Let $E$ be a smooth elliptic curve over algebraically closed field of characteristic zero, $\mathcal{L}$ is a line bundle over $E$, $\operatorname{deg}(\mathcal{L})=n \geq 1$. Let $\Gamma_n$ be a finite Heisenberg group i.e. group generated by $\epsilon_1$, $\epsilon_2$ and $\delta$ with relations $\epsilon_1^n=\epsilon_2^n=\delta^n=1$, $\epsilon_1 \epsilon_2 =\delta \epsilon_2 \epsilon_1$. 

Where I can find a proof that $\Gamma_n$ acts irreducibly on $H^0(E, \mathcal{L})$? Is it true for higher rank vector bundles over $E$?