"Spooky" Science Points Towards Quantum Computing 294
Stony Stevenson writes to tell us that University of Michigan physicists have been able to establish an "entanglement" between two atoms trapped more than a meter apart in different enclosures using light. This shows how two different atoms can have a sort of communication, something Einstein referred to as 'spooky action-at-a-distance'. "By manipulating the photons emitted from each of the two atoms and guiding them to interact along a fibre-optic thread, the researchers were able to detect the resulting photon clicks and entangle the atoms. Professor Monroe explained that the fibre-optic thread was necessary to establish entanglement of the atoms. But the fibre could be severed and the two atoms would remain entangled, even if one were 'carefully taken to Jupiter'."
Entanglement and causality? (Score:3, Interesting)
My arm-chair understand of Entanglement suggests that it should violate causality. Consider the following thought experiment.
We have two pairs of quantum mechanically entangled electrons. We sent a single electron from each pair five light minutes in to space. A long with a small machine that measures that's designed to react when it an electron comes "de-entangled". When it senses this, it immediately the spin of the electron in the other pair.
Here on earth we have a Tsar Bombe linked to one of the electrons from one of the pairs. Five meters away, the other electron is linked to a button. When a person presses the button, it measures one of the electron, thus breaking its entanglement. That instantly breaks the entanglement of the other electron live light minutes away. The machine then breaks the entanglement of the other pair thus instantly triggering the Tsar Bombe destroying the hut and everything in 100 Sq miles.
The problem is that, as I understand it, this would happen ten minutes before I press the button. Whoops! You see, when I de-entangle the first electron the disentanglement on the other side happens five minutes in my past. When the machine disentangles the second electron, the other electron is five minutes in its past. Totalling to ten minutes. Can you see what I'm getting at? I'm assuming this argument isn't new - What mistake have I made here?
Simon.
Re: (Score:2)
or maybe both buttons are pushed 5 minutes into the future.
Re: (Score:3, Insightful)
Re:Entanglement and causality? (Score:5, Funny)
(Sorry, couldn't resist...)
Re: (Score:2, Informative)
Re:Entanglement and causality? (Score:5, Informative)
The only thing the machine can measure is the electron's spin in either of two axis. Now, say you measure it in the left-right axis and its spin comes up as left. What do you know now? You do know that if the corresponding entangled particle has been measured in the left-right axis, it would have come up as right. But this does not tell you whether it has actually been measured. There is no way to tell whether the other party has measured their particle. No information has been transferred. You can't violate causality, even with quantum entanglement.
Re:Entanglement and causality? (Score:5, Interesting)
And IMHO, that's the 'weirdest' part: an interaction which an instantaneous non-local effect *but* that cannot be used to communicate faster than C??
Strange, very strange.
Re: (Score:3, Interesting)
And you'd think with that inherent self-contradiction, physicists would acknowledge that there's something fundamentally fscked with their understanding of the universe.
Yeah, they'll tell you that faster-than-C communication breaks causality and "allows things to happen before they're caused".
So you tell them that no, in an objective reference frame, event A happens befo
Re:Entanglement and causality? (Score:5, Insightful)
"Half of what we know about physics is wrong. The trouble is, we don't know which half." -Gary Skouson (AFAIK)
Re: (Score:3, Insightful)
Re:Entanglement and causality? (Score:4, Informative)
Now, extend this to entangled photons. You entangle two photons that are polarized up-down. You separate the photons by some distance. If you measure the polarization up-down, with 100% probability, you will discover that the polarization is up-down. No information transfered, nothing learned. Why? You already knew that the probability was 100% of being up down. Now, let's say that you measure the polarization at 45 degrees. With 50% probability, the polarization will be at 45 degrees instead of -45 degrees. Again, no information transfered. All you know now is that both particles have the same polarization. If someone else was holding on to the other entangled photon, they cannot know that the photon has "resolved" itself to a particular polarization value after the first photon has been measured. If someone told them the polarization of the first photon, then they could predict the value of the photon that they currently have, but that first requires someone to tell them (at the speed of light) what the polarization of their photon is. Again, no information transfered.
