Physicists Create Quantum Link Between Photons That Don't Exist at the Same Time
wired.com
wired.com
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 linear description of events describes the time-order that the actual experiment used.
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.)
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.
Every time I read an article discussing quantum mechanics, particularly new results in the field, I get more and more of the feeling that we are just missing something. According to the article entanglement can occur on the scale of lightyears but those entanglements cannot be used to transmit information faster than the speed of light.
The linked article on Schrodinger's Hat seems to be violate another rule about observation, but there's always this caveat that prevents it from violating some quantum principal.
So what is this strange operation? There exist two nice "superposition over all states" quantum states for the three bits held by the three players:
+++ = 000 + 001 + 010 + 011 + ... + 111
−−− = 000 − 001 − 010 + 011 + ... − 111
(+++) + (−−−) = 000 + 011 + 101 + 110
Here's where you need the complex numbers. Each of the "make the sum odd" people maps (+),(−) → (+),i(−). This is called a phase rotation, and you might know i² = -1 in the complex plane. These separate acts shift the global state to:
(+++) + (−−−) → (+++) + i²(−−−) = (+++) − (−−−)
(+++) − (−−−) = 001 + 010 + 100 + 111
As for proving that classical probabilities cannot do this, take six random variables no matter their joint distribution, call them Ao, Ae, Bo, Be, and Co, Ce -- what Alice, Bob, and Carol do when they're told to make the sum odd or even, respectively. The problem asks to make Ao + Bo + Ce ≡ Ao + Be + Co ≡ Ae + Bo + Co ≡ 1 (mod 2) while Ae + Be + Ce ≡ 0 (mod 2). Adding those four equations together gives 2 * (Ao + Bo + Co + Ae + Be + Ce) ≡ 3 (mod 2), but 3 isn't even. So it's not possible to satisfy all four equations all of the time with classical probabilities.
(Most physicists, and most philosophers, believe that the right theory of time is a B-theory of time: once every event is lain down in a four-dimensional spacetime diagram, there is nothing else to describe; no facts, in particular, about which of these different times is really now, the time at which the whole universe is.)
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 me really wants to believe we're missing something fundamental that would make all of this so much easier to accept and understand.
[edit: removed wikipedia link... too many spoilers :(]
Ontologically, there is not reason for us not to discover other things about nature that are (more) fundamental.
In "Many Worlds", the "something" would be the particular universe that "this instance" of ourselves ended up in.
Once you get the math (and it really kind of clicks all at once), it's not too hard. The problem is that the math has a lot of historical baggage and the ways of translating equations into prose are not the most effective I think.
The whole "uncertainty" thing bothers me. I think there's better ways of expressing this concept to the public.
I am, however, bothered by the fact that QM is not deterministic. Most experiments have ruled out loopholes for a deterministic universe, but it still just weirds me out that randomness is inherent to the measurement of observables.
(The result is that either which quantities we choose to measure must be deterministic, or physics must be nonlocal (i.e. what we do in one place can change what we measure in another place even if the two experiments happen simultaneously), or the results of our measurements must be nondeterministic)
It's like a ball a top of the totally symmetric hill. If you consider phase space it contains ball rolling down in all the possible directions but noone claims that ball rolls in all of them in alternate universes. It rolls down in exactly one way though we might never have any idea why it chose this one.
FWIW, if one bites the bullet and assumes the equations mean what they appear to mean then it's a lot simpler than you might think. My first thought on looking at the diagram in the article was "well duh".
Now that's a pretty bold claim, and I have no idea if it is correct or not. But it's a position held by at least one fairly well known and reputable physicist.
[1]: http://www.amazon.com/gp/product/0465092756
[2]: http://www.amazon.com/The-Trouble-With-Physics-Science/dp/06...
The basic idea is that observed decoherence is not caused by wave collapse, but is caused by interaction with noise. For example, consider the double-slit experiment. If you allow light to pass through two slits, it forms an interference pattern. If you put a detector in one of the slits, the pattern collapses.
The Copenhagen interpretation says this is because the observation collapses the waveform of the photon. The Bohm interpretation says that the photon interacted with your detector and its phase was knocked out of sync.
I think it is called entanglement teleportation.
So yes, they can. Which leaves me wondering which option is right: A. I'm missing something major here. B. They can measure 1 and 4 and then decide whether to use their box on 2 and 3 and contradict causality. C. This experiment isn't actually exciting at all and the 'entanglement' isn't the good kind.
Of course it's possible that 2&3 entanglement trick doesn't always work and it never works if preceeding 1&4 measurment proved that they were unentangled. But this is some serious cheating. :)