Write the spectral decomposition of the one-qubit density operator as . For a word , the tensor product vector
is an eigenvector of , with eigenvalue
These product vectors form an orthonormal eigenbasis, allowing arbitrary orthonormal choices within degeneracies. Using and ,
Thus the Von Neumann entropy is
This calculation uses the spectral ensemble of the memoryless quantum information source; the originally emitted vectors need not equal these eigenvectors.

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