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Comment by Rooster61 | Hacker News Reader

So, something that always bothered me, and my admittedly ignorant understanding of QM as a non-physicist. I understand that the quantum state of a particle can be teleported, and that classical information must be passed to observe the particle and collapse the state correctly.

My question is, if we can't observe the particle, how do we know that that particle is entangled with the original one?

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We take a process that produces entangled pairs, and send the 2 particles to different places. As long as we don't measure their state (whatever that means - TBD :) ), they remain entangled.

But that begs the question of how we know that we have produced an entangled pair. Do we not need a metric to use to make sure they are entangled?

The act of measurement on the second particle destroys the entagled state of the pair. That effect can be measured on the first particle's side (which may be far away... on the other side of the galaxy maybe).

But to see/interpret the effect on the first particle, you need to know what the measurement on the second was, and that information needs to be transferred across the galaxy by conventional means.

Honesty quantum mechanics sounds more like a bug in the universe or some quirk we just don’t understand yet.

Your question boils down to whether or not the quantum circuit that entangles the qubits works correctly. You can simply test the reliability of the quantum circuit by running it multiple times and checking that the outcomes behave as you expect. For example, if your circuit is supposed to generate the entagled pair [00 or 11 with equal probability] you would expect measurements to either be 00 or 11 and never 01 or 10.

Given an unknown state, it's not possible to know if the parts are entangled. There is no experiment to determine this. The "facts" of quantum have this weird disconnect with reality.

If you have access to many copies of the same state, then you can do tomography to learn if the state is entangled or not. But this is a different situation to what you are asking in your question. Actually I'm not entirely sure what your question is asking, but I won't delete this comment just yet.

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