So what is entanglement useful for then? It could be used as a powerful method of sharing a secret. Suppose I give you a cloud of entangled photons. If I don't know anything about the photons, then their polarizations will be completely random. I could then say that each time I resolve a photon's polarization, I will send you a message that I have read the value of the photon. So, I read the polarization of one photon causing its field distribution to collapse to the value I have measured. I then send you a message saying I have read the first value. At this point, you read the value of the corresponding entangled photon. You know that we have the same values, and so we have our first bit of the secret key. If we repeat this process for each entangled photon, we would end up with a random secret key that we both share that has never been sent across the transmission medium.
Re:Entanglement and causality? (Score:5, Interesting)
That's not spooky, bizarre, or even strange. It's not counterintuitive. So how is it different than quantum entanglement? I do not know, but I would like to.
Re:Entanglement and causality? (Score:5, Informative)
Re: (Score:2)
Re:Entanglement and causality? (Score:5, Informative)
What's even weirder is that in the quantum mechanical world, it's not that your picking two particles that are either in one state or the other with equal probability and it turns out that you always pick up opposite states. Rather it's that you have two particles that are both in both possible states at the same time. When you measure the particle it collapses into one of the two known states, but up until then it is in a superposition of both. And when you do that to one of the two entangled particles, the other particle will also collapse into one of the two states at the exact same time and you will know exactly which one the other particle will be in based on what state your own particle is in.
Re:Entanglement and causality? (Score:5, Insightful)
So you take your boxes too each side of the world and look in one that sets that ball to say red, the other turns blue instantly, and when you say instantly you really mean it, it is faster than light, faster than what should be the infinite speed, it is instant.
That is weird.
However, your example is accurate in describing why quantum entanglement doesn't break causality. You see you can't predict what colour the ball is going to be so you can't go to one end with eight boxes and say 'right ill make this byte the number 172.' then set your balls to 10101100 leading to the other boxes instanteously being set as well.
All you can do is measure the 8 boxes find out which are red and blue at either end confirm that they are entangled, thats it. No information transfer no causality breaking.
This is also why the initial posts idea falls down. You might know which particle is entangled with which but you can't measure its status without breaking the entanglement. So you could say tell the person 'measure it in 10 minutes and see if its broken down.' and yes you confirm that the entanglement breaks down instantaneously but you rather defeat the point by already giving the information. Either that or the person can guess when it breaks down but measuring it causes it to break down and bam you defeat the point again.
Entanglement has some kind of instant effect but it can not be used to send information and thus causality is preserved.
blood flow trauma (Score:4, Interesting)
The "faster than the speed of light" thing surprises me. Not because of how c functions in relationship to matter and energy, but because the physicists, whose discipline has now had a full 100 years to digest these complexities, and personally, eight or more years of post-secondary education hammering home the need to state things carefully, fail to state that the fact of the violation of the speed of light for an effect can not itself be established at faster than the speed of light.
Two physicists in a similar reference frame measure two entangled particles in different light cones (any interaction would therefore need to travel faster than ligth). The entanglement effect says that if one measures red, the other measures blue. How do they confirm this? The information about their measurements must travel *at the speed of light* until information from the distinct measurements meets up. At *this point in time* they know if the entaglement effect conformed with theory or did not conform with theory. They can't posssibly determine this conclusion faster than the speed of light between the positions where the measurements were taken.
It interests me that the effect can travel faster than light, but the conclusion about the effect can not, yet I've never seen a physicist discuss this. The discussion always goes entanglement, faster than light, spooky, bada bing. It's possible that the entanglement effect doesn't resolve itself until information about the two experimental measurements (which converges in obedience with the speed of light) actually meets up. Perhaps the disentanglement takes place only *after* the results of the two experiments meets up. That would involve the experiment (and experimenters) having become entangled in the experiment. Weird? In the realm of the very tiny, that's never stopped mother nature before.
