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Measurement and collapse
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Observation yields one eigenvalue with Born probability.
Measuring an observable yields one eigenvalue with probability given by the Born rule, and the state then becomes the corresponding eigenstate. How and why the collapse occurs is the measurement problem.
Measuring spin up/down on a superposition gives 50/50.
Repeated measurement gives the same result.
In more detail
Measurement in quantum mechanics projects a state onto one of the eigenstates of the measured observable, and this is called collapse. Repeated measurements then give the same result.
Real-world uses
- Quantum sensors rely on measurement to extract information.
- Quantum cryptography uses measurement to detect eavesdropping.
- Physicists explain why measuring a quantum system gives one definite answer, with probabilities set by the wavefunction.
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Frequently asked questions
What is wavefunction collapse?
The sudden change of the state to an eigenstate upon measurement, which is a postulate in standard quantum theory.
Is collapse physical?
Interpretations differ on this point, and it remains a topic of debate.
What is wavefunction collapse?
The idea that a measurement yields a single result and the state is then the eigenstate for that result, though interpretations differ on what this means.
Related topics in Quantum Mechanics
Variational principle · Dirac notation · Superposition · Schrödinger's cat · Entanglement · Bell's inequalities