Light falling on matter usually scatters off it or heats it up. But when there are many photons and they are very energetic, two quanta of light can collide and vanish, leaving two particles in their place: an electron and its antimatter twin, the positron. This process, predicted back in 1934, has never been reproduced in the lab with real photons. The reason is simple: you need a dense and energetic beam of light, plus a detector that can spot the produced particles against the noise.
The new work proposes a surprisingly elegant path. An ultra-powerful laser pulse hits a target made of plastic foam, and a magnetic field spontaneously arises in the plasma, twisting around the beam axis. This field acts like a funnel: positrons born deep inside do not fly off in all directions; instead, they are turned around and travel back toward the laser. Moreover, the funnel not only redirects the particles but also squeezes them into a narrow cone, much like a water jet gathers swimmers into a stream.
Supercomputer simulations show that if the target is made two-layered, first sparse then dense, the positron yield jumps dramatically. In the sparse layer, the laser has time to create the magnetic funnel, while in the dense layer many pairs are born. As a result, the backward signal reaches $10^6$ MeV^-1 sr^-1, which is hundreds of times brighter than previous schemes. Such a signal falls within the range where positrons have been detected in other experiments.
This is not just a theoretical trick. The setup can be built on existing petawatt laser facilities, and the two-layer foam target can be replaced by a pre-plasma created by the laser itself. If the experiment succeeds, physicists will for the first time see light turning into matter in its pure form. The magnetic funnel that spontaneously arises in plasma and focuses particles could also prove useful for other pair-production processes in strong fields.
Positrons born in the plasma are turned 180 degrees and fly toward the laser beam, like swimmers carried back to shore by a current.
The idea of creating matter from light resembles science fiction plots, for example, in Arthur C. Clarke's novel 'The Light of Other Days', where light is used to create matter. In reality, the Breit-Wheeler process is the literal conversion of photon energy into particle mass.