The research describes "photonic Landau levels." This is similar to how electrons are confined to certain energy levels when subjected to magnetic fields, but applied to photons. By confining photons to these levels, the crystal can effectively "trap" light in place. The light is still present and retains its energy, but it doesn't move forward; it's localized within a very small area of the crystal. The photons are not moving from one place to another as they normally would—they are caught in a sort of "light trap" created by the specific structural properties of the photonic crystal.

You can theoretically have an unlimited number of them in a space due to this, but due to other properties of bosons (like photons), there are other limits. In particular, if you accumulate enough of them they spontaneously spawn new particles!

They aren't really physical particles in the classical sense, they cross quantum fields, and to the extent that those occupy space, the photon isn't in any one position, but merely crossing it or spread out across it.

The trapped light does not take up physical space in the same way that matter does and can lead to an increase in the local energy density within the photonic crystal. However, it is not clear how this energy could be efficiently extracted and used for practical applications. Also, the confined light is subject to energy loss through various mechanisms, such as absorption by the material or scattering and these losses would limit the amount of energy that can be effectively stored in the system over time.