Matricial ranges, dilations, and unital contractive maps

Abstract

Let $J_n$ be the Jordan block of size $n$ with all eigen values zero. Arveson introduced the notion of the matricial range of an operator in his remarkable article called Subalgebras of $C^*$-algebras II (Acta Math, 128, 1972) and established that every unital positive map on the operator system generated by $J_2$ is completely positive. This describes the matricial range of $J_2$ as the set of all matrices with numerical radius at most $\frac{1}{2}$. Later, Choi and Li generalize this result of Arveson and prove that every unital positive map on the operator system generated by any $2\times 2$ matrix or any $3\times3$ matrix with a reducing subspace is completely positive. After fifty years of the above result of Arveson, the matricial range of $J_n$ for $n\geq 3$ has not been characterized. This article aims to investigate this long-standing open problem for $n=3$. We begin by establishing a structure theorem for a dilation of an operator $B$ satisfying $BB^*+B^*B=I$ and then investigate whether every $B\in\mathbb{M}_n$ satisfying $BB^*+B^*B\leq I_n$ admits a dilation $\widetilde{B}$ for which $\widetilde{B}\widetilde{B}^*+\widetilde{B}^*\widetilde{B}=I$. This study plays the central role to the development of this paper. We use this to prove that every unital contractive map on the operator system generated by $J_3$ is $2$-positive and obtain some partial results towards characterizing the matricial range of $J_3$. Next, we study unital contractive maps on operator systems generated by $4\times 4$ normal matrices, and show that this is equivalent to studying a unital contractive map on the operator system generated by $T=\text{diag}(\lambda,-1,i,-i)$, where $\Re{(\lambda)}\geq 0$. We prove that every unital contractive map on the operator system generated by $T=\text{diag}(1,-1,i,-i)$ is completely positive.