On the extension of positive maps to Haagerup non-commutative $L^p$-spaces

Abstract: Let $M$ be a von Neumann algebra, let $\varphi$ be a normal faithful state on $M$ and let $L^p(M,\varphi)$ be the associated Haagerup non-commutative $L^p$-spaces, for $1\leq p\leq\infty$. Let $D\in L^1(M,\varphi)$ be the density of $\varphi$. Given a positive map $T\colon M\to M$ such that $\varphi\circ T\leq C_1\varphi$ for some $C_1\geq 0$, we study the boundedness of the $L^p$-extension $T_{p,\theta}\colon D^{\frac{1-\theta}{p}} M D^{\frac{\theta}{p}}\to L^p(M,\varphi)$ which maps $D^{\frac{1-\theta}{p}} x D^{\frac{\theta}{p}}$ to $D^{\frac{1-\theta}{p}} T(x) D^{\frac{\theta}{p}}$ for all $x\in M$. Haagerup-Junge-Xu showed that $T_{p,\frac12}$ is always bounded and left open the question whether $T_{p,\theta}$ is bounded for $\theta\not=\frac12$. We show that for any $1\leq p<2$ and any $\theta\in [0,2^{-1}(1-\sqrt{p-1})]\cup[2^{-1}(1+\sqrt{p-1}), 1]$, there exists a completely positive $T$ such that $T_{p,\theta}$ is unbounded. We also show that if $T$ is $2$-positive, then $T_{p,\theta}$ is bounded provided that $p\geq 2$ or $1\leq p<2$ and $\theta\in[1-p/2,p/2]$.