Z-direction. Quantisation axis

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LagrangeEuler
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I'm a bit confused with this concept of quantisation axis. ##|+\rangle## and ##|-\rangle## spin. And operators ##\hat{S}^+=\hat{S}_x+i\hat{S}_y## and ##\hat{S}^-=\hat{S}_x-i\hat{S}_y## are rising and lowering operator of spin projection. Why z-axis. How is measured that?
 
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Why z-axis.
The coordinate system is arbitrary, you could use any other axis as well. The coordinate system is just a mathematical tool, not something physical.
How is measured that?
What?
 
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. Towards the end of the first lecture for the Qiskit Global Summer School 2025, Foundations of Quantum Mechanics, Olivia Lanes (Global Lead, Content and Education IBM) stated... Source: https://www.physicsforums.com/insights/quantum-entanglement-is-a-kinematic-fact-not-a-dynamical-effect/ by @RUTA
If we release an electron around a positively charged sphere, the initial state of electron is a linear combination of Hydrogen-like states. According to quantum mechanics, evolution of time would not change this initial state because the potential is time independent. However, classically we expect the electron to collide with the sphere. So, it seems that the quantum and classics predict different behaviours!
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