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Physicists Create Quantum Link Between Photons That Don't Exist at the Same Time

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11–20 of 56 posts

Re: Physicists Create Quantum Link Between Photons That Don't Exist at the Same Time

#11

Does this work only in one direction? Is there any reason to see the first measurement as causal? Could the second measurement be interpreted as causal that then propagates backward in time? I'm really trying to get at if the linear description of events in the article is just a convenient simplification, or if physicists thinks that there is time order here.

The first measurement is in no way causal. It's just that the whole point of the experiment was to show that entanglement occurs even when the photons don't coexist. If you didn't make the first measurement, then there wouldn't be anything different from the original experiment.

Re: Physicists Create Quantum Link Between Photons That Don't Exist at the Same Time

#12

Does this work only in one direction? Is there any reason to see the first measurement as causal? Could the second measurement be interpreted as causal that then propagates backward in time? I'm really trying to get at if the linear description of events in the article is just a convenient simplification, or if physicists thinks that there is time order here.

I don't know precisely what you're asking because the whole point is that there are three measurements (measure qubit 1, entangling-measure qubits 2 and 3, and measure qubit 3) whose time-order does not matter. There is a nice theorem in quantum computing that you can defer all measurements to the end of a set of operations if you replace all of the classical logic with appropriate quantum gates; conversely, you can measure as soon as you want if those quantum gates can be replaced with classical logic. Here, conveniently, there are no gates; so the theorem says that you get entangled results even though the entangled things don't exist simultaneously, because they could have if you'd pushed the measurement to the end of the experiment.

The linear description of events describes the time-order that the actual experiment used.

Re: Physicists Create Quantum Link Between Photons That Don't Exist at the Same Time

#13
post #2

At the risk of being "that guy," a few thoughts popped into my head after reading this article. I couldn't help but reflect on the possibility of us just completely misunderstanding how the universe works. It wouldn't be the first time in history. What if: Our concept of time and matter is wrong, or our idea of quantum physics is interesting and useful, but ultimately wrong. Perhaps I'm just a simpleton but part of m…

It is sobering to realize there are some things I'll never understand. But it's OK. Ignorance let's us have more fun speculating. I was wondering, are two entangled particles actually the same particle, where one or both "took a time machine" to get to the other place?

Re: Physicists Create Quantum Link Between Photons That Don't Exist at the Same Time

#16

Does this work only in one direction? Is there any reason to see the first measurement as causal? Could the second measurement be interpreted as causal that then propagates backward in time? I'm really trying to get at if the linear description of events in the article is just a convenient simplification, or if physicists thinks that there is time order here.

My best understanding is that in general, issues related purely to quantum entanglement are entirely unrelated to causality. As noted in the article, even the "spooky action at a distance" that made Einstein so uncomfortable cannot be used to transmit information faster than light. I haven't checked the math, but I assume that the same sort of properties mean that this system couldn't be used to transmit information into the past.

So issues of causality don't really arise: this is just a neat demonstration that the weird correlations implied by quantum mechanics can be separated in a "direction" that hadn't been tested before. (That is, there's definitely time passing in this system and there is definitely entanglement between objects existing at different times, but there's also no real tension between those facts.)

Re: Physicists Create Quantum Link Between Photons That Don't Exist at the Same Time

#17
post #2

At the risk of being "that guy," a few thoughts popped into my head after reading this article. I couldn't help but reflect on the possibility of us just completely misunderstanding how the universe works. It wouldn't be the first time in history. What if: Our concept of time and matter is wrong, or our idea of quantum physics is interesting and useful, but ultimately wrong. Perhaps I'm just a simpleton but part of m…

Anyone who claims they understand Quantum Physics, doesn't understand Quantum Physics – Feynman

Re: Physicists Create Quantum Link Between Photons That Don't Exist at the Same Time

#18
post #8
post #5

Earlier quoted context omitted.

Sure, but what are the odds the universe fundamentally defies intuitive logic versus the odds our small brains currently can't see the full picture but in the meantime have invented insane (but cool) mathematical machinations? I guess it's unknowable.

But what do you think physicists do all day? Exactly what you just described. Unfortunately we pretty much have the answer, and it's kind of a resounding NO. Look up "local hidden variable theory".

Lets not put pollute this discussion with that piece of wishful thinking :)

Re: Physicists Create Quantum Link Between Photons That Don't Exist at the Same Time

#19
post #5
post #3

Earlier quoted context omitted.

You're only the ten millionth person to voice that desire. Sadly, the universe is not obligated to be intuitive. At this point we probably have to accept that the parts of the universe that we didn't evolve to understand are free to be as utterly weird as they want to be.

Sure, but what are the odds the universe fundamentally defies intuitive logic versus the odds our small brains currently can't see the full picture but in the meantime have invented insane (but cool) mathematical machinations? I guess it's unknowable.

In that case it turns out that some extremely simple intuitive rules can generate some very unintuitive results. Those results may perhaps best be approximated by extremely complicated mathematics. c.f. chaos theory and turing equivalent 1-d cellular automata.

Re: Physicists Create Quantum Link Between Photons That Don't Exist at the Same Time

#20
post #16

Does this work only in one direction? Is there any reason to see the first measurement as causal? Could the second measurement be interpreted as causal that then propagates backward in time? I'm really trying to get at if the linear description of events in the article is just a convenient simplification, or if physicists thinks that there is time order here.

My best understanding is that in general, issues related purely to quantum entanglement are entirely unrelated to causality. As noted in the article, even the "spooky action at a distance" that made Einstein so uncomfortable cannot be used to transmit information faster than light. I haven't checked the math, but I assume that the same sort of properties mean that this system couldn't be used to transmit information…

Let me be a little more precise here. What you basically need to know here is that entanglement describes a correlation between measurements, and you cannot know whether that correlation exists until you collect those measurements together. This is the fundamental reason why it does not "transmit information" faster than light in a certain sense. However, that phrasing is misleading because you can use it to do things which would otherwise require transmitting information faster than light.

I like to illustrate this by a game based on "GHZ states" which I call "Betrayal." The idea is that there are 3 people working together, but I'm going to make one of them work at cross-purposes to the other two. If the team can recover gracefully from my mischief with high probability, then they all win a big cash prize.

The game is simple: the three people can prepare however they want in advance, but then they must go into different (relativistically-separated) rooms, look at a screen with words on it, and hit either a button labelled 0 or a button labeled 1. Then I collect these three numbers and sum them up, to get The Sum. So if Alice hits 1, and Bob hits 0, and Carol hits 1, then the Sum is 2.

On the screens in the rooms, I give them a task. Sometimes I do a "control" experiment: I tell all three of them "Make the sum even," and the team wins if it's even. Sometimes I create a traitor: I tell two of them, "make the sum odd", and one of them "make the sum even", and the team wins if it's odd.

Three classical players cannot beat this game 100% of the time, no matter how they prepare in advance. Three quantum players (i.e., three players sharing an entangled Greenberger-Horne–Zeilinger state) can. So if we repeat the game enough times, they can convince me that they can beat the game with higher probability than the classical limit, and thus win the big cash prize.

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