# Spin rotation measurement

#### gespex

Hello all,

In the following article I found through google, they explain how spin is measured:
http://news.softpedia.com/news/Why-Is-Quantum-Mechanics-So-Weird-40009.shtml

I understand how the Stern-Gerlach experiment works, but the article doesn't go into much detail about the next experiment where it describes the measurement of the spin being more than 360 degrees. They are talking about "rotating the external field", but what exactly does that mean? Does that mean a graduate bending of the magnetic field, such that halfway the north and south poles are reversed until the end where it is back to the starting magnetic field? Or is it done by some other method?

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#### Bill_K

Softpedia doesn't explain it very well. The point of Rabi's experiment was to accurately measure the magnetic moment. He had a Stern-Gerlach magnet which separated the spins followed by a second one to bring them back together. In between he subjected both beams to a rotating magnetic field with frequency ω. When ω was just the right frequency to cause a spin flip, the beams would fail to get all the way through.

#### gespex

Okay thanks for your answer. But what exactly is a rotating magnetic field in this context? Does it literally just mean a magnet whose north and south pole make a 360-degree rotation at a high speed, making a circle around the actual magnetic field?

#### sugar_scoot

If I have a strong Magnetic field B0 in the z direction then any atoms inside will have their magnetic dipoles align in the z direction, but will rotate in the x-y plane at a larmor frequency ω. Because the dipoles are rotating in the x-y plane with a frequency ω, you will need to apply a not as strong B1 field that is rotating at the same frequency ω for a short time to tip (flip) the dipole some small angle.

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