On a related point, I've never seen a physicist comment on whether it is possible to take two particles of unknown histories and prove they are not entangled. I suspect this can only be done by taking measurements which shuffle the quantum deck. Entangled particles are always introduced as an exceptional state of matter, produced painstakingly only in laboratory equipment for the purpose of conducting this experiment.
Is it not possible that most of the particles in the universe are entangled with most of the other particles of the universe? If there is no physical demonstration that two particles *are not* entangled, on what basis could you answer "no"? As a simpler case, is it possible to construct three particles A, AB, and B where AB is entangled with both A and B?
It just bugs me that the typical account of this effect rarely gets past the word spooky before exposition ceases, as if the very phrase "faster than light" causes some kind of cerebral blood flow trauma in any person who has devoted eight years of higher education in grappling with the consequences of E=mc^2.
Re: (Score:3, Informative)
Well, look harder. This effect is at the heart of a lot of interpretations of quantum mechanics.
In my preferred interpretation, the Many Minds Interpretation, there's nothing going at the speed of light. The fact that you'll find that the other one has measured the opposite of wha
In that case (Score:3, Interesting)
In this case, the observation would be the exact same as it the photon actually had a discrete property which caused it to choose one path as it hit the crysta
Re: (Score:2)
Entanglement would be more like put a stripe on each ball and throw each into a separate clothes dryer so the balls spin around. Now send one away on a rocket ship a thousand of light years away. Whatever orientation of the stripe on the one ball, the stripe on the other ball will be related in some manner.
Re:Entanglement and causality? (Score:4, Funny)
(Yeah, I know that doesn't really happen, but some bad explanations of entanglement could lead you to think that it could.)
Re: (Score:2)
You have 2 billiard balls, each can be either red or blue. Without looking at them you randomly place them in 2 separate boxes and ship them to opposite ends sides of the globe (east and west). First you take the east-side box, put it through a process where only a blue ball will come through, and a blue ball will always come through. Next you take the west-side box, put it through the same process where only a blue ball will come through, and no ball will ever come through, because measuring th
Re: (Score:2)
Re:Entanglement and causality? (Score:4, Insightful)
It's ``spooky'' to some since the ball decides -randomly- at the point of observation which color to display. The color is not known or set (or defined), in any way, before that observation. (so the `other' ball has no way of knowing what that -random- choice was, but somehow still manages to choose the proper color). [ie: in your example, the balls already have their color before they're separated; in quantum mechanics, they randomly choose the color upon observation].
First thing that pops to mind is ``how do they -know- that it's random?''; maybe the balls had their colors pre-set all along (like in your example). Well, there are various logical puzzles you can play where if things are -random- you'd get one result, and if things are pre-set, you'd get another result---and it does appear like the choice is -random- and not pre-set.
Google for ``Free Will Theorem''; it's a fun read
There's a lot of stuff about "no hidden variables" (ie: it's not that ``there's something [a deeper knowledge of things] we don't understand yet'' that's hidden from us... it's that the choice truly is random (there are no `hidden variables'); and somehow the other particle knows about that random choice at faster than speed of light). You cannot use this to send information though (since the choice is random---you only know what the other particle's choice is... but you can't force it to choose something in particular).
To resolve the confusion (and how I like to view things), it helps to picture the two particles as really being different sides of the -same- particle, that, from our perspective, just exists [we can observe] at two different locations. Picture the world from the particle's perspective---if you're moving at the speed of light, time stands still for you, therefore, from your perspective, you can traverse the universe at infinite speed---from your perspective, you can instantly react to events anywhere in the universe (from the outsider's perspective, they just see you as moving at the speed of light...). I guess it's one of those things that are hard to explain, but easy to visualize.
