- #1
tjackson3
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Homework Statement
For a particle of mass m moving in the potential [tex]V(x) = \frac{1}{2}m\omega^2x^2[/tex] (i.e. a harmonic oscillator), it is often convenient to express the position and momentum operators in terms of the ladder operators [tex]a_{\pm}[/tex]:
[tex]x = \sqrt{\frac{\hbar}{2m\omega}}(a_+ + a_-)[/tex]
[tex]p = i\sqrt{\frac{\hbar m \omega}{2}}(a_+ - a_-)[/tex]
Evaluate [tex]<x>,<x^2>,\sigma_x,<p>,<p^2>,\sigma_p[/tex] for the stationary state [tex]\psi_n[/tex], where n = 0 is the ground state, and check that the uncertainty principle is obeyed.
Note that I'm not asking you guys to help with all six of those; I think just a nudge in the right direction on one of them is all I need. Also note that, according to my professor, it is entirely possible to fully complete this problem without evaluating a single integral.
Homework Equations
[tex]\psi_0 = A_0e^{-\frac{m\omega}{2\hbar}x^2}[/tex]
[tex]\psi_n = A_n(a_+)^ne^{-\frac{m\omega}{2\hbar}x^2}[/tex]
[tex]A_n = \left(\frac{m\omega}{\pi\hbar}\right)^{1/4} \frac{(-i)^n}{\sqrt{n!(\hbar\omega)^n}}[/tex]
[tex]a_{\pm} = \frac{1}{\sqrt{2m}}\left(\frac{\hbar}{i}\frac{d}{dx} \pm im\omega x\right)[/tex]
[tex]a_-a_+ - a_+a_- = \hbar\omega[/tex]
[tex]a_-\psi_0 = 0[/tex]
[tex]a_+\psi_n = i\sqrt{(n+1)\hbar\omega}\psi_{n+1}[/tex]
[tex]a_-\psi_n = -i\sqrt{n\hbar\omega}\psi_{n-1}[/tex]
And using this,
[tex]\psi_n = A_n(a_+)^ne^{-\frac{m\omega}{2\hbar}x^2}[/tex]
The Attempt at a Solution
All I'm asking for help on is finding <x>. I think if I can do that, the rest will come fairly easily (if not quickly). At first I tried to do it in the most brute force way possible, using the last equation there in section 2, and literally taking [tex]\psi_n^*x\psi_n[/tex], but that went nowhere, due to the computational difficulties and the difficulty in taking the complex conjugate of that expression in a nice, closed form.
The more promising route seemed to be to find <x> for n = 0 and to use that somehow to generate <x> for some general n. <x> = 0 for n = 0 by parity. Unsure of where to go from there, I tried to use that to find <x> at n = 1. Thus I have (note that [tex]\psi^*_0 = \psi_0[/tex]):
[tex]\[a_+\psi_0 = i\sqrt{\hbar\omega}\psi_1\]
\[\psi_1 = \frac{a_+\psi_0}{i\sqrt{\hbar\omega}}\]\[\psi_1^*x\psi_1 = \frac{a^*_+\psi_0}{-i\sqrt{\hbar\omega}}\sqrt{\frac{\hbar}{2m\omega}}(a_+ + a_-)\frac{a_+\psi_0}{i\sqrt{\hbar\omega}}\][/tex]
I'm stuck here, though, for a couple of reasons. Since operators don't necessarily commute, I'm not sure exactly what liberties I can take in rearranging this into a more convenient form. Additionally, since [tex]a_+[/tex] is an operator, I don't know if it even makes sense to talk about its complex conjugate. Also, short of induction, I don't know how to use this to come up with a formula for a general n.
Thank you so much!
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