Any pure state |𝚿⟩ of a compound system S = (S₁, S₂ ) with the state space H = H₁ ⊗ H₂ determines a kind of covariance operator D_𝚿 acting in the Cartesian product H = H × H₂. If this operator is positively defined, then it determines a random field valued in H. In this case compound quantum system S can be treated as a classical random field system whose configuration space is not tensor, but Cartesian product space. It happensthat̂ D_𝚿 ≥ 0 and a subquantum process exists if and only if quantum state |𝚿⟩ is not entangled. The technical framework used in this note is already presented by von Neumann