Better local hidden variable models for two-qubit Werner states and an upper bound on the Grothendieck constant $K_G(3)$

TL;DR

This paper develops improved local hidden variable models for two-qubit Werner states, establishing a new upper bound on the Grothendieck constant $K_G(3)$ and demonstrating the models' applicability to various measurement types.

Contribution

The authors present new analytical LHV models for Werner states, leading to a tighter bound on $K_G(3)$ and adaptable techniques for other entangled states.

Findings

Correlations are local for Werner states with $v \,\approx\, 0.683$.

New upper bound on Grothendieck constant $K_G(3) \leq 1.4644$.

LHV models constructed for arbitrary POVMs at $v \approx 0.4553$.

Abstract

We consider the problem of reproducing the correlations obtained by arbitrary local projective measurements on the two-qubit Werner state $\rho = v |\psi_- > <\psi_- | + (1- v ) \frac{1}{4}$ via a local hidden variable (LHV) model, where $|\psi_- >$ denotes the singlet state. We show analytically that these correlations are local for $ v = 999\times689\times{10^{-6}}$ $\cos^4(\pi/50) \simeq 0.6829$. In turn, as this problem is closely related to a purely mathematical one formulated by Grothendieck, our result implies a new bound on the Grothendieck constant $K_G(3) \leq 1/v \simeq 1.4644$. We also present a LHV model for reproducing the statistics of arbitrary POVMs on the Werner state for $v \simeq 0.4553$. The techniques we develop can be adapted to construct LHV models for other entangled states, as well as bounding other Grothendieck constants.