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We consider the problem of reproducing the correlations obtained by arbitrarylocal 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 thesecorrelations 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 purelymathematical one formulated by Grothendieck, our result implies a new bound onthe Grothendieck constant $K_G(3) \leq 1/v \simeq 1.4644$. We also present aLHV model for reproducing the statistics of arbitrary POVMs on the Werner statefor $v \simeq 0.4553$. The techniques we develop can be adapted to constructLHV models for other entangled states, as well as bounding other Grothendieckconstants.

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