# Spin 1/2 rotation and informetry

1. Apr 19, 2010

### Cosmossos

Here is the problem:

How can relate the magnetic field and the rotations and then the lenght of the area? I only know that 2pi rotation give the initial state a pi phase but that's all.
where the hell I get the length from?

thanks

2. Apr 19, 2010

### nickjer

This is just a time evolution problem. You know the initial wavevector will evolve in time according to:

$$|\psi(t)\rangle = \exp(\frac{-iHt}{\hbar})|\psi(0)\rangle$$

Use the hamiltonian for a spin in a magnetic field. It will look very similar to the rotation generator. Also you know the time the particle is in the magnetic field knowing its velocity.

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