Physicists Create Quantum Link Between Photons That Don't Exist at the Same Time(wired.com)
wired.com
Physicists Create Quantum Link Between Photons That Don't Exist at the Same Time
http://www.wired.com/wiredscience/2013/05/quantum-linked-photons/
55 comments
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.
The linear description of events describes the time-order that the actual experiment used.
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.)
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.)
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.
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.
What makes the quantum players able to beat the limitation of the classical players with multiple trials?
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.
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.
It's just that they have access to an operation which classically doesn't exist, because their probabilities are complex numbers rather than real numbers. (Just as importantly, there are known limits to how great their correlation can be; the nice thing about Betrayal is that you can quickly prove that six classical random variables don't work no matter how they're jointly distributed.)
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:
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:
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.
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.
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.
If you don't measure the first photon then the 4th one could be anything and the measurement would not do anything to the 1st photon since it does not exist any more.
If one abandons the idea that there is a privileged 'now' (what philosophers call the A-theory of time, which is already difficult to make compatible with special relativity) this effect is not weirder than simultaneous entanglement -- admittedly, this is already sufficiently weird.
(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.)
(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.)
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 me really wants to believe we're missing something fundamental that would make all of this so much easier to accept and understand.
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.
At a mathematical level, it's pretty elegant how quantum mechanics works. At the level of the article, not so much.
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.
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.
There are certainly interpretations of QM that are deterministic. See many worlds, for example.
Personally, I've never found that a comforting alternative.
It just because people tell about it as if all the possible divergent trajectories through phase space happen not just ours. What they actually mean is that the possibilities are vast and we have no idea why reality chose exactly this path because it's not in any way priviledged.
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.
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.
That sounds exactly like non-determinism, which is what we were trying to avoid.
Do you have any sources for experiments ruling out our universe being deterministic? I'm not questioning you. I honestly have always been intrigued with the idea. Thanks.
The standard answer is the Bell inequalities, but if you only care about deterministic vs. nondeterministic then Conway's "free will theorem" is a bit simpler.
(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)
(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)
Nothing says we live in a local universe.
There's no proof, sure. But nonlocality is very counterintuitive, arguably more so than nondeterminism (and there are no familiar classical/macroscopic phenomena that exhibit nonlocality, whereas many familiar things at least appear nondeterministic in practice).
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.
please read Anathem http://www.amazon.com/Anathem-Neal-Stephenson/dp/0061694940/... (if you've not already) I think you will enjoy it :)
[edit: removed wikipedia link... too many spoilers :(]
[edit: removed wikipedia link... too many spoilers :(]
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.
I can't see any basis on which to assign the probabilities you ask for. A better question might be: on what basis do you believe that the universe should be intuitive? Why should it be so?
You may not be so "out there" at all. There were two books that came out a few years ago, both basically lambasting the state of physics - especially string theory. One was titled Not Even Wrong[1] and the other was titled The Trouble With Physics[2]. As best as I can tell, of the two (and I'm afraid I can't remember which it was now) made the case that we might be completely wrong about the nature of time. The idea was that the reason we have had to resort to such (seemingly) byzantine theories as string theory, without seeming to make much progress on deciding which string theory is correct (or if it's correct at all) is tied up in this misunderstanding of time. IOW, before we make any really fundamental breakthroughs in physics, we will have to re-conceptualize time itself.
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...
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...
I think the fundamental problem is the idea that there is more than one thing. As soon as you see there is only one thing, it's no surprise that everything is related. http://philosophy.stackexchange.com/q/4751/2992
But in that case, wouldn't physics be non-local after all? Do you believe that it is non-local?
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?
This train of thought has to have something to it. There is obviously something that is "the same". Perhaps it is something more fundamental than the photons that "generates" the photons. Or, if it doesn't "generate" them, it determines the state of them at least.
In "Many Worlds", the "something" would be the particular universe that "this instance" of ourselves ended up in.
In "Many Worlds", the "something" would be the particular universe that "this instance" of ourselves ended up in.
We're most certainly missing a lot of things. But quantum mechanics is in no way wrong. It may not be complete, but not wrong.
Ontologically, there is not reason for us not to discover other things about nature that are (more) fundamental.
Ontologically, there is not reason for us not to discover other things about nature that are (more) fundamental.
You sound far too certain for my liking. But IANAP.
It's possible (in fact almost certain) that we don't know anything, but it's unlikely that we're missing something that would make it easier to accept: any true theory of physics would have to explain nonintuitive experimental outcomes like the bell inequalities or the "free will theorem".
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".
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".
Don't worry, in 500 years, after everything that is now studied is well understood, there will be one or more underlying principles discovered which will simplify it all.
I'm a little baffled when articles come out claiming some weird relativity-breaking quantum effect and fail to mention that the no-collapse interpretation of quantum mechanics explains the effect easily without breaking relativity.
What's the easy no-collapse interpretation explanation?
There are a few, the most realistic is the Bohm interpretation: http://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory
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.
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'm guessing Ez is referring to the Many Worlds Interpretation.
original article is here: http://arxiv.org/abs/1209.4191
Could they possibly create 1-2 and 3-4 entangled pairs at the same time, measure 1 & 4 and then use this "projective measurement" trick to entangle 3 and 4? If so, could this affect the measurements of 1 and 4 in the past?
yes, i think, if you mean 2 and 3. you have 1-2 and 3-4, then you entangle 1-4 which I think should lead to 2-3 entanglement without the 2-3 couple ever interacting at all.
I think it is called entanglement teleportation.
I think it is called entanglement teleportation.
Yeah, I've meant entangling 2 & 3 pair and measuring 1 & 4. But my concern is if we can still use this trick after measuring 1 & 4?
>For example, last year a team showed that entanglement swapping still works even if they make the projective measurement after they’ve already measured the polarizations of photons 1 and 4.
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.
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.
This can't possibly work because it would mean that you cou
ld measure 1&4 and get result that proves that they were not entangled and then entangle 2&3.
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. :)
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. :)
I am not sure I understand you.
I hope that can make things clear: http://imgur.com/5mfB4m0 .
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The question now is, how to use this to mine Bitcoins?
I love how at the end of all these physics articles there is always some guy who is like "O HELL YEA WE CAN USE THIS FOR QUANTUM COMPUTERS!"
Those are probably the magic words that make the state security apparatus throw wads of cash at you.
Let's use this to keep a client in sync with the server. And also keep multiple instances of server in synch with each other.
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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.