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Quantum Interference Between Light Sources Separated by 150M Kilometers (aps.org)
58 points by wwarner 59 days ago | hide | past | web | favorite | 24 comments

150 million kilometers = 150 gigameters

that helped

or about 1 au

"The experiment can be further extended to a larger scale using photons from distant stars and open a new route to quantum optics experiments at an astronomical scale."

This would be interesting as a distant star would exhibit red-shift unlike something as close as our sun. So until further data such as this, we just have the single experiment. Which from my limited understanding, would give the data to draw some solid conclusions. Exciting stuff, and I could only speculate upon the results, though somebody more up upon the matter may well have some better insights.

Though it may well be a way to measure distances more accurately would be my limited take away from this. Maybe not, but certainly something going on here that will enhance our understanding of physics on all levels.

An ars-technica writeup of the original paper: https://arstechnica.com/science/2019/08/identical-photons-ge...

Ok, does anyone have a simple guide to what it means?

Yeah, I'm really waiting for some of the physics experts on HN to give their take on this. I have a limited understanding of the basic idea behind entanglement, but I can't extend it to this case.

I gather this experiment somehow entangles photons from the sun with photons produced by the experimenters, but they're already here when that happens right? So how does the distance come into play. I think I'm probably looking at it too "classically".

The diagrams helped clear it up for me (I think.) Yes, they are entangling them on Earth and this seems to be testing the idea that an atom created here is the same/similar enough to one from the sun for this to take place. "Figure 4: Entanglement fidelity measurement of the entangled photon pairs with no common history." It's something I would have assumed as a novice but it's interesting that they thought to test it. If you couldn't entangle them, what would that mean?

It looks like the keyword in the title was light "sources" that are 150 Million Kilometers apart. I didn't catch that at first.

> created here is the same/similar enough to one from the sun for this to take place

Exactly. They are testing if photons on Earth are the same as those produced in the Sun - a very simple test of the translation invariance of the laws of quantum physics.

> If you couldn't entangle them, what would that mean?

If such experiments failed despite repeated efforts, then maybe the laws of quantum physics are not invariant under translation as far as 150M kilometers.

Note that we communicate with Voyager crafts for instance using classical EM waves, so presumably the laws of classical EM work the same as far as the edge of the solar system.

I'm not an expert, but I thought the paper was more novel than that, in that it challenges my picture of entanglement. The paper demontrates entangled behavior with photons with no common history. They are "identical" in their conserved properties, rather than "entangled" which to me implies that their conserved properties sum to the values that existed before the event that produced the entangled pair.

To me, the surprise is that identical pairs always take the same path.

This might challenge your picture of entanglement, but definitely not that of a physicist. Otherwise, I can't make head or tail of your statements.

I'll try to restate. I was under the impression that entanglement implied what the paper calls "common history", such as when an excited calcium atom decays to its ground state and produces two entangled photons. I also understood entanglement to be particles with some opposite properties, so that the sum of say the angular momentum of the system before and after the decay remains constant. So it was news to me that (a) photons without a common history, and (b) photons indistinguishable in all degrees of freedom could be entangled.

> So it was news to me that (a) photons without a common history, and (b) photons indistinguishable in all degrees of freedom could be entangled.

Oh no. It is definitely possible to entangle two photons from two different sources. Say two hydrogen atoms emit a photon each; then you just need a non-linear crystal to entangle them, or use linear crystals and post-select good results. Here is a recent example for the latter for quantum repeaters [1], though this has been done for decades now. And making two different photon sources indistinguishable (or controllably-distinguishable) is like the first thing you do in a quantum optics lab.

> I also understood entanglement...

That's not a correct way to think of entanglement. Two particles are (maximally) entangled if

(1) you do the same measurement on the two particles, the results are (perfectly) correlated.

(2) if you do orthogonal measurements on the two particles, the results are (perfectly) uncorrelated.

And all this happens in a way that no local classical theory can explain.

[1] https://arxiv.org/pdf/1908.05351.pdf

Thanks, ya done me a solid!

I have a limited understanding, but I think this is just further reproduction of the same science. I believe there's some controversy over non-locality in that entanglement and spooky action at a distance is still a result of hidden local variables. By increasing the distance between the sources of the photons, you would give more evidence to nonlocality. Please correct me if this is offbase.

"Good" [1] Bell experiments have already been performed by several groups, giving us high confidence that any local hidden variables theory is incorrect. So, there is very little controversy over the matter. Moreover, the Bell test here is not a "good" Bell test, since the results were post-selected i.e. only some experimental data points were selectively used.

The important part of the paper is the first result about two-photon interference. It shows that two photons from Sun and Earth can be made indistinguishable from each other and hence show maximum interference. This is evidence for the postulate of quantum theory that all photons (or other fundamental particles) can be made identical to each other.

[1] that take care of all prerequisite conditions of Bell's theorem. Also called loop-hole free Bell experiments. See https://en.wikipedia.org/wiki/Bell_test_experiments#Hensen_e...

Non-locality means that the way the probability of presence (wave function) of a photon is determined is by trying out all possible paths through the entire universe at once rather than only in regions nearby the photons?

The path integral formulation is the most prevalent and successful version of quantum physics (this is what you describe in the second half). Non locality is different; it supposes that there are hidden mechanisms we have not uncovered (and may but be able to uncover) instead which describes our world. It must be non local to get the observed experimental results. Non local means data about the universe is teleporting faster than (conventional) causality allows i.e. the speed of light

Either case seems to require faster-than-light or instant communication between alternative paths that are being tried out, so the distinction is whether the combination of different paths is mediated by another faster-than-light (non-local) particle or whether the combination is axiomatic to the laws of the universe without a particle mediating it?

No, the path integral formulation does not require faster than light communication. Time is an explicit parameter

Imagine entangled data transmission... having a matrix of entangled particles, using known truths to illuminate what has up until now have been unknowable things. Wow, it’s an amazing time to be alive.

A bug in the simulator?

This wasn't a simulation, unless your going down the universe is a simulation avenue and this is a bug in the matrix route.

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