Tight Hardness of the Non-commutative Grothendieck Problem

TL;DR

This paper establishes the NP-hardness of approximating the non-commutative Grothendieck problem within a certain factor, matching the best known algorithms, and provides a new hardness result for the commutative case.

Contribution

It proves a tight NP-hardness of approximation for the non-commutative Grothendieck problem and offers a new hardness result for the commutative Little Grothendieck problem.

Findings

NP-hardness of approximating within 1/2 + ε for the non-commutative Grothendieck problem

Matching the approximation ratio of existing algorithms

New NP-hardness result for the commutative Little Grothendieck problem

Abstract

$\newcommand{\eps}{\varepsilon} $We prove that for any $\eps > 0$ it is $\textsf{NP}$-hard to approximate the non-commutative Grothendieck problem to within a factor $1/2 + \eps$, which matches the approximation ratio of the algorithm of Naor, Regev, and Vidick (STOC'13). Our proof uses an embedding of $\ell_2$ into the space of matrices endowed with the trace norm with the property that the image of standard basis vectors is longer than that of unit vectors with no large coordinates. We also observe that one can obtain a tight $\textsf{NP}$-hardness result for the commutative Little Grothendieck problem; previously, this was only known based on the Unique Games Conjecture (Khot and Naor, Mathematika 2009).