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## Main Question or Discussion Point

I know that if you have two entangled particles and measure ones spin, the other one must have the opposite spin, but what if you have 3 particles entangled with each other, what would happen?

- Thread starter Honorable_Death
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I know that if you have two entangled particles and measure ones spin, the other one must have the opposite spin, but what if you have 3 particles entangled with each other, what would happen?

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Zz.

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In the case of multi-particle states, things are a lot more complicated. There are several inequivalent types of multi-particle entanglement that cannot be converted into each other by local operations and classical communication. Multi-particle states can also be used to derive very elegant proofs of Bell's theorem that don't involve inequalities.

Multi-particle entangled states are actually pretty generic in modern physics, occuring as ground states of many natural many-body Hamiltonians. However, it is only recently that people have begun to use entanglement theory to study the properties of these systems. Older treatments were all cast in terms of correlation functions, which are not completely unrelated to entanglement, but they do not clearly distinguish classical correlations from "genuinely quantum" correlations due to entanglement. In particular, entanglement studies have lead to new algorithms for simulating some of these systems on classical computers, which people did not know how to do before. This is currently a very active area of research.

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DrChinese

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FYI: Entangled photon spin is not always anti-correlated. For instance, with Parametric Down Conversion (PDC), there is Type I and Type II. Type I photon pairs have the same polarization, and that is perpendicular (orthogonal) to the input photons. Type II photon pairs consist of one perpendicular and one parallel to the input beam.Honorable_Death said:I know that if you have two entangled particles and measure ones spin, the other one must have the opposite spin, but what if you have 3 particles entangled with each other, what would happen?

There are a variety of ways to get multiples (>2) which are entangled. With PDC, you can get this when 2 input photons degenerate into 2 pairs within a narrow time window. This is a spontaneous result, one which occurs only occasionally. In one experiment, they got 4 entangled photons out once in 24 seconds on average.

Reference: "Experimental observation of four-photon entanglement from down-conversion".