Slashdot Mirror
Further Advances In Quantum Computing
Porfiry writes: "Scientists at the U.S. Department of Energy's Los Alamos National Laboratory have taken another step forward in the quest for a quantum-based computer by demonstrating the existence of a physical state immune to certain types of information-corrupting "noise," which could otherwise disrupt computations based on quantum states. The essential phenomenon that the Los Alamos team demonstrated is a state in what is called a "decoherence-free subspace." The researchers showed this state's existence using entangled photons, paired particles of light whose conditions are intimately linked."
He responded "well, yes and no...".
--
Extremely challenging, like in "it can't work and it won't ever work..."
...which makes for nice sounding rhetoric despite its being false. (Normally I hate being baited by trolls, but it's morning and I haven't finished my coffee...).
A quick search of the Physical Review Letters web site shows 20+ letters in the last five years alone deomonstrating the preparation of entangled quantum states in the laboratory. Furthermore, quantum computation (an application of Grover's algoritm--see, e.g., "Experimental Implementation of Fast Quantum Searching" by Chuang et al., Physical Review Letters Volume 80, Issue 15, pp. 3408-3411) has been demonstrated in the laboratory, so your claims of quantum computation being a mere "mathematical abstraction" do not appear to be valid.
I'm curious what motivates your objection to quantum mechanics. Do you reject the mathematical theory of quantum mechanics (in all of its various guises) which has held up rather well to experimental validation, or is it instead that the heuristic, post-Copenhagen interpretation of the theory (i.e. "spooky action at a distance") rubs you the wrong way? If the latter, then I think your objections are more semantic than substance.
This means that if the spin of one particle is measured in any direction (say out of X or Y or Z for cartesian coordinates), then the spin for the other particle is going to be opposite that measured for the first particle, BUT ONLY IF IT'S MEASURED IN THE SAME DIRECTION. So if you measure the z component of particle 1, you get either h-bar/2 or -h-bar/2, and you know that particle 2, if measured in the z direction, gives the opposite one. This will work if both measurements are in the x, or y, or any other combination of directions. But they must be the same direction.
One fundamental aspect of spin is that spin operators in different directions don't commute. that is, if one measures the spin in one direction, say Z, then another direction, say X, and then measures the Z direction spin again, it won't necessarily be the same. That is, measuring the X direction between the two Z measurements changed the state of the system.
So the part of this thought experiment that bothered Einstein and company is that if one can see that if both particles are entangled such that any spin measurement made will be opposite the other particle's measurement, providing the spin direction being measured is the same, then this implies that there are some sort of hidden variables in nature to account for this. Namely, the particles are entangled in seemingly all directions, until that first measurement is made. Surely, then, nature must possess some knowledge about all three orthogonal directions simultaneously.
But what Bohr and Heisenberg maintained is that one cannot simultaneously measure the X,Y,Z spins. That is, we CANNOT ask about measurements that could be made but were not made, we can only talk about those measurements that were made.
So it's a bit different than the analogy the article gives about two pennies, one being heads up and one being heads down, because if your penny is heads up, it'll always be heads up, as that is not a fundamental spin-1/2 particle.
sorry if this post makes ZERO sense, i'm just blabbering about what was pretty cool in quantum class. hopefully tomorrow we'll learn s'more to make it make more sense.
make world, not war
"Well, this guy in overalls showed up and said he needed to take the drive out to be cleaned. I'm a scientist, for Christ's sake, not a technician. How was I supposed to know," one scientist, who spoke to us on condition of anonymity told us.
Officials at Los Alamos are confident that, by following the lead of the State Department and offering a $25,000 reward, they will soon recover their lost data.
"Otherwise," said the scientist, "we are, like, so totally fucked. I mean, this project has been hell! Sixty-hour weeks for six months is harsh!"
sig not found
Look, Quantum Theory is a theory just like any theory except that it tends to explain a few things that more rudimentary theories cannot. It is not the ultimate reality, though. All of these theories are just casting our eyes further away from the shadow we perceive as reality and towards what is casting that shadow.
I did research for a paper on quantum computing a couple of years ago. There have been demonstrated uses of quantum effects in this field, as well as the dozens of other fields where quantum theory is applied.
In fact, they have been able to use NMR to glean the bulk spin of the composing atoms of a liquid and perform simple operations. In another angle of quantum computing, they've been able to use lasers to super-cool cesium atoms and manipulate their quantum states to the same effect.
All of this is pointing to the fact that quantum theory correctly predicted the ability of such quantum computing. It is enabling the theories from long ago, as well as newer ones, to finally be applied. It has the potential to disrupt (and even re-invent, but that's another story) encryption as we know it.
Furthermore, quantum entanglement is not a requirement for these processes.
