I'm aware that this is a very basic question, yet I hope to get a non-trivial answer(adsbygoogle = window.adsbygoogle || []).push({});

Let's assume to have an apparatus(to make measurements) that is oriented in space.A

We first point it along thezaxis and measure a spin σ_{z}= 1.

Then we rotate the apparatus through an angle of ½π radians (90 degrees), so thatpoints along theAxaxis and we measure the σ_{x}component of the spin.

Classically, we would expect to get zero but we know that the apparatus will give either σ_{x}= 1 or σ_{x}= -1 and we can only foresee that theaverageof these kind of repeated measurements will be zero.

What I'm unclear about is why we conclude that determinism has broken down, what would make us sure that the result of the second measurement is completely random and there is no more fundamental principle that determines that, even though such a mechanism is unknwon?

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# Spins and Qubits

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