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I have a question about the EPR phenomenon of the quantum-mechanics. When you observe the EPR phenomenon, you separate two spins in the singlet state. Then, you and your fellow independently observe the direction of each spin, and you compare the directions.

If one spin is in New York, and another is, for example, in Tokyo, how do you assign the directions? Because the earth is round, I do not know which direction is opposite to another.

In reality, it is difficult to make such a large separation without losing correlation. However, in principle, it is irrelevant how large the distance is.

Thank you for your attention.

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In practice, this is accomplished by simply testing to find which relative angles provide the highest expected correlation (or anti-correlation). There are a variety of things that can affect a particle’s spin in flight that allow it to remain entangled.

This is noticeable, for example, when entangled photons are routed through fiber. Compensation is made in experiments in which there is over 100 kilometers of rolled fiber, i.e. a lot of twists.

https://arxiv.org/abs/0801.3620

There have also been ground to satellite tests of entanglement, with similar issues of alignment. These have been accomplished over distance of greater than 1000 kilometers.

https://arxiv.org/abs/1707.00934

So not exactly from Tokyo to New York, but the principle is the same. “Opposite” is defined by relative angles and statistical correlation.

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  • $\begingroup$ Thank you so much. That is just I wanted to know. $\endgroup$
    – Masato Morifuji
    Commented Aug 31, 2023 at 23:01

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