Computers That Solve Problems Without Being On 144
Iron Monkey writes: "Nature has this article about how quantum computers can theoretically solve problems without ever actually being turned on! Maybe California can use a few of these to solve their energy crisis - the ultimate in conservation."
The Mind Boggles (Score:1)
I wonder if I will be able to look at pr0n without actually paying...
Victor
wow (Score:1)
vapor-state computing... (Score:1)
Delightful.
As any marketer knows, vaporware is the best product, because nobody knows its real limitations, only its theoretical ones. "There's no theoretical reason it can't sort a list in constant time... ergo it must be able to! Stop the presses!"
Call it vapor-state computing.
--G
Re:Get your Physics right. It has to be ON. (Score:1)
-David T. C.
There's no such thing as a free number-crunch (Score:1)
Full text at the LANL preprint archive [lanl.gov].
Warning: requires some background in quantum computation.
Of particular interest is Chapter 6, where they say the following:
Our starting point was the enticing notion of being able to run a quantum computer `for free'. The quotation marks here were very necessary, however, for it is not at all clear what if anything comes for free. A CF protocol requires the computer to be present, and due time must allowed for the machine not to run.
They go on to explain why any attempt to use the times when the computer is "off" for other computation will ruin the original computation. The bottom line is that the story is fascinating and the model may be useful for analyzing various phenomena, but apparently this trick will not save any physical resources beyond "standard" quantum computation.
Of more pseudo-practical interest is the issue of interaction-free measurement (the "bomb testing problem" mentioned in the paper). For a nice introduction see The Tao of Interaction-Free Measurements [lanl.gov].
I hate to say it, but... (Score:1)
--
Matt
What ever "everyone" did this? (Score:1)
Once all the other worlds caught on, no one would be running the problem, so everyone will just sit there waiting for someone else to do their work.
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Re:Get your Physics right. It has to be ON. (Score:1)
Think of it like setting a whole row of coins on their edge -- neither heads nor tails.
Yes, it takes energy to put them their in the first place -- just like it takes energy to create an entangled pair of photons to begin with.
Whack the table they are sitting on, and they drop over onto one side -- heads or tails. Measure the outcome.
In the normal Universe, whacking the table constitutes energy and your thus is your power source.
Your confusion arises from the fact that Quantum isn't the normal Universe. The act of observing the states causes the result.
In the end, technically, you are right. The act of observing itself imparts energy to the system -- thus is the power source. However, it is so small as to be unable to be measured by our current abilities. This results in the scientists not wanting to turn a 1-page article into a 30-page explanation of superposition, Heisenberg's Uncertainty Principle and other techno babble.
--
Charles E. Hill
Weird idea (Score:1)
I don't know if this solution will work in hot California, but here is a weird idea:
All the energy going into the CPU's are emitted as heat. This mean that having 300W of CPU's crunching data is equivalent to having a 300W electric heater oven turned on. Now, the obvious is, turn the oven off and start your computers.
Lets extend this to replacing peoples ovens with arrays of CPU connected to a common network. The result should be something like distributed.net or SETI@home network.
With this model all the energy 'wasted' in order to heat peoples homes could be put into good use cracking huge complex computational problems, or it could be distributed to home users.
Open for flame.
Ecological effects (Score:1)
It highlights a meaning of the phrase "It's the little things in life that make the diffenence" and that won't be far from the truth.
Of course this is only a small step on a path which has been running for years, computing has been really pushing for speed. Especially when programmers had to account for each byte it used to squeze as much from a machine as possible.
This could also be useful in robotic technology where programmers always have to think of how long can the robot last before it has to get to the recharging station.
it's almost like having a mini power station in you computer.
You could imaging (although a little exagerated)
---
"Hey look Sir. No wires and it just runs and runs." says the inventer to his employer
"Can I take it home to test?" asks his boss
"Sure" replies the inventer "and dont forget it."
Next morning the boss returns "Well I can't believe it ran so long, it certainly kept the kids busy."
the inventor thinks 'bet they messed it up like usual'
"what was really odd was it really seemed to act strange." continued the boss "Well I guess it must be the quantum physics I remember the quantum world is supposed to have strange things in."
