Quantum expanders and growth of group representations

Abstract

Let $\pi$ be a finite dimensional unitary representation of a group $G$ with a generating symmetric $n$-element set $S\subset G$. Fix $\vp>0$. Assume that the spectrum of $|S|^{-1}\sum_{s\in S} \pi(s) \otimes \overline{\pi(s)}$ is included in $ [-1, 1-\vp]$ (so there is a spectral gap $\ge \vp$). Let $r'_N(\pi)$ be the number of distinct irreducible representations of dimension $\le N$ that appear in $\pi$. Then let $R_{n,\vp}'(N)=\sup r'_N(\pi)$ where the supremum runs over all $\pi$ with ${n,\vp}$ fixed. We prove that there are positive constants $\delta_\vp$ and $c_\vp$ such that, for all sufficiently large integer $n$ (i.e. $n\ge n_0$ with $n_0$ depending on $\vp$) and for all $N\ge 1$, we have $\exp{\delta_\vp nN^2} \le R'_{n,\vp}(N)\le \exp{c_\vp nN^2}$. The same bounds hold if, in $r'_N(\pi)$, we count only the number of distinct irreducible representations of dimension exactly $= N$.