| course |
course_year |
question_number |
tags |
title |
year |
Quantum Mechanics |
IB |
71 |
IB |
2016 |
Quantum Mechanics |
|
Paper 2, Section II, B |
2016 |
The one dimensional quantum harmonic oscillator has Hamiltonian
$$\hat{H}=\frac{1}{2 m} \hat{p}^{2}+\frac{1}{2} m \omega^{2} \hat{x}^{2},$$
where $m$ and $\omega$ are real positive constants and $\hat{x}$ and $\hat{p}$ are the standard position and momentum operators satisfying the commutation relation $[\hat{x}, \hat{p}]=i \hbar$. Consider the operators
$$\hat{A}=\hat{p}-i m \omega \hat{x} \quad \text { and } \quad \hat{B}=\hat{p}+i m \omega \hat{x} .$$
(a) Show that
$$\hat{B} \hat{A}=2 m\left(\hat{H}-\frac{1}{2} \hbar \omega\right) \quad \text { and } \quad \hat{A} \hat{B}=2 m\left(\hat{H}+\frac{1}{2} \hbar \omega\right) .$$
(b) Suppose that $\phi$ is an eigenfunction of $\hat{H}$ with eigenvalue $E$. Show that $\hat{A} \phi$ is then also an eigenfunction of $\hat{H}$ and that its corresponding eigenvalue is $E-\hbar \omega$.
(c) Show that for any normalisable wavefunctions $\chi$ and $\psi$,
$$\int_{-\infty}^{\infty} \chi^{}(\hat{A} \psi) d x=\int_{-\infty}^{\infty}(\hat{B} \chi)^{} \psi d x$$
[You may assume that the operators $\hat{x}$ and $\hat{p}$ are Hermitian.]
(d) With $\phi$ as in (b), obtain an expression for the norm of $\hat{A} \phi$ in terms of $E$ and the norm of $\phi$. [The squared norm of any wavefunction $\psi$ is $\int_{-\infty}^{\infty}|\psi|^{2} d x$.]
(e) Show that all eigenvalues of $\hat{H}$ are non-negative.
(f) Using the above results, deduce that each eigenvalue $E$ of $\hat{H}$ must be of the form $E=\left(n+\frac{1}{2}\right) \hbar \omega$ for some non-negative integer $n$.