Quantum computing: state collapsing to + or - after measurement


I'm going through the quantum computing course and it's a challenge. I was more or less able to follow, but reaching the chapter about cryptography and communication, I read something that to me seems to contradict the fundamentals of the course introduced in the first chapters.

My understanding: Any qubit state can be measured as |0> or |1>, and the measurement then collapses into the measured value in a fully deterministic manner. Therefore, no matter what the qubit's state was, once measured it can only be a "pure" |0> or |1>.

What we read in the communication chapter: " Alice will either get |+> or |-> when she measures, which will collapse Bob's qubit into that state as well. " (the implication is that Bob can then measure and get a |0> or |1> with a 50/50 chance) That doesn't compute for me. Since |+> and |-> are superposition states, where the qubit could be |0> or |1> with a 50/50 chance, how can a measurement make the qubit collapse to that state, instead of |0> or |1> ? How can Alice measure anything else than |0> or |1> ??

Any clarifications would be much appreciated !

Note by Pierric Descamps
1 week, 1 day ago

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