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"Spooky" Science Points Towards Quantum Computing
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'."
An ansible is a device described in science fiction for superluminal communication. It's usually portrayed as a pair (or more) of devices closely connected, as if separated from a common origin.
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.
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make install -not war
Formosa's Law
> a small machine that measures that's designed to react when it an electron comes "de-entangled" That's your mistake. There's no possible way to detect that an electron has suddenly become "de-entangled".
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.
What's purple and commutes? An Abelian grape.
if I read the article correctly, the fact that they managed to entangle the particles at a macroscopic distance.
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No. You can't transfer information across an entanglement. Faster than light communication is as impossible as it ever was; and causality has not yet been knowingly violated.
What's purple and commutes? An Abelian grape.
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.
Curiosity was framed, Ignorance killed the cat.
I've said this a few times now, but I'll repeat it: You Can't Transmit Information Across A Quantum Entanglement. (Usual caveats: to the best if our knowledge at the present time).
What's purple and commutes? An Abelian grape.
Here is a relevant article:
Entanglement interpretation of black hole entropy in string theory
Nanosecond clocks are easy (particle and nuke physics experiments commonly use NIM logic units (http://www.lecroy.com/lrs/dsheets/365al.htm as an example) with gate pulse widths of 5-10 ns, and rise times
of less than 2ns. That's COTS and not trying hard. If you need faster you can clip the output by playing games with feedback.
Also, experiment trigger timing is generally adjusted to around 1ns by using different lenght interconnect coax.
Also, you can get COTS time to digital converters (TDCS) with resolution down to around 25 ps.
That's not even touching what the laser researches can do -- orders of magnitude faster.
So yeah you can measure a 1 ns flight time w/o really trying.
] - [
Fig. 1 Above
a photon bouncing back and fourth between two mirrors on Earth.
It was entangled with another photon and is a pair.
] - [
Fig. 2
the other electron from the pair bouncing back and fourth on my ship.
(The pair was created in an entangled state on Earth and then just separated: 1 for Earth, 1 for the ship)
Now, I intercept the photon from fig. 1 in such a way to make a very obvious change to it. i.e change its electron spin dramatically etc..
] * [ Earth
Fig. 3 electrons in a mad spin!
] * [ Ship
Fig. 4 electrons also in a wild spin because of quantum entanglement
NOTE: The above photons are now NO LONGER entangled. What we measure when we measure the ship's photon will be
just an independent photon.
BUT, it will be an independent photon with it's electrons in a wild spin.
So what? So, now we know it's been tampered with through the Quantum Bridge.
THUS, we HAVE actually received data.
The only time we need to even know about relativity is if the ship is traveling at a sufficient speed
i.e. a fraction of the speed of light even.
Think of it this way. A little example
On Earth we agree that if the electrons are spinning wildly, the ship should come home!
Just before each hour Earth time, we either 1) do nothing or 2) upset a photon.
There has to be a different photon for every hour. Maybe we have a vast warehouse of them.
Say we are on mission hour 451. We want the ship to come home.
On the hour, we upset photon 451.
Paired photon 451 on the ship has it's electrons in a wild spin.
Just after the hour on the ship they upset the corresponding photon 451
The electrons are in a wild spin. Message = "we gotta go home"
Ship goes home. Note that the photon is useless now. It's entangled state is gone.
The next hour, a different pair must be used.
If I had mod points I'd struggle between modding this AC insightful and funny.
Skiffy is Spiffy, but Ort is tort.
As *both* a geek and a sports fan, it's because The #5 (out of 110 Division 1-A teams) ranked University of Michigan football team lost to Appalachian State last Saturday, 34-32. UM is the first ranked team (e.g., Top 20) in the 100+ year history of college football to lose to a Division I-AA team.
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.
What's purple and commutes? An Abelian grape.
It's more like you have a bag of blue and red billiard balls, you pull out two randomly without looking at either ball's color, place each in a box and ship them halfway across the world. The two boxes are opened up and observed, and each time one box contains a red ball the other box will always contain a blue ball.
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.
The laws of probability forbid it!
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: 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: 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.
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.)
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.
However, when I first read this article, I didn't understand any of it, because it assumes lots of knowledge about Physics. I finally understood it when I read this lecture. It starts by showing how to mathematically represent a quantum state (e.g., spin) and in the last section it answers exactly your question.
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 what you measured, despite it not be predetermined, is in MMI not any more surprising than the fact that a star which was several light years on your left is now several light years on the right after you turn around, despite the fact that there wasn't enough time for it to travel with light speed from several light years to your left to the same distance on the right. It's your turning around that "moved" the star without actually affecting it, and it's your measurement (which means becoming yourself entangled with the object), that is which "changed" the remote particle without actually affecting it. The price this comes with is to accept that there's a "parallel you" which got the exact opposite result, and with whom you'll never get contact. And that the observed facts are indeed only defined relative to the observer.
