I don't know how they did it, but verifying Bells inequality would do it if you made many pairs

Easy-peasy. They will periodically see if the photons, when measured had already been measured by an eavesdropper. In quantum world measurement will affect the entangled system, so it’s impossible for someone to measure (read) the data in between without it being detected at both ends.

> so it’s impossible for someone to measure (read) the data in between without it being detected at both ends.

You give too little information for this to be understandable.

That's one of the fundamentals of quantum mechanics. How that works on the intuitive level, is still being debated. Most of all the mathematics and the experimental results support that with an extremely high level confidence.

Not sure if a HN comment is a right to go back to the fundamental here, either, I'll try to re-iterate it again: reading the quantum information means to "measure it". Measurement alters the state of the system. If someone then "measures" it (i.e. "reads", i.e "eavesdrops"), the communicating parties will find out.

I'm under the impression you can't. You create an environment that is more likely to keep the sets of particles from interacting with anything else and then you create an entangled pair. To confirm the entanglement I believe would take several measurements of several pairs of particles and these measurements would result in a probability that they were still entangled, not a certainty.

I found this explanation:

https://www.quora.com/Is-it-possible-to-detect-if-a-particle...

In my opinion, it's not the entanglement that is interesting or confusing, it's the super positions of both particles before they are measured that is hard to explain... How can two particles be in super positions of two states but then when you measure them they are always complimentary? Either it was "spooky action at a distance" ie (particle A communicated it's measurement result to particle B) or there is "hidden variables" inside the super positions that determines the result of the measurements.

Perhaps by checking their Instagram status.

Check correlation via interference. They'll give consistent results if entangled, and random results if they're not.