Excluded states in entangled systems: technical and conceptual aspects

The impossibility of ascribing definite states to the constituents of an entangled system restricts the scope of Pauli's principle in this context. We analyze the conceptual and physical aspects of the problem by studying the actual scope of the principle and the possibility of extending the concept of exclusion in correlated systems. When the entanglement is weak, as in the archetypical case of the Helium atom, the principle can be applied in an approximated way. The concept of non-complete set of properties plays a crucial role in the argument, which also clarifies the physical meaning of the atomic quantum numbers; they are a multi-particle property, not an one-electron one. In contrast, for strong entanglement the excluded states are independent of the principle. We describe some of these states, many times determined by symmetries of the multi-fermion state.