"Sir I think you'll find it's this loose wire. Thats why I don't test these things with the kids that quickly. They always fiddle with anything they can lay their hands on." cautioning his boss.
Re:The true effect of quantum computers (Score:1)
I don't think that I lambasted the original poster. He posted misinformation, I pointed it out. IMO, that's the point of having an open forum: I can post my bit of knowledge on something, and if someone else knows better, they can correct me. As you did.
BTW, I did know that entanglement doesn't give you complete knowledge of the other particle's state. AFAIK, only one part of the state (e.g. phase or angular momentum) will be entangled.
I still don't think my description was as inaccurate as his, but that my just be stubbornness speaking.
Re:Safety Warning (Score:1)
icanneverbereached@sogoaway.com aint my address.
Re:Schrödinger's computer (Score:1)
Does this work for PMs? (Score:1)
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Oh good. (Score:1)
I know, I know, I'm ignorant. The article did a poor job trying to explain this as anything but "some surrealist form of computer science".
Oh, and I can finally use this quote:
"In theory, there is no difference between theory and practice. But, in practice, there is. " - Jan L.A. van de Snepscheut
i have read a story like this (Score:1)
I'm not too sure I understand all of what you're getting at, so I'm not sure the story is actually relevent. It went something like this:
Folks fiddling around in a computer research lab working on quantum physics/chaos/computations stuff (oh, you know...;). They talk about spooky action at a distance, weird interlinks between representations of the universe (equations) and the universe (causal links I mean). Anyway, some dude is testing out the head professor's ideas, and suddenly the head prof notices that the weather has changed dramatically. Something about noticing how hot it has gotten in her lab. She looks out the window and realizes that the sun is a lot brighter than it should be. She also notices smoke in the distance. (!) Neat stuff. Maybe someone out there has read this and has some bibliographic info to supply? Hint hint.
____________________(
Re:Deep Thought (Score:1)
I maintain that:
Chaos is the future and beyond it is freedom
Solving problems without being turned on (Score:1)
An old computer of mine is already capable of solving problems without being turned on.
You see, there's this door in my house that won't stay closed... :-)
Super eurobeat from Avex and Konami unite in your DANCE!
Re:What ever "everyone" did this? (Score:1)
Quantum possibilities and the halting problem? (Score:1)
Then the halting problem can be solved too if you just use quantum computing to "pick" a solution out of the box. The problem is when you compare the solution to the initial problem and realize it doesnt work in *our* alternative universe. Why? in alternate universes you might have both solutions be the case, but not in ours.
I smell new theoretical problems brewing for quantum computing already
ah well... (Score:1)
A musical number without any music, and an artist without an audience going through the motions without any instruments... *sigh* I'd say it sounds like my love life, only I have an organ.
Re:Whose policy are you smoking? (Score:1)
Sorry.. whatever was I thinking?
Must be because I'm one of those damn Canadian types. Does anyone have any information as to whether conservation is uncanadian? I guess it must be, since we're the ones selling all this energy to California...
Re:From the submission queue... (Score:1)
This must have something to do with the quantum superposition of the two submissions.
You see, the submissions were in a superimposed state until someone actually looked at them - at which time one of them was accepted, and one was rejected.
Re:And that's why I hate reading things on "Nature (Score:1)
Does a MC (modern computer) have to check the work of a QC in order to ensure the QC got it right? For example, instead of letting the MC use the Number Field Sieve algorithm to factor primes, the QC tells the MC what the most probable answer is, then the MC makes sure that this answer meets all the qualities of the correct result. This is obviously still a huge gain for factoring primes, but there have got to be a huge amount of algorithms wherein testing if the QC has the right answer would be more expensive than the MC just finding it on its own due to communication/conversion costs between the two different platforms/processors.
Will MCs need to check the work of QCs?
Why would you need to solve the energy crisis (Score:1)
Mac Doorstop (Score:1)
Does this mean that Apple invented quantum computing?
Re:DMCA will make this illegal. (Score:1)
Just think, in a quantum scheme there may be a parallel universe where Jack Valenti is a benevolent benefactor to free speech. weird stuff, this many worlds theory.
