Dilations of $q$-commuting unitaries

Abstract

Let $q = e^{i \theta} \in \mathbb{T}$ (where $\theta \in \mathbb{R}$), and let $u,v$ be $q$-commuting unitaries, i.e., $u$ and $v$ are unitaries such that $vu = quv$. In this paper we find the optimal constant $c = c_\theta$ such that $u,v$ can be dilated to a pair of operators $c U, c V$, where $U$ and $V$ are commuting unitaries. We show that \[ c_\theta = \frac{4}{\|u_\theta+u_\theta^*+v_\theta+v_\theta^*\|}, \] where $u_\theta, v_\theta$ are the universal $q$-commuting pair of unitaries, and we give numerical estimates for the above quantity. In the course of our proof, we also consider dilating $q$-commuting unitaries to scalar multiples of $q'$-commuting unitaries. The techniques that we develop allow us to give new and simple "dilation theoretic" proofs of well known results regarding the continuity of the field of rotations algebras. In particular, for the so-called "Almost Mathieu Operator" $h_\theta = u_\theta+u_\theta^*+v_\theta+v_\theta^*$, we recover the fact that the norm $\|h_\theta\|$ is a Lipshitz continuous function of $\theta$, as well as the result that the spectrum $\sigma(h_\theta)$ is a $\frac{1}{2}$-H\"older continuous function in $\theta$ with respect to the Hausdorff metric. In fact, we obtain this H\"older continuity of the spectrum for every selfadjoint $*$-polynomial $p(u_\theta,v_\theta)$, which in turn endows the rotation algebras with the natural structure of a continuous field of C*-algebras.