# Exp operator expansions

1. Oct 19, 2006

### MaverickMenzies

Does anyone of an exp operator expansion identity that for operators of the form:

$$\exp \left ( \hat{A} \hat{B} \right ) [\tex] For example, I know of the BHC decomposition: [tex] e^{\hat{A} + \hat{B}} = e^{-\frac{ [ \hat{A}, \hat{B} ]}{2}} e^{\hat{A}} e^{\hat{B}} [\tex] I've heard that there exists such an expansion for the first case but i can't find it anywhere. I'd be grateful if anyone could supply me a reference or at least point me to it. Cheers. Last edited by a moderator: Oct 19, 2006 2. Oct 19, 2006 ### StatMechGuy If they commute, then yes, each term in the series is just [tex]A^n B^n$$. Otherwise you have to just calculate a LOT of commutators and hope something nice works out. What specific problem were you trying to tackle?

3. Oct 19, 2006

### MaverickMenzies

I am trying to decompose the Cross Kerr unitray transformation i.e. the unitary operator:

$\exp \left ( i \varphi \hat{n}_{1} \hat{n}_{2} \right )$

Where the operators in question are photon number operators in two different radiation modes.

I understand that there exists a decomposition (besides the taylor series expansion) of this operator using the Hubbard-Stratonovich decomposition. However, this decomposition requires that the creation and annihilation operators are fermomic. I, however, need a corresponding decomposition for bosonic operators.

Any suggestions?

4. Oct 20, 2006

### Epicurus

The formula for exponential operator expansions of this sort is called the Baker-Campbell-Haussdorf formula which holds regradless if the operators are bosonic or fermionic and is a result of the mentioned taylor series expansion.

5. Oct 20, 2006

Staff Emeritus

Too quick off the mark, Epicurus. The OP already knew that:

and knew that BCH didn't handle the case he was asking about.

6. Oct 20, 2006

### Severian

Could you use the operator product expansion, and then the BCH formula?

7. Oct 24, 2006

### MaverickMenzies

I don't see how. The BHC formula states that:

[TEX]e^{\hat{A} + \hat{B}} = e^{- [ \hat{A}, \hat{B} ]/2} e^{\hat{A}} e^{\hat{B}}[/TEX]

when [TEX][\hat{A},[\hat{A},\hat{B}]] = [\hat{B},[\hat{A},\hat{B}]] [/TEX]

i.e. it refers to the sum of operators in an expotential. I want to decompose a product of (commuting operators).

8. Oct 24, 2006

### reilly

Yes, BCH, or CBH or whatever you want to call it, states that

exp(A +B) = exp(A)exp(B)exp([A,B]/2),

and this hold only when [A,[A,B]] = [B,[A,B]] = 0.

There's nothing in this theorem that precludes fermion operators.
(See Mandel and Wolf, Optical Coherence and Quantum Optics, p 319-320 for a detailed discussion.)

Regards,
Reilly Atkinson