B Quantum Mechanics: Superposition of 2 States?

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Assuming Quantum mechanics is a probability theory that describes something real (which should be our first presumption), is there ever only a superposition of 2 states?

The reason I ask this is because:

For calculating the probability of an outcome you square the sum of the probability amplitudes.

If you have outcomes with probability amplitudes a, b, c; then you would get a probability of aa + ab + ac + bb + ba + bc + cc + ca + cb. You don't ever see a term like abc or aab for example.

So assuming QM is a realistic probability theory, you are multiplying all of the ways A can happen plus all of the ways B can happen plus all of the ways C can happen. Assuming these different terms are treated as actual realistic instance possibilities, doesn't this imply that when considering one instance a superposition of only aa or ab or ac or ba or bb or bc or ca or cb or cc is actually possible? In other words you don't actually see superpositions beyond two states at a time.
 
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kurt101 said:
Assuming Quantum mechanics is a probability theory that describes something real (which should be our first presumption), is there ever only a superposition of 2 states?

The quick answer, for some quantum systems, no -- there can be more than 2 states as part of the superposition.
 
StevieTNZ said:
The quick answer, for some quantum systems, no -- there can be more than 2 states as part of the superposition.
Can you provide a reference, example, or anything that might me be able to understand the longer answer? And are you implying that the Born rule is violated or just my interpretation of it is?
 
kurt101 said:
If you have outcomes with probability amplitudes a, b, c; then you would get a probability of aa + ab + ac + bb + ba + bc + cc + ca + cb.

No, you don't. I've already corrected you on this in the other thread where you made this claim; see here:

https://www.physicsforums.com/threa...y-qm-is-incomplete.956222/page-3#post-6064334

Since your OP is based on a mistaken premise, I am closing this thread.
 
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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