# What is the spatial mode of light or the spatial mode of a massive particle?

I'm extremely confused by what physicists mean by the spatial mode of light. I am also equally if not more confused by what the spatial mode of a massive particle is. Can anyone help me out by providing an explanation? (I'm also a visual learner, so diagrams will help)

Spatial mode of light makes no sense to me because I don't understand what its referring to!

I'm thinking that the spatial mode of a massive particle is like the eigenstate of an atom. At every energy level, there is a corresponding eigen state and this is the atom's spatial distribution. I'm guessing that taking a particle in general also has eigen states, and to distinguish them from atoms, they call them spatial modes?

A spatial mode is just what it sounds like, a state that is localised in space. For a massive particle this is easy to visualise: it is a localised wavepacket. There is no need for such a wavepacket to be an eigenstate of the Hamiltonian. For example, in section 2 of the paper you linked, Heaney considers a confining potential split into two regions $A$ and $B$. The ground state wavefunction of the Hamiltonian can be considered as a coherent sum of two distinct spatial modes, one localised in $A$ and one localised in $B$.