Re:ah well... - 4' 33" (Score:1)
I suppose that the real purpose is the similiar to the strange physics of the article, or exploring the Antarctic; you might not want to experience the results of these musical experiments yourself, but its important that someone tries to find new things. Stockhausen isn't incredibly listenable either, but without these people we would never have found techno for instance.
Re:Whose policy are you smoking? (Score:1)
Re:i have read a story like this (Score:1)
For instance you get a Quantum computer that is doing all the math for a hurricane forcast. One particle of air is measured EXACTLY in time and space. The entirety of the hurricane model should then collapse. Using the principles of quantum articles effecting each other at a distance it would be concieveable that the hurricane would then be *forced* to follow the path that the computer set for it. If you had umbictious amounts of these systems around doing everything and more then computers do today AND they were all connected then it would be possible that this one waveform collapsing would solve space-time co-ordinates in other systems. This would collapse those computers waveforms and have a domino effect. Some sort of quantum enviromental damage in which the effect would be a completely collapsed (or partially collapsed, could make a better story) timeline that could not be deviated from. All unknowns would be able to be easily worked out.
Agian I would like to say that while I can't be sure (IANAQP IANA Quantum Physisist) I really doubt that anyt hing could ever work this way. If it did Newton would have destroyed the universe when he figured out that the apple just hit his head ;)
If that's true... (Score:1)
I can see it now... (Score:1)
Q3A (Score:1)
"Fourty-two million frags in two seconds. Excellent! Please power up to start the game."
Re:Quantum Computer Questions (Score:1)
so does this mean... (Score:1)
Way to go, science! (Score:1)
If this doesn't sound like vaporware I don't know what does...
On the other hand, I often solve problems by procrastinating so long that there's no point in doing the task any longer. This may in fact be an optimal solution for many problems. Way to go, scientists, for figuring out something I knew instinctively when I was 12.
Fiction Source: Permutation City by Greg Egan (Score:1)
Let's say that a quantum computer can simulate the results of a computer twice its complexity which simulates a computer twice its complexity...
And the original computer doesn't even need to be on!
Now imagine the virtual worlds within it evolving.
That's just the tip of the iceberg in this story. Accepting the virtual q-computers is a big "if" to swallow, but it all hangs together beautifully.
Read it. Now, before it becomes reality and less interesting.
Joel
Re:Schrödinger's computer (Score:1)
=Blue
The inevitable problem. (Score:1)
Instead trying to find answers for our questions, we will have to find questions for our answers.
Re:Example of counterfactuals in QM (Score:2)
The obfuscation just kills me. It seems the more contorted the interpretation, the more likely it is to be published.
Re:Example of counterfactuals in QM (Score:2)
Hence my question: is this configuration considered publishable? It's a no-brainer. It has no new physics, and adds nothing to the interpretation. It's a simple retelling of the old two-slit experiment, and it coins a new obfuscatory word to boot.
Here's my "counterfactual" experiment:
Set up two slits. Put a bomb with a (transparent) electron detector trigger over one of the slits. Do the two-slit experiment. If the bomb is a dud, the detector will not interact with the particles' de Broglie waves, and an interference pattern will appear. If the bomb is not a dud, the interference pattern will disappear (or the bomb will go off).
For each electron going through the apparatus, you have the same 50/50 chance of detonation if the detector is working. It's harder to measure the interference pattern, since you need more than one electron, but the mechanics are the same. You still have a non-zero chance of finding that the detector is working without activating it (if all the electrons go through the other slit, and you get enough to see that the interference pattern isn't there).
Re:I really don't grok this at all. (Score:2)
Think of it as a very complicated resonator, which settles very quickly into a stable pattern of motion.
Re:On Einstein (Score:2)
Einstein mainly objected to the idea that QM, which is a form of statistical mechanics, is proof that the universe is inherently statistical and non-deterministic. While there's no reason why it couldn't be, there's nothing in QM to sway things one way or another.
Re:To quote from the referenced Nature article.... (Score:2)
Re:And that's why I hate reading things on "Nature (Score:2)
Then your algorithm/program is wrong, just like with conventional computers. From what I have gathered about quantum computing, the trick is to find an algorithm that will give the correct solution (or one of a set of correct solutions) with the propability 1 and 0 for all incorrect ones.