You seem to believe that reality is what you see. You cannot see quantum states from our macroscopic world. Nor can you see relativity in action. Hundreds of years ago, people couldn't "see" gravity, either. And long before that, people couldn't even "see" air. Please, see the light. Quantum theory, like all theory, is a mathematical abstraction of how the world works. And each successive model is getting closer and closer to what can actually be experimentally observed.
In 1935, Einstein met with Boris Podolsky and Nathan Rosen to formulate a theory which basically said that particles intrinsically possess certain properties before these properties are measured. a "side effect" of the theory is known as the Einstein Podolsky Rosen Paradox (EPR Paradox).
Suppose you entangled a pair of photons polarised at 90 degrees to each other. You can't know what the polarisations are until you measure them; they could be vertical, horizontal or any angle in between. All you do know for sure is that they are perpendicular to each other. You send these photons off in different directions. At some point as they shoot off into the distance the photons will run into polarising filters you've cunningly put in their path.
Suppose one photon passes straight through a vertically aligned filter. It must be vertically polarised, so its partner must be horizontally polarised. The second photon would therefore pass through any horizontal filter in its way, but not through a vertical filter. So far so good. One photon is vertically polarised, the other is horizontally polarised, so they are at right angles as they should be, and all's well with the world.
Not quite. Until the first photon hits the filter, you have no idea whether it will go through or not. And for that matter, the photon doesn't know, what sort of filter it is going to hit until it gets there. Since you know nothing about either photon's individual polarisation until you make a measurement, you only know that the odds of it going through are fifty-fifty, no matter what angle the filter is set at. So the second photon can't know what the first photon will do until it actually does it. Yet the actions of the first photon determine the actions of the second. The second photon has to get some sort of tip-off from the first, even though they are physically a long way from each other.
What's more, this tip-off has to be instantaneous, because it has to work even if the two photons hit their filters at exactly the same time. It's impossible to predict what either photon will do, and yet the two of them must act in concert so that their polarisations have the correct relationship to each other. This is the "spookiness" that Einstein, Podolsky, and Rosen took such exception to.
- "Hear that?! The percolations are imminent! Cease your ingress!"
physical state immune to certain types of information-corrupting "noise,"
In the corporate world, we call this "management"
Dirty Pirate Hooker
Sure, quantum computing can factor enormous numbers really fast, but its been pointed out a number of times that as Quantum Computing Taketh Away, it also Giveth:
Encryption Destroyed and Resurrected
As mentioned above, the classic problem that a quantum computer is ideally suited for is cracking encryption codes, which relies on factoring large numbers. The strength of an encryption code is measured by the number of bits that needs to be factored. For example, it is illegal in the United States to export encryption technology using more than 40 bits (56 bits if you give a key to law-enforcement authorities). A 40-bit encryption method is not very secure. In September 1997, Ian Goldberg, a University of California at Berkeley graduate student, was able to crack a 40-bit code in three and a half hours using a network of 250 small computers.15 A 56-bit code is a bit better (16 bits better, actually). Ten months later, John Gilmore, a computer privacy activist, and Paul Kocher, an encryption expert, were able to break the 56-bit code in 56 hours using a specially designed computer that cost them $250,000 to build. But a quantum computer can easily factor any sized number (within its capacity). Quantum computing technology would essentially destroy digital encryption.
But as technology takes away, it also gives. A related quantum effect can provide a new method of encryption that can never be broken. Again, keep in mind that, in view of the Law of Accelerating Returns, "never" is not as long as it used to be.
This effect is called quantum entanglement. Einstein, who was not a fan of quantum mechanics, had a different name for it, calling it "spooky action at a distance." The phenomenon was recently demonstrated by Dr. Nicolas Gisin of the University of Geneva in a recent experiment across the city of Geneva.16 Dr. Gisin sent twin photons in opposite directions through optical fibers. Once the photons were about seven miles apart, they each encountered a glass plate from which they could either bounce off or pass through. Thus, they were each forced to make a decision to choose among two equally probable pathways. Since there was no possible communication link between the two photons, classical physics would predict that their decisions would be independent. But they both made the same decision. And they did so at the same instant in time, so even if there were an unknown communication path between them, there was not enough time for a message to travel from one photon to the other at the speed of light. The two particles were quantum entangled and communicated instantly with each other regardless of their separation. The effect was reliably repeated over many such photon pairs.
The apparent communication between the two photons takes place at a speed far greater than the speed of light. In theory, the speed is infinite in that the decoherence of the two photon travel decisions, according to quantum theory, takes place at exactly the same instant. Dr. Gisin's experiment was sufficiently sensitive to demonstrate the communication was at least ten thousand times faster than the speed of light.
