What are some ways of inducing spin polarization?

Question

I saw a talk today and they mentioned how nitrogen-vacancy diamond centers can be used to optically induce spin polarization and now I wonder what other ways there are to induce a spin polarization.

Thank you!

Answer 1

The classic examples of spin polarisation apparatus are the Stern-Gerlach magnets. These produce an inhomogeneous magnetic field that causes up- and down- polarised spins to move in opposite directions. The magnets can be used to split an unpolarised sample into two spatially separated and oppositely polarised samples. The Stern-Gerlach experiment was one of the great experimental demonstrations of energy quantisation in quantum mechanics.

An example closer to the one you suggest would involve a spin that is confined somehow, for example in an atom. Hyperfine levels of trapped ions are often used as qubits for storing quantum information; this is currently one of the most advanced of the many proposed quantum computation architectures. The spin polarisation of an electron with respect to the nucleus can be changed by pumping with coherent electromagnetic radiation, much like the situation you described with a spin trapped in an NV centre.

Answer 2

You can also produce spin polarization inside a semiconductor like GaAs or GaAsN by shining circularly polarized light. There are two types of holes in the valance band - heavy hole and light hole. Apparently, this excitation of the electron from valance band can be either from heavy holes or light holes depending on the polarization of the light illuminating the material. And this excitation process is spin dependent since the total spin angular momentum of heavy holes and light holes are different.

A more common way is by electric injection using a ferromagnetic metal contact to a semiconductor.

Look into these and related sources for a more accurate and detailed discussion: http://arxiv.org/abs/cond-mat/0508222

http://prl.aps.org/abstract/PRL/v87/i1/e016601