Maybe I read it too quickly but it looks like this is what happens:
Nothing ever detects photon A.
So I’m seeing people entangling photons, then moving far away from each other, then one of the people illuminating a novel message, then the other person instantly reading that message, with no intermediate time needed to wait for FTL concerns to catch up…
Did I read it too quickly?
I’ve read that article a couple times, but can’t quite figure out the setup. It does seem, though, there are actually four photons involved. Two pairs of paired red and infrared photons. The infrared photons are the only ones that might hit the target, and the red ones are the only ones detected. But that’s as much as I can figure out.
If you go to the link to the University of Vienna press release, there’s an image of the experimental setup, but nothing is labeled or explained. For example, where is the “cat” in that image, and what do the three colors represent. Maybe someone can explain it in a little more detail.
There’s a whole lot of quantum weirdness going on, and I’m not sure exactly how it works. But here’s the basic setup:[ol][li]A (green) light beam is split into two entangled photons, A and B.[]Photons A and B are fed through a “non-linear crystal” (NL1 & NL2 in the diagram), which turns each of them into two photons, an infrared and a red photon. Call them A[sub]I[/sub], A[sub]R[/sub], B[sub]I[/sub] and B[sub]R[/sub], respectively.[]Photon A[sub]I[/sub] interacts with the object to be imaged (O in the diagram).[]Photons A[sub]I[/sub] and B[sub]I[/sub] are recombined into a single beam, in such a way that it is impossible to tell whether the photons in that beam came from A (the “imaging” path) or B (the “non-imaging” path.)[]Through the magic of quantum mechanics, this information about the above-mentioned photon paths has to go somewhere. Since both A[sub]I[/sub] and B[sub]I[/sub] are entangled with A[sub]R[/sub] and B[sub]R[/sub], that’s where the information ends up.When A[sub]R[/sub] and B[sub]R[/sub] are recombined & interfered with each other, the cat is revealed.[/ol]This doesn’t violate FTL transmission of information in an obvious way. You could imagine taking the diagram in ZenBeam’s link and splitting the apparatus along the dotted line, making a “transmitter” on the left and a “receiver” on the right. But if you put these two halves many thousands of light-years apart, you’d still have to wait for the photons A[sub]R[/sub] and B[sub]R[/sub] to get from the transmitter to the received before you could figure out what the image was.[/li]
I think. This is pretty woolly and complicated stuff, and I can’t say I’ve fully deciphered it yet. But it’s pretty damn cool all the same.
For anyone else confused, there aren’t any diagrams in my link. I found the diagrams MikeS is referring to, in the article at nature.com (linked to from the Science Daily article). I hadn’t clicked on that link before.
ETA: and it really helps to know the path starts with the green line, and not the other way around as I was assuming.
Whoops, sorry for the confusion. The diagram at nature.com is the one I was working from (actually, I was working from the PDF of the paper itself, but the diagram on the website is the same.)
