 A hydrogen atom may be represented in ordinary non-relativistic quantum mechanics as a quantum system composed of two subsystems: an electron e and a nuclear proton p. When isolated, its quantum state may be represented by a vector \(\Psi\) in a space H constructed as a tensor product of spaces \(H_{p}\) and \(H_{e}\) used to represent states of \(e, p\) respectively. The states of \(e, p\) are then defined as entangled if and only if   