  Our investigation aims to study the specific role played by entanglement in
the quantum computation process, by elaborating an entangled spin model
developed within the 'hidden measurement approach' to quantum mechanics. We
show that an arbitrary tensor product state for the entity consisting of two
entangled qubits can be described in a complete way by a specific internal
constraint between the ray and density states of the two qubits. For the
individual qubits we use a sphere model representation, which is a
generalization of the Bloch or Pauli representation, where also the collapse
and noncollapse measurements are represented. We identify a parameter r in
[0,1], arising from the Schmidt diagonal decomposition, that is a measure of
the amount of entanglement, such that for r = 0 the system is in the singlet
state with 'maximal' entanglement, and for r = 1 the system is in a pure
product state.
