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Electronic Colloquium on Computational Complexity

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TR17-011 | 22nd January 2017 22:01

Quantum entanglement, sum of squares, and the log rank conjecture



For every constant $\epsilon>0$, we give an $\exp(\tilde{O}(\sqrt{n}))$-time algorithm for the $1$ vs $1-\epsilon$ Best Separable State (BSS) problem of distinguishing, given an $n^2\times n^2$ matrix $M$ corresponding to a quantum measurement, between the case that there is a separable (i.e., non-entangled) state $\rho$ that $M$ accepts with probability $1$, and the case that every separable state is accepted with probability at most $1-\epsilon$.
Equivalently, our algorithm takes the description of a subspace $W \subseteq \mathbb{F}^{n^2}$ (where $\mathbb{F}$ can be either the real or complex field) and distinguishes between the case that $W$ contains a rank one matrix, and the case that every rank one matrix is at least $\epsilon$ far (in $\ell_2$ distance) from $W$.

To the best of our knowledge, this is the first improvement over the brute-force $\exp(n)$-time algorithm for this problem.
Our algorithm is based on the *sum-of-squares* hierarchy and its analysis is inspired by Lovett's proof (STOC '14, JACM '16) that the communication complexity of every rank-$n$ Boolean matrix is bounded by $\tilde{O}(\sqrt{n})$.

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