- #1

- 3

- 1

- Homework Statement
- The oscillator is in the state ##\lvert \psi (t)\rangle = \dfrac{1}{\sqrt{2}} \left( e^{-i (n-\frac{1}{2})\omega t}\lvert n-1 \rangle + e^{-i (n+\frac{1}{2})\omega t}\lvert n \rangle \right)##.

Calculate the amplitude of oscillation of a classical oscillator of this frequency and energy ##E = \langle\psi (t)\rvert H \lvert\psi (t)\rangle## and show that it differs from your result for ##\langle\psi (t)\rvert x \lvert\psi (t)\rangle## by a factor independent of ##n##.

- Relevant Equations
- ##\lvert \psi (t)\rangle = \dfrac{1}{\sqrt{2}} \left( e^{-i (n-\frac{1}{2})\omega t}\lvert n-1 \rangle + e^{-i (n+\frac{1}{2})\omega t}\lvert n \rangle \right)##

Using the ladder operators I can easily compute ##E = \langle H\rangle = \hbar \omega n##, so I can find the amplitude of the classical oscillator, as ##E = \frac{1}{2} m \omega^2 x_{max}^2##, thus, ##x_{max} = \sqrt{\dfrac{2 E}{m \omega^2}} = \sqrt{\dfrac{2\hbar n}{m \omega}}##.

The expectation value of ##x## can be also easily computed using the ladder operators. I find ##\langle x\rangle = \sqrt{\dfrac{2\hbar n}{m \omega}}\cos{\omega t}##. This is clearly a problem, as I find that ##\langle x\rangle## is time dependent (and the classical solution isn't!). The difference is ##x_{max} - \langle x\rangle = \sqrt{\dfrac{2\hbar n}{m \omega}} (1 - \cos{\omega t})##, which

Can someone help me find where I have made a mistake?

The expectation value of ##x## can be also easily computed using the ladder operators. I find ##\langle x\rangle = \sqrt{\dfrac{2\hbar n}{m \omega}}\cos{\omega t}##. This is clearly a problem, as I find that ##\langle x\rangle## is time dependent (and the classical solution isn't!). The difference is ##x_{max} - \langle x\rangle = \sqrt{\dfrac{2\hbar n}{m \omega}} (1 - \cos{\omega t})##, which

*isn't*independent of ##n##, as the exercise statement says.Can someone help me find where I have made a mistake?

Last edited: