Hidden Bell correlations in the four-level atom
  We extend the Bell inequality known for two qubits to the four-level atom, including an artificial atom realized by the superconducting circuit, and qudit with j=3/2. We formulate the extended inequality as the inequality valid for an arbitrary Hermitian nonnegative 4x4-matrix with unit trace for both separable and entangled matrices. 
Margarita A. Man'ko and Vladimir I. Man'ko
quant-ph