Re: (Score:2)
Entanglement means the ball wasn't either color until you looked (actually it was in the state of being both colors), and looking at it made the ball the other color.
I've got no idea how they actually proved that, but I don't even get regular physics most of the time.
Re: (Score:3, Informative)
Exactly! That's the question everybody should ask when they hear about "spooky action", but for some reason, I have rarely seen it asked.
The answer is: there's a difference that can be seen in the thought experiment proposed by Einstein and some other people, which is explained in this Wikipedia article: EPR paradox [wikipedia.org].
However, when I first read this article, I didn't understand any of it, because it assumes lots o
Re: (Score:2)
If your initial 8 were NOT 11001100, then you would NOT have meaningful signal in the rest.
If it WERE 11001100, then you would *very likely* have meaningful signal in the rest.
Could that work?
Only partly correct (Score:2)
Suppose we look at photons with entangled polarities instead. At least in theory we ought to be able to use birefringence to select photons based on known polarity properties. Thus we ought to be able to know what the polarity was supposed to be before we rotate it. Thus the noncommu
Re: (Score:2)
Re:Entanglement and causality? (Score:5, Funny)
Re: (Score:2)
My arm-chair understand of Entanglement suggests that it should violate causality. Consider the following thought experiment.
We have two pairs of quantum mechanically entangled electrons. We sent a single electron from each pair five light minutes in to space. A long with a small machine that measures that's designed to react when it an electron comes "de-entangled". When it senses this, it immediately the spin of the electron in the other pair.
Here on earth we have a Tsar Bombe linked to one of the electro
Re:Entanglement and causality? (Score:5, Informative)
Ok, your comment is badly mangled, but I think I get the gist of it and I'll try to explain.
The problem is that we can't currently control what state the two disentangle into, we can merely guarantee that they share a state in common. Special relativity doesn't explicitly deny something happening faster than the speed of light, just data being transmitted faster than that limit. Because we can't determine anything from the two entangled electrons other than they share a common state, we can't actually get any data out of the system, thus there is no discrepancy. There's also the fact that determining if they are entangled is itself a measurement and thus the act of checking for entanglement breaks the entanglement. We can only verify they are entangled by checking after the fact that they both have the same state when we measure them, otherwise there is no way to know if they are entangled or not.
Polarization communication scheme? (Score:2)
This has been the central issue to me since I first started studying quantum mechanics. The other is, why exactly do we need to? Given an entangled pair of photons, measuring ones polarization will tell you what the others polarization is. So, at this point we now know what the polarization of each is. Are the photons still entangled after the first was measured? If so, then the following experiment can be setup...
1. Place an entangled
Re: (Score:2)
Are the photons still entangled after the first was measured?
This all goes back to Schroedinger's cat. Once you've measured one of the photons they are both in the same state, and are no longer entangled, any further change to one of them will not be reflected in the other. The whole thing works because in an entangled state there's a probability that the photon is in either state, and until we measure it we don't know exactly which state that happens to be. Quantum theory states that until you measure it, it actually exists in both states at the same time, and it'
Re: (Score:3, Informative)
Ok. Since now you measured the photon polarization, the photons cease to be entangled. Therefore you just have generated a photon of random spin (well, actually one randomly selected of two spins, where th
Re: (Score:2)
Re: (Score:2)
Re: (Score:2)
Re: (Score:2)
Quantum bomb detector.
Re: (Score:2)
So:
Calculate the entanglemant on 1 end.
Light transmits the information to the other end. (5 minutes used.)
Second end is entangled with the first.
Push button - instant boom
Image of the boom 5 minutes later at the button.
Re: (Score:2)
A long with a small machine that measures that's designed to react when it an electron comes "de-entangled". When it senses this, it immediately the spin of the electron in the other pair.
