-wave microwaves to bursts of highly powerful X-rays. The only ones we've had trouble with have been the ultra-short wavelengths making up the gamma ray part of the spectrum. Now, that might be set to change.
A new mathematical model devised by University of California Riverside physicist Allen Mills shows how a gamma ray laser might work, by using positronium: hydrogen-like particles consisting of an electron and its positively charged antimatter partner, a positron.
By packing the short-lived partnerships in helium and dropping the temperature, it just might be possible to tame the gamma rays resulting from the matter-antimatter collision into the orderly queues of a laser beam.
"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.
If those numbers can be turned into a practical demonstration, Mills may have solved what was once described as one of the greatest challenges in modern physics. But to understand why this is such a huge challenge, we need to understand what's so special about lasers in the first place.
The word 'laser' is actually an acronym, standing for Light Amplification by Stimulated Emission of Radiation. In ordinary visible light, its wavelengths are all over the place, and they don't tend to match up.
But a laser beam is created by stimulating the electrons in a particular material into emitting the exact same wavelengths of light and giving them an energy boost, which lines them all up so that their crests and waves match up perfectly - this is called coherence.
That coherence is what keeps the light waves from interfering with one another and spilling out of line, so you end up with a beam of concentrated light you can easily shine across the room (possibly to entertain your cat).
We've been able to do this process with relatively long wavelengths of light since the 1960s. By the 1970s, engineers were creating lasers with UV light, down to a tiny 110 nanometres in length.
And then we hit a wall.
Finding the right materials for generating and harnessing shorter and shorter wavelengths has been difficult enough. But smaller waves means a narrower period of excitement for the light-generating electrons, an issue that both demands an increasing amount of power to be supplied to the laser's amplification process while spreading the spectrum of light.
For these reasons, the push for lasers based on ever-smaller wavelengths has been slow going. X-ray lasers only became a reality in the mid-1980s, at first rumoured as part of the US 'Star Wars' strategic defence program before finally being confirmed in later experiments.
Most attempts at developing a gamma ray laser have focussed on cooling the light-generating atoms right down to near absolute zero, at which point they all adopt the same quantum signatures and act like one single super particle.
The clever part of Mills' approach is to mix the light-emitting positronium particles with helium, which repels the exotic electron-positron pair and pushes them together to form a dense, stable cluster that becomes the basis of a condensate.
On paper it all seems to add up. The next step is for Mills to run experiments at his Positron Laboratory at UC Riverside in an effort to generate sufficient amounts of this exotic form of matter.
"Near term results of our experiments could be the observation of positronium tunnelling 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," says Mills.
It's a problem well worth cracking. In theory, a gamma ray laser would give us unprecedented resolution in novel forms of imaging technology, new kinds of propulsion system, and maybe… if you had one big enough… a way to generate your own backyard black hole.
This research was published in Physical Review A.