but it is possible to generalize the discussion to generic superpositions of Bell states $${\ensuremath{|\Psi\rangle}} = \int\,d^3{\ensuremath{\textbf p}}_1 \,\,\int\,d^3{\ensuremath{\textbf p}}_2\, C_\mu({\ensuremath{\textbf p}}_1,n_1;{\ensuremath{\textbf p}}_2,n_2
){\ensuremath{|B^\mu({\ensuremath{\textbf p}}_1, n_1;{\ensuremath{\textbf p}}_2, n_2)\rangle}}.$$ Moreover, it is actually possible to think of any two-particle state, [*entangled or otherwise*]{}, of the form $${\ensuremath{|\Psi\rangle}} = \int\,d^3{\ensuremath{\textbf p}}_1 \,\,\int\,d^3{\ensuremath{\textbf p}}_2\, f({\ensuremath{\textbf p}}_1,s_1,n_1
;{\ensuremath{\textbf p}}_2,s_2,n_2)a^\dag({\ensuremath{\textbf p}}_1,s_1,n_1)a^\dag({\ensuremath{\textbf p}}_2,s_2,n_2){\ensuremath{|0\rangle}}$$ in terms of Bell states. Clearly a relation between the coefficients given by $$f({\ensuremath{\textbf p}}_1,s_1,n_1;{\ensuremath{\textbf p}}_2,s_2,n_2) =
C_\mu({\ensuremath{\textbf p}}_1,n_1;{\ensuremath{\textbf p}}_2,n_2)({\ensuremath{\tilde\sigma}}^\mu\sigma^2)_{s_1 s_2}$$ works. The relation is invertible as $$C_\mu({\ensuremath{\textbf p}}_1,n_1;{\ensuremath{\textbf p}}_2,n_2) =
\frac{1}{2}\eta_{\mu\nu}f({\ensuremath{\textbf p}}_1,s_1,n_1;{\ensuremath{\textbf p}}_2,s_2,n_2)(\sigma^2{\ensuremath{\tilde\sigma}}^\nu)_{s_2
s_1}$$ Thus $C_\mu({\ensuremath{\textbf p}}_1,n_1;{\ensuremath{\textbf p}}_2,n_2)$ can be written down given $f({\ensuremath{\textbf p}}_1,s_1,n_1;{\ensuremath{\textbf p}}_2,s_2,n_2)$, and vice versa. Note however that in quantum field theory all states transform unitarily $({\ensuremath{|\Psi\rangle}}' = U(\Lambda){\ensuremath{|\Psi\rangle}})$ under Lorentz transformations, no matter how complicated the superposition is. The coefficients $f({\ensuremath{\textbf p}}_1,s_1,n_1;{\ensuremath{\textbf p}}_2,s_2,n_2)$ and $C_\mu({\ensuremath{\textbf p}}_1,n_1;{\ensuremath{\textbf p}}_2,n_2)$ are not operator-valued, and commute with $U(\Lambda)$.

Reduced density matrices, identical particles, and Lorentz-invariance of von Neumann entropy
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We next introduce reduced density matrices and their properties. While it is possible to generalize, we shall choose to concentrate on systems with identical spin $\frac{1}{2}$ massive fermions e.g. electrons.

Given an ${{\ensuremath{\mathbb N}}}$-particle system of identical particles with density matrix 