Free groups and quasidiagonality

Abstract

We use free groups to settle a couple questions about the values of the Pimsner-Popa-Voiculescu modulus of quasidiagonality for a set of operators $\Omega$, denoted by qd$(\Omega)$. Along the way we deduce information about the operator space structure of finite dimensional subspaces of $\mathbb{C}[\mathbb{F}_d]\subseteq C^*_{\ell^p}(\mathbb{F}_d)$ where $C^*_{\ell^p}(\mathbb{F}_d)$ is the so-called $\ell^p$-completion of $\mathbb{C}[\mathbb{F}_d].$ Roughly speaking, we use free groups and qd$(\Omega)$ to put a quantitative face on the two known qualitative obstructions to quasidiagonality; absence of an amenable trace or the presence of a proper isometry. The modulus of quasidiagonality for a proper isometry is equal to 1. We show that qd$(\{\lambda_a,\lambda_b\})\in [1/2,\sqrt{3}/2]$ where $a$ and $b$ are free group generators and $\lambda$ is the left regular representation. In another direction, we use certain $\ell^p$ representations of free groups constructed by Pytlik and Szwarc and a recent result of Ruan and Wiersma to show that qd$(\Omega)$ may be positive, yet arbitrarily close to zero when $\Omega$ is a set of unitaries.