Generalized Bell inequalities and frustrated spin systems
  We find a close correspondence between generalized Bell inequalities of a special kind and certain frustrated spin systems. For example, the Clauser-Horn-Shimony-Holt inequality corresponds to the frustrated square with the signs +++- for the nearest neighbor interaction between the spins. Similarly, the Pearle-Braunstein-Cave inequality corresponds to a frustrated even ring with the corresponding signs +...+-. Upon this correspondence, the violation of such inequalities by the entangled singlet state in quantum mechanics is equivalent to the spin system possessing a classical coplanar ground state, the energy of which is lower than the Ising ground state's energy. We propose a scheme which generates new inequalities and give further examples, the frustrated hexagon with additional diagonal bonds and the frustrated hypercubes in n=3,4,5 dimensions. Surprisingly, the hypercube in n=4 dimensions yields an inequality which is not violated by the singlet state. We extend the correspondence to other entangled states and XXZ-models of spin systems. 
Heinz-J\"urgen Schmidt