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1 year ago in Physics , Quantum Mechanics By Rinku

When is rotation of the quantization axis allowed in physics?

This isn't just a textbook technicality. In experimental setups like NMR or quantum computing with trapped ions, the alignment of quantization axes with external fields is crucial. I need to clarify the principles symmetry considerations, invariance requirements to explain when such a rotation is a valid operation versus when it constitutes a real physical change.

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By Rahul K Answered 9 months ago

Having worked with spin systems in both academic and applied settings, I can say the key is symmetry. You are allowed to rotate the quantization axis if the Hamiltonian of your isolated system is rotationally invariant meaning there's no preferred direction, like in free space. In practice, I've seen the axis fixed by an external field (B-field in NMR, crystal field in solids). Rotating the axis there changes the physics because you're redefining it relative to that breaking field. The allowed rotation is a unitary transformation on the state, reflecting a change in our description, not the system's state, provided the symmetry holds.

 

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