I'm studying isogeny graphs and in particolar isogeny graphs of supersingular elliptic curves.
In a supersingular isogeny graph $\mathcal{G}_\ell(p)$ nodes are supersingular elliptic curvers $E$ over $\mathbb{F}_{p^2}$, up to isomorphism, and edges are $\ell$-isogenies between them.
When $p\neq \ell$, the graph is connected. Moreover, it is a $\ell+1$-regular graph and the number of its nodes is $\sim p/12$.
I understood all these properties, but I can't find a proof of the following statement
Supersingular isogeny graphs are expander graphs
Do you have any references where the proof is carried out with all the steps? Or is this thing trivial (but I can't understand it)?
Thank you for help.