How do you detect de-entanglement? Only way I can see is to observe both particle states and look for quantum correlation. One is five light minutes away, so it's going to take a while to measure this.Re:Entanglement and causality? (Score:5, Interesting)
The end result is that information transmitted through entanglement travels at the fastest speed allowed by conventional means. Until we create a warp drive that limit is the speed of light.
not really (Score:3, Interesting)
No, you do not need to transport it seperatly, per se. You only need to have the receive understand how to interpret the spins. This can even be done even if the spin direction is completely random.
FLASH was the original proposal. (Score:2)
Re: (Score:2)
Re: (Score:2)
To make a poor analogy: two people may be twins, but there is not, even in principle, any test that you can perform on one twin that will tell you if s/he *has* a twin (e.g. s/he may have been a singleton fetus, a twin who lost its partner in utero, or the other twin, light years away, may have died months or years
Re: (Score:2)
Starships, I don't need no stinking starships!
No, I've already arrived, yes I'm still home!
Your Thursday is my Wednesday, No Wait!
Wherever I go, then I am.
Entanglement and black holes... (Score:5, Interesting)
Re: (Score:3, Interesting)
So when you drop your entangled photo
Re: (Score:2)
Your suggestion brings up the subject of the Hawking Paradox, which is a thermodynamic problem involving the destruction of information in a black hole. There are some competing theories regarding this, one of which is that black holes don't truly exist.
Re:Entanglement and black holes... (Score:5, Informative)
Re: (Score:2)
Thanks, I got it now.
Re: (Score:2)
Re: (Score:2)
There is no response in the second atom. If two particles are entangled, no measurement or manipulation of one can change the measurement outcome statistics of the other. You just know that if you measure them a certain way, the results will be correlated. It can seem like a subtle difference, especially if you aren't familiar with some of the odd aspects of QM (i.e. you can't simultaneously know both position and momentum,
Re:Entanglement and black holes... (Score:4, Informative)
Greg Egan has a good version: paraphrased, you have a coin on Earth, and a coin on Mars. They're entangled. You flip them. You get random results.
Now you turn on a widget on Earth. You continue to flip them. You continue to get random results, at both ends. But now they're the same random results.
The key fact is: you don't know that this is happening, until you can get a communication from Earth to Mars or vice versa describing what the results are. Once you do, you can compare the results, and say: hey, during this time period both coins were producing identical results! Maybe the widget was turned on! Or it could be just chance, of course. The coins are random, after all.
So while it's interesting, it's not useful as a communications medium.
(It is, however, great for a means of generating encryption keys. Earth wants to send a message to Mars? Earth turns on the widget, waits a bit, turns it off again. It then sends a message saying, the sequence from X to Y is the encryption key, here's a message encrypted with it. During that period, the coin on Mars has produced the same random sequence of bits as the one on Earth --- so you get the same key at both ends, without having to transmit it! But you still haven't transferred any actual information until you transmit the encrypted message, via conventional means.)
Re: (Score:2)
It takes two to tangle... (Score:2)
Finally (Score:4, Funny)
Re:Finally (Score:5, Funny)
Re: (Score:2)
Re: (Score:3, Funny)
And then the moment she measures you up, it's over!
Quantum Internet??? (Score:2)
Ansible (Score:5, Informative)
I'm looking forward to a day when ansible devices are as common as symmetric key crypto, which will likely be the only way to secure their communications, other than the "conservation of info" already built in to quantum entanglement.
Re:Ansible (Score:5, Informative)
Re: (Score:2)
Re: (Score:3, Informative)
Re: (Score:3, Interesting)
Quantum Bluetooth? (Score:3, Funny)
Re: (Score:2)
It's a common misconception when anything gets published about quantum entaglement. *Especially* when the details get distorted and misinterpreted by a reporter. Here's an (admittedly flawed) analogy to give a better idea of what's really going on:
You have two pouches, each has a rock in it. By entangling them, you know that one *must* be white, and the other *must* be black--you just don't know which one until you look inside the pouches. You can send one pouch across the universe to your Aunt Tilly.
