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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."
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
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!"
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