To stop light, the German researchers use a technique called electromagnetically induced transparency (EIT).
They start with a cryogenically cooled opaque crystal of yttrium silicate doped with praseodymium. A control laser is fired at the crystal, triggering a complex quantum-level reaction that turns it transparent.
A second light source (the data/image source) is then beamed into the now-transparent crystal. The control laser is then turned off, turning the crystal opaque.
Not only does this leave the light trapped inside, but the opacity means that the light inside can no longer bounce around — the light, in a word, has been stopped.
With nowhere to go, the energy from the photons is picked up by atoms within the crystal, and the “data” carried by the photons is converted into atomic spin excitations.
To get the light back out of the crystal, the control laser is turned back on, and the spin excitations are emitted at photons. These atomic spins can maintain coherence (data integrity) for around a minute, after which the light pulse/image fizzles.