- #1

Hazz

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## Homework Statement

Hi, I am just trying to wrap my head around using path integrals and there are a few things that are confusing me. Specifically, I have seen examples in which you can use it to calculate the ground state shift in energy levels of a harmonic oscillator but I don't see how you can find the shift for any other energy level. I am aware you don't need to use a path integral for this but I am hoping to get a better understanding of them by doing this.

## Homework Equations

I know that you can calculate the energy shift in the ground state using the partition function by using

[itex] E_0=-lim_{\beta\rightarrow\infty}\frac{1}{\beta} log K_E[J] |_{J=0}[/itex]

I know if I am using the Hamiltonian

[itex]\mathcal{H}=\frac{\hat{p}^2}{2m}+\frac{1}{2}m\omega^2\hat{q}^2+\gamma\hat{q}^4[/itex]

then to first order you can just write ## K_E[J] ## as

[itex] K_E[J]=K_E^0[J]-\gamma \int d\tau (\frac{\delta}{\delta J(\tau)})^4K^0_E[J]|_{J=0}[/itex]

where

[itex] K_E^0[J]=\frac{C}{2}\int d\tau d\tau'J(\tau)G(\tau,\tau')J(\tau')[/itex]

and G is the green's function.

## The Attempt at a Solution

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So you can evaluate this all and work it out for the ground state which I don't have a problem with, but what confuses me is how this works for anything that isn't the ground state. Clearly the ##\beta\rightarrow\infty## limit is always going to put you there.

I understand that you probably need to use

[itex] \langle 0 |\hat{a}e^{-\beta H} \hat{a}^\dagger |0\rangle [/itex]

In some capacity but I don't really see where to proceed from there. Do you have to worry about commuting ##\hat{a}^\dagger## through the exponential?

I don't really know where to look for guidance with this sort of thing so any help would be appreciate. Thank you very much!