| course |
course_year |
question_number |
tags |
title |
year |
Quantum Mechanics |
IB |
37 |
IB |
2002 |
Quantum Mechanics |
|
2.II.18D |
2002 |
Consider a quantum mechanical particle moving in an upside-down harmonic oscillator potential. Its wavefunction $\Psi(x, t)$ evolves according to the time-dependent Schrödinger equation,
$$i \hbar \frac{\partial \Psi}{\partial t}=-\frac{\hbar^{2}}{2} \frac{\partial^{2} \Psi}{\partial x^{2}}-\frac{1}{2} x^{2} \Psi$$
(a) Verify that
$$\Psi(x, t)=A(t) e^{-B(t) x^{2}}$$
is a solution of equation (1), provided that
$$\frac{d A}{d t}=-i \hbar A B$$
and
$$\frac{d B}{d t}=-\frac{i}{2 \hbar}-2 i \hbar B^{2}$$
(b) Verify that $B=\frac{1}{2 \hbar} \tan (\phi-i t)$ provides a solution to (3), where $\phi$ is an arbitrary real constant.
(c) The expectation value of an operator $\mathcal{O}$ at time $t$ is
$$\langle\mathcal{O}\rangle(t) \equiv \int_{-\infty}^{\infty} d x \Psi^{*}(x, t) \mathcal{O} \Psi(x, t),$$
where $\Psi(x, t)$ is the normalised wave function. Show that for $\Psi(x, t)$ given by (2),
$$\left\langle x^{2}\right\rangle=\frac{1}{4 \operatorname{Re}(B)}, \quad\left\langle p^{2}\right\rangle=4 \hbar^{2}|B|^{2}\left\langle x^{2}\right\rangle$$
Hence show that as $t \rightarrow \infty$,
$$\left\langle x^{2}\right\rangle \approx\left\langle p^{2}\right\rangle \approx \frac{\hbar}{4 \sin 2 \phi} e^{2 t}$$
[Hint: You may use
$$\frac{\int_{-\infty}^{\infty} d x e^{-C x^{2}} x^{2}}{\int_{-\infty}^{\infty} d x e^{-C x^{2}}}=\frac{1}{2 C}$$