A "Photon Machine Gun" For Quantum Computers 143
An anonymous reader writes "Generating entangled photons in a reliable way is impossible right now, stalling the development of the optical quantum computers that would use entangled photons as quantum bits (qubits). Because entangled photons can only be produced at random — which takes time — the most powerful optical quantum computing device use only 6 qubits. UK and Israeli quantum physicists have designed a blueprint for a 'quantum machine gun' that fires out barrages of entangled photons on demand. They think within a few years this device will be built, and could lead to quantum computing using 20 to 30 qubits. Every additional qubit doubles the computing power, so these quantum computers could outperform any existing classical computer, the researchers say. The quantum machine gun is described as 'one of the most exciting theoretical proposals I've read in five years' by a leading quantum physicist." The research was published in Physical Review Letters earlier this month.
Re:Not particularly useful against an insurgency (Score:5, Informative)
Harmful harmful force? Dude i think you need to re-evaluate your worldview if you want to blame the group being constantly attacked and threatened with the explicit goal of genocide for everything wrong. The mere presence of jews in the middle east produces the reaction you see from Hamas and friends, whether or not Israel was officially a state would have fuck all to do with anything other than the success of those attempts at genocide.
Hell Hamas' own govt charter explicitly blames the jews (merchants of death) for everything from the french and russian revolutions to both world wars while outright demanding the death of every jew and anyone who refuses to participate in said genocide.
Re:Qubit does not double power in traditional sens (Score:3, Informative)
If you are very unfortunate, n qubits can map 2^n -1 bits. -1 because 2^0 = 1, and that'd just be weird.
If this is the case, then a 6 qubit machine maps 63 bits, but 20 would map 1,048,575 bits (1 Mbit of information) and 30 would map 1 Gbit of information.
Re:Explain the hype, please? (Score:4, Informative)
Ok, I just wrote a lengthy reply, and then by accident hit "refresh", and all the text was gone :-(
Therefore here the short version:
Re:Explain the hype, please? (Score:3, Informative)
Bad analogy time.
The simplest way to factor a large number is to just try to divide it by 2, by 3, etc. Once you've divided it, you now have 2 smaller numbers to factor. Repeat until you get a prime. This takes a long time for a large number because you have to try it over and over again.
With a quantum computer you can do all of these computations in parallel, and then arrange for all of the non-factors to cancel each other out, meaning that you can only measure a legitimate factoring. (Getting all of the non-answers to cancel out is the trick in quantum computing, and it isn't a particularly easy one to pull off. These are not general purpose computers!) If it keeps giving you 1*n as your factoring, eventually you conclude it is prime. Otherwise the first time it gives you something else, you've broken the number down into 2 easier ones.
To break RSA you only have to factor one number. So everyone cites that as the classic problem. But you can't factor a number you can't put in. With 20-30 qbits you can only input 20-30 bit numbers so you can't factor anything bigger than that. By contrast a motivated person these days with a few PCs and a few months to devote to it can factor a general 600 bit number. Most people's codes are 1024 bits or longer.
Therefore this research is cool, but any claim of an immediate threat to cryptography is waaay overblown.
Re:For certain problems. (Score:2, Informative)
Of course, as of yet it isn't even known if a classical computer can calculate NP complete problems in polynomial time. P!=NP is still a conjecture.
BTW, the correct arXiv reference is arXiv:quant-ph/0601151 [arxiv.org]. After all, there's also astro-ph/0601151, cond-mat/0601151, hep-ph/0601151, hep-th/0601151, math/0601151 and physics/0601151, none of which are relevant here.