Could extra dimensions of string theory explain (Score:2, Insightful)
spooky action from a distance (Score:4, Funny)
Re: (Score:2)
The big problem with entanglement. (Score:3, Interesting)
"a sort of communication" (Score:5, Insightful)
appalachianstate? (Score:2)
Re: (Score:2, Interesting)
why is "appalachianstate" a tag? the article mentions nothing about it....
It's a subtle zing at University of Michigan. The physicists are from there, their football team lost to much-weaker Appalachian State Saturday in what's arguably the biggest upset in college football history. Since U-M is often perceived as arrogant people feel they got their comeuppance.
(Yeah yeah, off-topic. Still a great news item though. Such was the delight of rivals Ohio State and Michigan State that students from there were emailing one of Appalachian State's players, asking to be added to his f
Re: (Score:3, Informative)
For a more geek-friendly comparison, UM's loss was as shocking as if the MPAA and RIAA announced that all the movies and music they "owned" were going to be released into the public domain next Monday.
Cheers.
Re: (Score:2)
(btw, when vick was still at VT, we all knew he was an asshole douchebag. didn't surprise me at all to see the dog fighting ring thing)
my sis went to app-state tho....maybe i should take them more seriously now.
Because.. (Score:2)
The two events aren't really connected aside from involving UMich. Welcome to popular culture.
Re: (Score:3, Funny)
So in summary... (Score:2, Funny)
2. Transport one of them to Jupiter (Or your favorite planetary body, Pluto excluded)
3. Detonate a bomb at the other atoms location
4. ???
5. PROFIT!
Re:FedEx, UPS, etc. are gonna make a fortune (Score:5, Funny)
Probably not.
Re:FedEx, UPS, etc. are gonna make a fortune (Score:4, Informative)
Before complaining, please know what you are talking about... A quick search on wikipedia would tell you: Einstein received his Nobel Prize for works on Quantum Theory!
http://en.wikipedia.org/wiki/Albert_Einstein [wikipedia.org]: Einstein received the 1921 Nobel Prize in Physics "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect."
http://en.wikipedia.org/wiki/Photoelectric_effect [wikipedia.org]: The photoelectric effect is a quantum electronic phenomenon in which electrons are emitted from matter after the absorption of energy from electromagnetic radiation such as x-rays or visible light. (...) The photoelectric effect helped further wave-particle duality, whereby physical systems (such as photons, in this case) display both wave-like and particle-like properties, a concept that was used in quantum mechanics. Albert Einstein mathematically explained the photoelectric effect and extended the work on quanta that Max Planck developed.
Comment removed (Score:5, Funny)
I found a better company (Score:5, Funny)
Re: (Score:3, Funny)
Re:I found a better company (Score:5, Funny)
I believe that's grounds for a permanent ban from Slashdot...
Re:FedEx, UPS, etc. are gonna make a fortune (Score:5, Funny)
Re: (Score:3, Informative)
Re: (Score:2)
Re: (Score:2)
Re: (Score:2)
Re:Someone explain this to me... (Score:5, Informative)
Re: (Score:2)
Got it... we need to do it on the sly. Kind of like how Arthur Dent was able to fly.
When people say physics is tricky, they ain't kidding!
Re:Someone explain this to me... (Score:4, Interesting)
BTW, I've heard this question posed more often as a pair of scissors with the blades as long as the Solar System. Close the short end and the tips should be moving faster than light. Except they don't, because as you get further out to the tips it requires more and more energy to move them faster. They'll get close, but never exceed c.
Re: (Score:2)
Re: (Score:2)
Re: (Score:2)
Hold on a minute, I need to go check on my cat - its been in a box with a radioactive isotope for ages, so I need to simultaneously feed it and dig it a grave.
Re: (Score:2)
Re: (Score:3, Interesting)
If you could send something out faster than the speed of light, then you can truly send things into the past and there by violate causality. If you want to know why this is, study Minkowskian geometry, and particularly its Lorentian coordin