Completely bounded maps into certain Hilbertian operator spaces

Abstract

We prove a factorization of completely bounded maps from a $C^{\ast }$-algebra $A$ (or an exact operator space $E\subset A$) to ${\ell }_2$ equipped with the operator space structure of $(C,R{)}_{\theta }$ ($0<\theta <1$) obtained by complex interpolation between the column and row Hilbert spaces. More precisely, if $F$ denotes ${\ell }_2$ equipped with the operator space structure of $(C,R{)}_{\theta }$, then $u:A\to F$ is completely bounded iff there are states $f,g$ on $A$ and $C>0$ such that \[ \forall a\in A\quad \|ua\|^2\le C f(a^*a)^{1-θ}g(aa^*)^θ.\] This extends the case $\theta =1/2$ treated in a recent paper with Shlyakhtenko. The constants we obtain tend to 1 when $\theta \to 0$ or $\theta \to 1$. We use analogues of "free Gaussian" families in non semifinite von Neumann algebras. As an application, we obtain that, if $0<\theta <1$, $(C,R{)}_{\theta }$ does not embed completely isomorphically into the predual of a semifinite von Neumann algebra. Moreover, we characterize the subspaces $S\subset R\oplus C$ such that the dual operator space $S^{\ast }$ embeds (completely isomorphically) into $M_{\ast }$ for some semifinite von neumann algebra $M$: the only possibilities are $S=R$, $S=C$, $S=R\cap C$ and direct sums built out of these three spaces. We also discuss when $S\subset R\oplus C$ is injective, and give a simpler proof of a result due to Oikhberg on this question. In the appendix, we present a proof of Junge's theorem that $OH$ embeds completely isomorphically into a non-commutative $L_1$-space. The main idea is similar to Junge's, but we base the argument on complex interpolation and Shlyakhtenko's generalized circular systems (or ``generalized free Gaussian"), that somewhat unifies Junge's ideas with those of our work with Shlyakhtenko.

Minor corrections of misprints and addition of an introduction