Proof of difficulty? (Score:2)
They're saying that because any quantum state in the computer can be reduced to the null state, an answer can be found before the computation is made. In other words, the quantum state you must input into the computer to program a calculation can instead be used to predict the answer.
If you know how to program a quantum computer, you theoretically can already know what the answer is.
Now, it may take more effort to derive the answer from the input states than to just plug it into the computer and let it churn away. So the computer itself will probably not be superfluous. But it's still an interesting theorem. It seems to be saying that programming a quantum computer will be approximately as difficult as solving the problem you want to program!
From the submission queue... (Score:2)
Re:From the submission queue... (Score:2)
Re: (Score:2)
had that for years... (Score:2)
My computer is off, and solves the problem of my backdoor falling closed.
Also, a computer that's turned off, and dropped from the empire state building on pauly shore's head solves a LOT of problems.
//rdj
The next step ... (Score:2)
Although there might be minor limitations. But better not mention them, to avoid confusing the readers of nature. One of those limitations might even be, that you have to solve the problem (by whatever means) just to know how to extract and interpret the information.
The question is, does that quantum computer to be built and run in another world have to appear on a budget in this world, and where the money actually goes. But that will only be solved by quantum accounting.
Re:The true effect of quantum computers (Score:2)
However, there are some things that a QC doesn't do any better than a classical computer. In fact, most things, a classical computer does just as well as a QC. So the likelyhood of ever wanting to replace your computer with a QC is pretty small.
QCs have some real downsides that make implementing a general purpose one impractical or expensive. For one things, because of the no-cloning theorem (which states that you can't duplicate a quantum state without destroying the orignial), you cannot do a fan-out (connect one gate output to several gate inputs). There are ways around this, but I doubt it will ever be worth it.
Note that a classical computer is one that implements a turing machine, and a quantum computer is one that has a time-evolution operator given by the Schroding equation. "classical computers" implemented with silicon are obviously quantum mechanical devices. Classical computers will eventually have to face the quantum indeterminism of the extremely small scale, but they will still be classical comptuers -- even if they aren't implemented as silicon based semiconductors.
Re:Schrödinger's computer (Score:2)
Schrödinger 95, 98, 98SE, and ME. Going Down in TRUE RANDOM FASHION since day one!
"Everything you know is wrong. (And stupid.)"
Deep Thought (Score:2)
Re:And that's why I hate reading things on "Nature (Score:2)
It's better than that: Nature summarise an article, then /. gives us a precis of the Nature article. According to the abstract [royalsoc.ac.uk] of the original article, 'one can discover the outcome of [a] computation [...] without running the computer', and, of course, 'there are some limits on the information that can be obtained from them'.
Nature seems to have turned this into 'it should be possible to determine the outcome of a computation while the machine stays off', and not told us anything about what the limits are.
May be the Nature reporter didn't want to observe the original article for fear of collapsing its waveform.
It would be dead handy if they could extend their research to show that you don't even have to build the quantum computer in the first place. Hey, you probably didn't even need to know the results of the calculation, do you?
-- Andrem
Re:Get your Physics right. It has to be ON. (Score:2)
--
Mod this up for God's sake...it's the original (Score:2)
--
Re:I really don't grok this at all. (Score:2)
I think the article is saying that if the correct answer happens to be zero, then all the states with 1's in will cancel each other out, so it will appear that the computer is doing nothing between starting the program and outputting its result. However, you don't know that the answer is going to be zero in advance, so you have to run the program anyway and wait.
I hope that makes it clearer !
Safety Warning (Score:2)
Quote (Score:2)
Extreme Programming for Quantum Computers (Score:2)
Extreme Programming for Quantum Computers:
Re:I really don't grok this at all. (Score:2)
Re:Parallel Worlds? (Score:2)
Re:The true effect of quantum computers (Score:2)
I wonder if that will be the eventual effect of quantum computing. Think about it. Currently all sorts of things that happen in our lives and determin the course of history flow through computers. Votes, Communication, work schedules, Weather forcasts ect. ect. ect. This trend continues with computerss for the next 50 years at which time quantum computers are/become the norm. So at that point everything from the train arriving on time to if this comp-country goes to war is worked out on a computer. Then someone gets the bright idea to look outside of a window and finds the exact co-ordinates of a passing pigeon. Since all the Quantum computers are tied together by the Internet 3 all probability waveforms collapse and the future of the universe is set in stone from that point foward.
