PC3130 AY23/24 Quiz 5
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PC3130 AY23/24
Quiz 5
Due in class, at the beginning of class on Mon 30 Oct, 2023
1. Perturbation theory: three level system
This question is adapted from Griffith’s Introduction to Quantum Mechanics.
Consider a quantum system with just three linearly independent states. Suppose the Hamiltonian, in the matrix form, is
(1)
where V0 is a real, positive constant, and E is some small positive number (E ≪ 1). We suppose that the part of the matrix arising from E is a perturbation.
(a) Write down the eigenvectors and eigenvalues of the unperturbed Hamiltonian (E = 0).
(b) Solve for the exact eigenvalues of H. Expand each of them as a power series in E up to second order.
(c) Use first- and second-order non-degenerate perturbation theory to find the approx- imate eigenvalue for the state that grows out of the non-degenerate eigenvector of H0 . Compare your results with those in part (b).
(d) Use degenerate perturbation theory to find the first-order correction to the two ini- tially degenerate eigenvalues. Compare your results with those in part (b).
(e) Find, using perturbation theory (refer to the Appendix below), the second-order cor- rections to the two initially degenerate eigenvalues. You are required to show your working clearly. Compare your results with those in part (b).
2023-10-31