A Refined Algorithm For the EPR model

Abstract

The Einstein-Podolsky-Rosen~(EPR) model is an analogous model of the anti-ferromagnetic Heisenberg model or the equivalent quantum maximum-cut problem, proposed by R. King two years ago. Adjacent qubits in the model prefer symmetric EPR/Bell parings rather than the antisymmetric one, in order to maximize the energy. Recently, two groups independently develop specific algorithms for the highest-energy state with approximation ratio $\frac{1+\sqrt{5}}{4}\approx.809$, based on maximum fractional matchings. Here we try to refine one of the two algorithms by devising homogeneous/quasi-homogeneous fractional matchings, with the aim to distribute quantum entanglement as much as possible. For regular graphs $G_d$, we immediately obtain increasing approximation ratios $r_d$ with $r_2=\frac{3+\sqrt{5}}{6}\approx.872$. For irregular graphs, we show such a refinement could still guarantee nice performance if the fractional matchings are chosen properly.