Introduction to Quantum Mechanics · Problem A.29

Afterword — David J. Griffiths

Physics — Quantum Mechanics · hermitian-transformations

Problem

Consider the following hermitian matrix: T=(2i1−i2i1−i2).\mathsf{T} = \begin{pmatrix} 2 & i & 1 \\ -i & 2 & i \\ 1 & -i & 2 \end{pmatrix}. (a) Calculate det⁡(T)\det(\mathsf{T}) and Tr(T)\text{Tr}(\mathsf{T}). (b) Find the eigenvalues of T\mathsf{T}. Check that their sum and product are consistent with (a), in the sense of Equation A.93. Write down the diagonalized version of T\mathsf{T}. (c) Find the eigenvectors of T\mathsf{T}. Within the degenerate sector, construct two linearly independent eigenvectors (it is this step that is always possible for a hermitian matrix, but not for an arbitrary matrix---contrast Problem A.19). Orthogonalize them, and check that both are orthogonal to the third. Normalize all three eigenvectors. (d) Construct the unitary matrix S\mathsf{S} that diagonalizes T\mathsf{T}, and show explicitly that the similarity transformation using S\mathsf{S} reduces T\mathsf{T} to the appropriate diagonal form.

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