  This paper studies $B(r)$, the space of $r$-tuples of $2 \times 2$ complex
matrices that generate $\operatorname{Mat}_{2 \times 2}(\mathbf C)$ as an
algebra, considered up to change-of-basis. We show that $B(2)$ is homotopy
equivalent to $S^1 \times^{\mathbf Z/2\mathbf Z} S^2$. For $r>2$, we determine
the rational cohomology of $B(r)$ for degrees less than $4r-6$. As an
application, we use the machinery of arXiv:2012.07900 to prove that for all
natural numbers $d$, there exists a ring $R$ of Krull dimension $d$ and a
degree-$2$ Azumaya algebra $A$ over $R$ that cannot be generated by fewer than
$2\lfloor d/4 \rfloor + 2$ elements.
