# Creation and annihilation operators

by Lindsayyyy
Tags: annihilation, creation, operators
 P: 213 Hi all 1. The problem statement, all variables and given/known data Show: $$(a^\dagger a)^2=a^\dagger a^\dagger a a +a^\dagger a$$ wheres: $$a= \lambda x +i \gamma p$$ $$a^\dagger= \lambda x -i \gamma p$$ 2. Relevant equations - 3. The attempt at a solution Well, I haven't got much. I just tried to use the stuff given, put it into my equation and solve it, but I don't get to the right side. I calculated a+a first $$a^\dagger a ={\lambda}^2x^2 + \frac {1}{2} I + \gamma^2 p^2$$ But when I now try to calculate the square of that term I get lost. If I square it I get to: $$(a^\dagger a)^2= \lambda^4x^4+\gamma^4p^4 +\lambda^2 \gamma^2 (x^2p^2+p^2x^2)-\lambda^2 x^2 -\gamma^2 p^2 +\frac 1 4 I$$ Can anyone help me with this? I don't know what to do now/ If I'm on the right way. Thanks for your help edit: I is the identity matrix
 P: 950 (a$^{+}$a)$^{2}$ = a$^{=}$aa$^{+}$a =a$^{+}$(aa$^{+}$)a. Now try to get aa$^{+}$ in terms of a$^{+}$a.
P: 213
 Quote by grzz (a$^{+}$a)$^{2}$ = a$^{=}$aa$^{+}$a =a$^{+}$(aa$^{+}$)a. Now try to get aa$^{+}$ in terms of a$^{+}$a.
what does your a^= mean? I don't know this sign.

edit:

$$aa^\dagger=\lambda^2 x^2 + \frac 1 2 I +\gamma^2 p^2$$

 P: 950 Creation and annihilation operators I used the plus sign instead of the dagger sign. You are supposed to see a with + as a superscript wherever you see a with ^ as a superscript. Sorry for the trouble in the notation used.
 P: 213 I see a = not a + but I think it's just a typing mistake. so what you mean $$...= a^\dagger a a^\dagger a=a^\dagger (aa^\dagger) a$$ is that what you meant? I calculated my aa+ (see my post) and a+a is the same, though there's a minus infront of the I
P: 624
 Quote by Lindsayyyy I calculated my aa+ (see my post) and a+a is the same, though there's a minus infront of the I
"The same"? That difference in sign makes all the difference. Can you now relate $aa^{\dagger}$ to $a^{\dagger}a$?
 P: 213 With "the same" I meant the rest of the terms. Unlucky word choice from my side I guess. What do you mean with "can you now relate aa+ to a+a" do you mean that I should calculate $$(aa^\dagger) (a^\dagger a)$$ thanks for the help
 P: 624 No, I meant the relationship between $aa^{\dagger}$ and $a^{\dagger}a$ ie express one of them in terms of the other.
 P: 950 Yes that is what I meant. Allow me to use a+ to mean a with + as a superscript. So aa+ = a+a + 1. Now one more step.
 P: 213 Do you mean something like $$a^\dagger a = 2\lambda^2 x^2 +2 \gamma^2 x^2 -a a^\dagger$$ edit: nevermind, fail lol. I guess you meant what grzz just said. I have to eat dinner now. I'll try later or tomorrow and come back when I have further questions (I'm pretty sure I will have some :( ) Thanks so far for the help guys.

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