# Publications and Preprints

On the maximal dimension of a completely entangled subspace for finite level quantum systems
by
K. R. Parthasarathy
Let $\mathcal{H} _i$ be a finite dimensional complex Hilbert space of dimension $d_i$ associated with a finite level quantum system $A_i$ for $i = i, 1,2, \ldots , k$. A subspace $S \subset \mathcal{H} = \mathcal{H} _{A_{1} A_{2}\ldots A_{k}} = \mathcal{H} _1 \otimes \mathcal{H} _2 \otimes \ldots \otimes \mathcal{H} _k$ is said to be {\it completely entangled} if it has no nonzero product vector of the form $u_1 \otimes u_2 \otimes \ldots \otimes u_k$ with $u_i$ in $\mathcal{H} _i$ for each $i$. Using the methods of elementary linear algebra and the intersection theorem for projective varieties in basic algebraic geometry we prove that $$\max _{S \in \mathcal{E} } \dim S = d_1 d_2\ldots d_k - (d_1 + \cdots + d_k) + k - 1$$ where $\mathcal{E}$ is the collection of all completely entangled subspaces. When $\mathcal{H} _1 = \mathcal{H} _2$ and $k = 2$ an explicit orthonormal basis of a maximal completely entangled subspace of $\mathcal{H} _1 \otimes \mathcal{H} _2$ is given. We also introduce a more delicate notion of a {\it perfectly entangled} subspace for a multipartite quantum system, construct an example using the theory of stabilizer quantum codes and pose a problem.

isid/ms/2004/06 [fulltext]