# if observing entangled particles causes the entanglement to break, how do we know that they were ever entangled?

I have a working understanding of physics, and am working through quantum entanglement. I read that when we observe one entangled particle, we then know the spin of the other.

I also know that when we observe particles, we break the entanglement. If as soon as we observe the "entangled particle" the entanglement breaks.So how do we know that they were ever related/entangled?

• If as soon as you spend money the money is gone, how to you know you ever had money? Aug 21, 2018 at 15:14
• Certain special materials/atoms when stimulated (by a photon) will emit 2 photons ( of lower energy of course ) that are of opposite polarization. Thus these photons are related to each other and the word we use is entangled. If you used this material and create photon pairs ( they emit in opposite directions ) they you have entangled photons. Scientists were excited, by measuring/destroying one photon you knew info about the other. Historically anytime we had info on a photon we had destroyed it, the remaining entangled photon is undestroyed yet we know its polarization! Aug 21, 2018 at 23:50
• I also know that when we observe particles, we break the entanglement. Maybe this is so in the Copenhagen interpretation. In the many-worlds interpretation, all that happens is that the observer becomes entangled as well.
– user4552
Aug 22, 2018 at 0:23

If you measure two entangled polarized photons using polarizing filters that are parallel, then they will always come out the same.

This, of course, requires that you perform the same experiment multiple times. For a single measurement you cannot tell.

Now, this result could be explained by hidden variables, so do a second experiment where the filters are slightly misaligned. The correlations will break Bell's inequality, and hence we have entanglement rather than hidden variables.

• This by itself is most definitely no proof of entanglement. Aug 21, 2018 at 15:01
• You're right. It could be hidden variables. But now add a second experiment that where the polarizing filters are slightly mis-aligned, and the result will break Bell's inequality, thus: entanglement. Thanks, I updated the answer. Aug 21, 2018 at 15:06
• I like this answer much much better now. My only nitpicks are that 1) "always come out the same" could be true or false depending on the specific entangled state of the particles and 2) "slightly misaligned" should be "aligned in some other direction which must be chosen carefully, depending on the specific entangled state of the particles". Better to say that there is some set of measurements you can do that will give you a collection of outcomes violating Bell's theorem. Aug 21, 2018 at 15:15
• Yes. I just hesitate to put in caveats that may obscure the main plot. But feel free to edit the answer if you like. Aug 21, 2018 at 15:17
• @bernander But the main plot is exactly not just to look for anticorrelated outcomes. Aug 21, 2018 at 15:18

It is not possible to prove entanglement by measuring a single entangled pair of particles. Instead, many identically-produced pairs must be measured to show that there is a correlation between the results of measurements on members of the pairs. Because of various experimental uncertainties, it is very rare to find that the measurements "always come out the same"; it's more accurate to say that they come out the same more often than would be expected if they are not entangled.

Certain special materials/atoms when stimulated (by a photon) will emit 2 photons ( of lower energy of course ) that are of opposite polarization. Thus these photons are related to each other and the word we use is entangled. If you used this material and create photon pairs ( they emit in opposite directions ) they you have entangled photons. Scientists were excited, by measuring/destroying one photon you knew info about the other. Historically anytime we had info on a photon we had destroyed it, the remaining entangled photon is undestroyed yet we know its polarization!

• The same is true for any conserved quantity in two body reactions. Aug 22, 2018 at 16:12
• It looks like entangled photons are not restricted to have opposite polarization. You can search for opposite polarization in this link. Actually, I feel like the term 'opposite' is better suited for spin. Sep 26, 2021 at 2:43