Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2016/iii/paper-323/4/ii/solution

The coefficient matrix in the computational basis is
For a bipartite pure state, the partial traces give and . Therefore the requested reduced density matrices are
The columns of are orthogonal. Their norms are and , so normalizing them gives the orthonormal set
With and , a Schmidt decomposition is
Expanding this expression reproduces the minus sign in the coefficient. The two reduced density matrices both have eigenvalues and , consistent with these Schmidt coefficients.

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