So, does this violate Einstein's Special Theory of Relativity, which postulates the speed of light as the fastest speed at which we can transmit information? The answer is no -- there is no information being communicated by the entangled photons. The decision of the photons is random -- a profound quantum randomness -- and randomness is precisely not information. Both the sender and the receiver of the message simultaneously access the identical random decisions of the entangled photons, which are used to encode and decode, respectively, the message. So we are communicating randomness -- not information -- at speeds far greater than the speed of light. The only way we could convert the random decisions of the photons into information is if we edited the random sequence of photon decisions. But editing this random sequence would require observing the photon decisions, which in turn would cause quantum decoherence, which would destroy the quantum entanglement. So Einstein's theory is preserved.
Even though we cannot instantly transmit information using quantum entanglement, transmitting randomness is still very useful. It allows us to resurrect the process of encryption that quantum computing would destroy. If the sender and receiver of a message are at the two ends of an optical fiber, they can use the precisely matched random decisions of a stream of quantum entangled photons to respectively encode and decode a message. Since the encryption is fundamentally random and nonrepeating, it cannot be broken. Eavesdropping would also be impossible, as this would cause quantum decoherence that could be detected at both ends. So privacy is preserved.
Note that in quantum encryption, we are transmitting the code instantly. The actual message will arrive much more slowly -- at only the speed of light.
-Ray Kurzweil, The Age of Spiritual Machines, pg. 115
OK, being as I am a reasearcher who has done work on decoherence-free subspaces (DFSs...they are also known as quantum error avoiding codes or noiseless subspaces...damn nomenclaturese) I thought I'd give all you netadmins a real simple explanation of what a DFS is. Of course, being a simple explanation, it will fuzz over a bit. But I thought I'd at least try!
Suppose you are trying to send some bits down a noisy communication channel (sending an email from Timbucktoo to Weed, CA). Now the noise will cause the bits that you send on one end of the line to sometimes come out different on the other end of the line. Many of you know how we get around this in real world situations: we use error correction. The basic idea of error correction is to use redundancy to transmit information. Thus, for example, instead of sending the bit 0 you might send ten 0's and instead of sending the bit 1 you might send ten 1's. If the channel isn't too noisy then the reciever can figure out what bit you ment to send by looking at the ten bits he recieves and deducing if of those ten more are 0's or 1's. Basically you can reduce the noise rate of information transmission at the cost of increasing the number of bits you need to send in order to transmit one bit of information. (Sorry for those of you who know this shit like the back of your hand).
Decoherence-free subspaces work on a similar "encode the information" (i.e. 0->ten 0's, 1->ten 1's), but they "use symmetry" to protect the information.
Suppose that after extensive testing of the phone line you are using you notice that if you send two bits down the line in rapid sucession the line either does nothing to these two bits or flips both of them. Thus for example, if you send 00, the reciever always gets either 00 (no error) or 11 (error!) and if you send 01, the reciever always gets either 01 (no error) or 10 (error!). Your phone line has a symmetry! How do exploit this symmetry?
Well, what you do is simply encode the information you want to send into the parity of the two bits. This simply means that if you want to send 0 down the line, you send 00 (or 11) and if you want to send 1 down the line, you send 01 or (10). Now the noise can flip 00 to 11 (or vice versa) but it cannot change 00 to 01. Thus the you can perfectly recover the information you sent down the line regardless of an error occuring. What is neat about this is that it doesn't depend on the strength of the noise (the probability that an error occurs, for example). By using the symmetry of the noise you can avoid the noise completely! Symmetry=>protection.
What I've explained to you is an example of a decoherence-free subsystem (a generalization of decoherence-free subspaces, but the same basic idea) in the real "classical" world. To build a quantum computer we need to deal with similar problems but in the "quantum" world.
When Peter Shor (quantum computing god) invented a quantum computing algorithm for factoring (the one that breaks RSA), one of the main problems in actually implementing such a computer was quickly understood to be noise. Noise in quantum system is called decoherence (at least by me) and is much more nasty than the classical noise you get when (say) you are talking on your cell phone. The problem with quantum systems is that if they interact with external systems they completely lose their quantum nature. And making this problem even harder, whenever you observe a quantum system it also loses its quantum nature.
But following his work in discovering the factoring algorithm, Peter Shor noticed that he could do error correction on quantum systems to avoid this decoherence problem (hence Peter Shor=quantum computing god). A huge host of people then developed the theory of quantum error correction which showed that the decoherence problem could be overcome. This is probably one of the most amazing new ideas of the past decade: that quantum information can be in principle be sheilded from its environement by suitable error correction.
Anyway, decoherence-free subspaces are like quantum error correction in that you encode quantum information, but they, like the example above, use the fact that often noise has some sort of symmetry. Think about it this way: decoherence of a quantum system is like you looking at the system (you are interacting with the system!). But say you have two atoms which are so close together to each other that you cannot distinguish atom A from atom B. Then there is a symmetry in the way in which observe the system: you cannot distiguish that atom A is on the left or if it is on the right. Such a symmetry can then be shown the produce encodings of information which are protected from your observation!
Ah well, I had to try. Thanks to anyone who made it this far without "man I want to kill this dork" thoughts.
dave bacon