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**1. Homework Statement**

We consider two spins 1/2, [itex] \vec{S_{1}}[/itex] and [itex]\vec{S_{2}}[/itex], coupled by an interaction of the form [itex]H=\alpha(t)\vec{S_{1}}*\vec{S_{2}}[/itex]. [itex]\alpha(t)[/itex] is a function of time who approches 0 for |t|-->infinity and takes appreciable values only in the interval of [itex][-\tau,\tau][/itex] near 0.

a) À t=-infinity, the system is in the state |+->. What is the probability of finding the particle in the state |-+> for t=infinity with the first order perturbation theory.

b) Prove that we can solve exactly this problem with the Schrodinger equation. Suggestion : you can start with the exact equation

[itex] $\displaystylei\hbar \frac{db_{n}}{dt}=\sum_{k} exp(iw_{nk}t)W_{nk}b_{k}(t)$ [/itex]

for the b(t) coefficients. Comparing this result with part a), what conditions is required so that perturbation theory is correct?

**2. Homework Equations**

[itex] $\displaystyle \frac{d^{2}b}{dt^{2}}= \frac{db_{n}}{dt}(\frac{d\alpha}{dt}\frac{1}{\alpha}+C_{1}\alpha)+C_{2}\alpha^2 b$ [/itex]

have the solution

[itex]$ b(t)=Aexp(C_{+}\int_{-inf}^{inf}\alpha(t')dt')+Bexp(C_{-}\int_{-infinity}^{t}\alpha(t')dt'$[/itex]

**3. The Attempt at a Solution**

a)

Ok. So, the first thing I did is write [itex]\vec{S_{1}}*\vec{S_{2}}=\frac{1}{2}(S_{1+}S_{2-}+S{1-}S{2+}+2S{1z}S{2z})[/itex].

I calculated that

[itex]\frac{1}{2}<-+|S_{1+}S_{2-}|+->=0[/itex]

[itex]\frac{1}{2}<-+|S_{1-}S_{2+}|+->=\frac{\hbar^2}{4}[/itex]

[itex]\frac{1}{2}<-+|Sz1Sz2|+->=0[/itex]

So, I used

[itex]P=\frac{1}{2}\left|\int_{-inf}^{inf}dt'exp(iwt')\alpha(t)<-+|\vec{S_{1}}*\vec{S_{2}}|+->|\right|^2[/itex]

which finally gave

[itex] P=\frac{\hbar^2}{4}\left| \int_{-inf}^{inf} dt' exp(iwt') \alpha(t') \right| ^2 [/itex]

That's as far as I can go as I don't know [itex] \alpha(t) [/itex].

b) Well... there's the problem. I start with the suggestion,

[itex] $\displaystylei\hbar \frac{db_{n}}{dt}=\sum_{k} exp(iw_{nk}t)W_{nk}b_{k}(t)$ [/itex]

I have no clue of where to start from that. Basically, I have to jump from the last equation I wrote to the equations written in the relevant equations. So, I'll try to make what I think, but this could be very wrong.

[itex] $\displaystylei\hbar \frac{db_{0}}{dt}=exp(iw_{01}t)W_{10}b_{1}(t)$ [/itex]

Let's say that, for the sake of me not writing more in this already long post, that [itex] W(t)=\alpha(t)*K [/itex]

We have [itex] $\displaystylei\hbar \frac{db_{0}}{dt}=exp(iw_{01}t)K\alpha(t)b_{1}(t)$ [/itex]

Now, I'm stuck. I can't take the derivative whit respect to time at each side, because I don't know the b's.

Have a clue? :S

Thank you for reading me!

Erythro73