Indeed, in the MMI, the entanglement never gets resolved. It only seems resolved to you because you yourself get entangled with the observed system, and therefore you observe only part of the complete state (your "branch" of reality). According to MMI, this is what gives the apparent (but not real) collapse of the quantum state.
It is not. You cannot prove entanglement on a single system, ever. That's because you cannot measure the unknown state of any single quantum system. This of course includes the entanglement. You need a set of identically prepared quantum systems to do that. It's not hard to see that: Imagine you get a spin-1/2-particle, measure its z-spin, and get that it's spin up. That may be because it was spin up before you measured. But equally well, it could have been polarized in x or y direction (in which case you had a chance of 1/2 to measure z-spin up). Or it could have been one particle of a bell pair. Or maybe it was polarized almost in positive z-direction (in which case it was very probable, but not sure that you'll measure z-spin up). But also maybe it was polarized almost in negative z direction. In which case it was unlikely, but not impossible to get z-spin up on your measurement. You see, there are plenty of possibilities. Unfortunately you cannot just get more information by making another measurement on the same electron, because your first measurement destroyed the original state, whatever it was. You need a second, independent electron to get more information about it. And indeed, you'll need quite a lot of identically prepared electrons to get a good notion of its state. That includes entanglement.
If you have a preparation procedure, then you can produce as many copies of the same state as you want. And with as many c
The Tao of math: The numbers you can count are not the real numbers.
No. To detect a change, you'd have to know the state before it was measured, and then would have to have a measurement result which is incompatible with that state. But with entangled particles, the observable you're measuring is undefined before your first measurement (that means, your first measurement cannot measure a change), and after the measurement, the particles are not entangled any more.
The fact that changing the one's particle's spin will not at all affect the other particle's spin. Only a measurement will (at least from the view of the one doing the measurement).
Measuring forces them into an eigenstate of the measured observable, i.e. into a state where the quantity you measure has a defined value. That defined value is what you get as measurement result.
The Tao of math: The numbers you can count are not the real numbers.
Quantum mechanics is hard for people to understand because the effects we observe at the quantum level are fundamentally different from our experience with the macroscopic world. Consider a photon's polarization. If you polarize that photon up-down, then with 100% probability, the photon is polarized up-down. If you attempt to measure the photon's polarization left-right, you will discover that with a 0% probability, it has that polarization. So far so good right? If, however, you measure the polarization of the photon at 45 degrees, you now have a 50% probability that is polarized in that direction and 50% probability that is polarized at -45 degrees.
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.
The only problem being that photons aren't the only type of particle that can be entangled. Electrons may be entangled - and they certainly do not travel at the speed of light. The easiest way to think of the "truly random" nature of a particle's property is by grasping the idea that a particle's properties are a superposition of possibilities that only collapses after one of the properties have been measured. Like a photon's polarization as mentioned in a previous post. If you know that a photon has been polarized up-down and measure the polarization at 45 degrees, there's a 50% probability that it is polarized in that direction. This is why if you put 3 polarization filters with the orientations: (-, \, |) in front of a lens, it will still pick up some light whereas if you put polarization filters with the orientations: (-, |), no light will pass through.
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 the two spins you select from are determined by your measurement device).
First, the photons are no longer entangled. Second, even if they were still entangled, the adjustment wouldn't affect anything observable on the other end. Only a measurement collapses the wave function. For example, say the entangled state says both photons are polarized the same way. Now you rotate one of the polarizations to make them polarized the opposite way it was before the manipulation. That means now the photons are still entangled, but in a way that now you always measure the opposite polarization on each side. That is, the polarization of the other photon was not changed the same way (it's hard to imagine the undefined polarization to be changed to another undefined polarization, but it's only the absolute polarization which is undefined; the relative polarization is well defined, and that is what is changed).
No. See above.
Because it shows quite clearly that neither classical particles not classical waves can completely describe the quantum mechanical observations.
That's not mysterious if you describe light as classical waves. Basically it's the Huygens principle: Each point of a wave front is origin of a new spherical wave. For an infinitely extended plane wave the "sideways" parts cancel out, but if there's an edge, on the "dark side" there's no light waves which could cancel them out (because those light waves are blocked).
Yes, if it were only the interference pattern alone, there would be nothing mysterious about it. Interference of light was long known, and was used as the "final proof" that light consists of waves. The mysterious is that at the same time, photons hit the screen one by one, which means they also show behaviour we expect from particles. Waves don't make discrete, localized spots. On the other hand, particles don't interfere. That is, if you view light as classical particles, you cannot explain the interference pattern (the photon sho
The Tao of math: The numbers you can count are not the real numbers.