For any semifinite von Neumann algebra ${ \mathcal M}$ and any $1\leq p<\infty$, we introduce a natutal $S^1$-valued noncommutative $L^p$-space $L^p({ \mathcal M};S^1)$. We say that a bounded map $T\colon L^p({ \mathcal M})\to L^p({ \mathcal N})$ is $S^1$-bounded (resp. $S^1$-contractive) if $T\otimes I_{ S^1}$ extends to a bounded (resp. contractive) map $T\overline{ \otimes} I_{ S^1}$ from $ L^p({ \mathcal M};S^1)$ into $L^p({ \mathcal N};S^1)$. We show that any completely positive map is $S^1$-bounded, with $\Vert T\overline{ \otimes} I_{ S^1}\Vert =\Vert T\Vert$. We use the above as a tool to investigate the separating maps $T\colon L^p({ \mathcal M})\to L^p({ \mathcal N})$ which admit a direct Yeadon type factorization, that is, maps for which there exist a $w^*$-continuous $*$-homomorphism $J\colon{ \mathcal M}\to{ \mathcal N}$, a partial isometry $w\in{ \mathcal N}$ and a positive operator $B$ affiliated with ${ \mathcal N}$ such that $w^*w=J(1)=s(B)$, $B$ commutes with the range of $J$, and $T(x)=wBJ(x)$ for any $x\in { \mathcal M}\cap L^p({ \mathcal M})$. Given a separating isometry $T\colon L^p({ \mathcal M})\to L^p({ \mathcal N})$, we show that $T$ is $S^1$-contractive if and only if it admits a direct Yeadon type factorization. We further show that if $p\not=2$, the above holds true if and only if $T$ is completely contractive.
Categories: math.OA