Before you flame yes I know that Quantum computing doesn't work like this. Still it is a good thoght experiment. Might make a good short story by an Author who may or may not be alive. (Dick perhaps ;)
On Einstein (Score:2)
Just because Einstein was brilliant doesn't mean he wasn't an idiot.
I've always get riled when people point to Einstein and say, "See, if Albert E. says it then it must be true."
Yes he made revolutionary contributions to Physics based on his ability to comprehend, categorize, and summarize work by previous Physicists in a way that even highschool students can understand, fundamentally. Relatvity permanently tweaked human cognitive faculties; he evolved human's thought process.
However, he still was a religious man, which is more psychological than scientific. I hardly think that refuting a promising, revolutionary discipline, such as QM, simply because of Einstein's deeply rooted fear of the unknown is absurd. We all know it's easier to solve problems in a petri dish than it is to solve problems in your personal life, that's why Albert E. abandoned his wife and kids for his first cousin! (And if I remember correctly, one son renounced him and the other went nuts. So is it OK to abuse your family if you're a genius?)
Even the smartest among us still try to put the religious blinders back on again. It's my job to call them on it. Sorry tb3, I'm not directing this at you, I'm just ranting.
-S
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Quantum Computing explained... (Score:2)
A simplistic comment to be sure, but the article also touches on the clasic example of Schrödinger's cat: Certainly one of the more trotesque example of quantum mechanics, out there, but I guess the value is you'll never forget it. I guess that makes a difference
--
IHBT, but YHL (Score:2)
> are both offensive to anyone who has the guts to think by themselves.
> fundamentally abhorrent ideas such as quantum mechanical tunneling
> or particle-wave duality do not even deserve to be commented on.
Newtonian mechanics state that barrier tunneling is impossible as a phenomenon, and Maxwell's stuff doesn't touch on it. This idea developed from Einstein's GR theory. Understanding barrier tunneling allowed for the eventual discovery of materials that allow barrier tunneling in predictable ways. When these materials allow the tunneling specifically of electrons, they are labelled semiconductors. These materials can be made into computing machines that allow you to post a message to Slashdot saying that the theory that decribes their behavior is bunk.
There, now, don't you feel dumb?
There is little "common sense" in modern physics, but that's only because the sense you decribe as "common" is based on macroscopic observation, and these rules break down badly as you go to extremes of smallness, or bigness, or fastness, or farness. If you push two big magnet "North" poles together, they push apart, even when they're touching. Push two protons together, however, and after they get within a certain distance, they attract each other. Abhorrent? Perhaps. Impossible? Tell the residents of Hiroshima that it's impossible.
Virg
Ouch! (Score:2)
Virg
Politicians (Score:2)
My .02,
Next step... (Score:2)
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Juno (Score:2)
Juno must be drooling over these quantum computers!
Re:Get your Physics right. It has to be ON. (Score:2)
The "quantum computer" they're talking about isn't analagous to a PC, and may never be. As I understand the state of the technology today, a quantum computer would have to be purposefully built to solve a single problem, but once constructed, could solve the problem without actually examining the data. See this article [teleport.com] about quantum bomb detection - it give an easier to understand example that describes the physics behind "conterfactual computation".
Re:Whose policy are you smoking? (Score:2)
"The president believes that it's an American way of life, that it should be the goal of policy-makers to protect the American way of life. The American way of life is a blessed one."
Wow. Let's see here...since Roe V. Wade abortion has also been an "American way of life," but I don't see him doing anything to protect it. Blessed my ass...
That just pisses me off when bone-headed leaders like Dubby can't figure out that change happens and that even "The American Way of Life" needs to change sometimes...
This is nothing new... (Score:2)
Zeroes only? (Score:2)
"When I was young, we didn't have any o those sissy objects and icons. We had only zeroes and ones. My first assignment was a database management program, and I had to use only zeroes."
