# Calculating a matrix element & first order shift

1. Nov 12, 2014

### StephvsEinst

So I saw this a moment ago:

How can the second and third terms yield (n+1)+(n-1)=2n and not (n+1)+n=2n+1?
PS: I solved the problem by using [a,a(+)]=1.

Sorry, this is very simple but I cannot figure out what I did wrong.

2. Nov 12, 2014

### dextercioby

It's not clear to me what A|n> is equal to. This is very important, since (dropping the finesse of domain issues linked to unbounded operators on Hilbert space)

$$\langle n, A^{\dagger} A n\rangle = \langle A^{\dagger\dagger} n, A n\rangle = \langle A n, A n\rangle$$

$$\langle n, A A^{\dagger} n\rangle = \langle A^{\dagger} n, A^{\dagger} n\rangle = \sqrt{n+1}^2 = n+1$$

Last edited: Nov 12, 2014
3. Nov 12, 2014