Re:no peeking (Score:4, Informative)
First, let's look at a fair attempt to explain why quantum indeterminacy is not just the same thing as classical indeterminacy (like your two particles, which by your question were presumably determinate in the classical model, at least until they became entangled). You seem to be reasoning much as the following note claims early quantum physicists tried to, when they first grappled with Heisenberg's uncertainty principle and the question of knowing the position and velocity of an electron simultaneously. I give you someone deliberately trying to put the concept in normal, natural language and not use any actual math:
http://www.uhh.hawaii.edu/~ronald/310/Quanta.htm [hawaii.edu]
One point is, the interpretation that we can't know both position and velocity at the same instant, therefore the electron doesn't have both at the same instant, doesn't explain that thing you refer to as "with no regard to distance". This is what sometimes gets called "Spooky action" and is related to non-locality in general. Starting from the interpretation that it's not our not knowing that causes the indeterminacy but the indeterminacy which causes our not knowing turns out to be putting the horse back in front of the cart. Once people started working from the idea that the indeterminacy is fundamental and not like your example of the balls (where there is a definite color for each, and the observer just doesn't know it yet), they started making progress on figuring out how entanglement could be faster than light.
http://www.absoluteastronomy.com/topics/Quantum_indeterminacy [absoluteastronomy.com]
This is about what non-locality really means: One consequence is that we can't assign a local cause (such as: a localized observer hasn't looked yet) to explain why something on the quantum level is determinate, or we lose the ability to explain how the faster than light part happens.
Just as the original QM problem was about determining position and velocity, talking about "non-localizable" (position), and instantanious/faster than light (velocity) is two ends of the same stick. The more you prove that the action happens much faster than the limitation of light-speed, the more you can't claim the action is caused by anything in a particular locale.
Re:Explain the hype, please? (Score:3, Informative)
Ok, I just wrote a lengthy reply, and then by accident hit "refresh", and all the text was gone :-(
You're welcome. [mozilla.org]
Re:no peeking (Score:3, Informative)
OK, you measure photon phase with a polarized lens. The way you measure the phase is to pass the photon through a polarized lens at an arbitrary angle. Unfortunately, all you can measure about the phase is whether the phase matches that of the lens - either the photon makes it through the lens, which means the photon had the phase of the lens, or it doesn't, which means the photon had a phase at 90 degrees to the lens. There's lots more to say about this, but I think this is enough to explain the answer to your question.
If you measure the phase of a photon with random phase (whether it's just classical random meaning it has a phase, but you don't know what it is, or if it's quantum random, meaning it's unknowable), half the time the measurement will say it's in phase, and half the time it will say it's 90 degrees out of phase. It will never say anything else (obviously).
If you have two photons with entangled phases, and you set up lenses at 90 degrees to one another, you will always (guaranteed, at least up to how accurately you can get the lenses at true 90 degrees to one another) get corresponding phase readings. Note that the actual angle of the two lenses doesn't matter - just their relative angle.
If the entangled photons had an initial phase, there's no way that could happen. If the angle happened to be 45 degrees off from your lenses, each of the photons would only get through half the time, and both would be random, not correlated. Either the photons somehow "know" what angle the lens you're going to use to measure them is at the time the photons are emitted (even though the lens might not be set up then), or they "communicate" about what angle the lens is after they hit it. (Or they have an infinite number of correlated states telling them how to react to every possible angle, or...)
In any case, the phase is not the expected classical relation.
Re:Qubit does not double power in traditional sens (Score:2, Informative)
Re:For certain problems. (Score:1, Informative)
The polynomial time quantum factoring algorithm is by Peter Shor. Grover's algorithm is for searching an unsorted database in time proportional to the square root of the number of items in the database.
Neither Shor's algorithm for quantum factoring nor Grover's algorithm for unsorted searching are guaranteed to find the correct solution. They are only guaranteed to find the correct solution with very high probability.
The relationship between BQP and NP, is still unknown, but its answer does not directly apply to the P=NP problem. That is even if quantum computers can solve NP complete problems in polynomial time, NP complete problems may still not be solvable in polynomial time on classical computers. However, the converse is true. If P=NP then NP is contained in BQP.
Re:no peeking (Score:1, Informative)
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Re:Not particularly useful against an insurgency (Score:3, Informative)
http://lmgtfy.com/?q=hamas+charter [lmgtfy.com]