"You had zeroes? We had to use the letter 'O'."
Seriously speaking, how would the user know what was the problem he got the answer for? Maybe they would get 'segmentation fault, zeroes dumped' on every problem.
Without even being made? (Score:2)
Just by meditating deeply enough, I should get Quake running on this mother any minute now!
Re:Quantum Computer Questions (Score:2)
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Re:Emperors New Clothes... (Score:2)
Hell, even Einstein didn't buy it. He said, "God does not play dice with the universe."
I can't even point you to a good introductory source, off-hand. All my textbooks were very dry reading.
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Re:Get your Physics right. It has to be ON. (Score:2)
Re:The next step ... (Score:2)
Or if a problem could be contructed where a match would only exist in another world where they are also running this process, you could form some sort of communications link.
Or at least one that was not provably just an illusion of quantum mathematics...
The true effect of quantum computers (Score:2)
Quantum computation, however, is much more complex and much more interesting. Quantum computers are based on the concept of quantum entanglement, the ability of a quantum state to exist in a superposition of all of its mutually exclusive states: It's a 1 and a 0. However, this is not as easy to use as one might think. While it's true that if you have n quantum logic gates you have the ability to input 2^n data values simultaneously (as opposed to only 1 piece of data if you have n digital logic gates), this is not going to be the end of classical computing for a few reasons. First, quantum computers have to be perfectly reversible. That means for every output there's an input and vice versa. And there has to be no way of knowing the initial states of the data. You don't process data, you process probabilities in a quantum computer; if you know exactly what any one value is throughout the computation, you can find out all of the values: the superposition ends and you're stuck with a useless chunk of machinery. This means YOU CAN ONLY GET ONE RESULT FROM ANY QUANTUM COMPUTATION, THE END RESULT. You can't see what the data in the middle is or the computer becomes useless. (Landauer's principle makes heat loss data loss. When your processor gets hot, it's losing data. If the same thing happened to a quantum computer, it wouldn't be quantum anymore.) Decoherence is what happens when you randomly lose data to the environment by design, not by choice, and the superposition ends. This is bad for Q.C. Oh, and quantum computers can only do *some* things faster, like prime factorization and discrete logarithms. Not multiplication or addition. Plus, the circuits that would do basic arithmetic would be bigger and slower than what you've currently got.
So what does this all mean? It means that quantum computers are going to provide some advantages (real quick big number factorization), and some disadvantages (that whole RSA standard). The most realistic initial use of quantum computers will be as add-ons to existing super-computers to resolve certain types of NP-Complete headaches that regular math can't simplify yet. At best they will someday be an add-on to your PC; but they will never replace the digital computer.~
If you want more info, check out http://www.qubit.org [qubit.org], it's got some decent tutorials.
Example of counterfactuals in QM (Score:3)
Suppose you have a bomb with a trigger on its tip, so sensitive that a single photon hitting it will make it explode. Suppose further that you have some good bombs and some duds with stuck triggers that don't move. Can you tell which bombs are good and which aren't? Well sure, just hit it with a photon and see if it blows up. But can you do it without blowing up the bomb? Yes! (Well, probabilistically yes.)
Here's how it works: shoot a laser beam through a beamsplitter, bounce the two beams off of mirrors (one of which is attached to the trigger of a bomb) which redirect the beams back into another beamsplitter, with detectors on the other side. You won't understand the setup without looking at the picture [hampshire.edu] -- I will be referring to it.
A laser beam is a coherent superposition of a bunch of photons. What happens if you turn the intensity down so much that you're getting single photons out? When the photon reaches the first beamsplitter, you might think that the photon either goes one way or the other. But quantum mechanics says that the photon enters a superposition state in which there is a 50% probability that it took the upper path and a 50% probability that took the lower path -- you won't know until you collapse the wavefunction by measuring which path it went through (by putting a detector along the path or something). (I'm being vague here: it's not that it went one way but you don't know until you measure it; rather, "which way it went" is simply undefined until you measure it, and if you don't measure it then it's never defined.)
So anyway, the photon hits the first beamsplitter and enters a superposition. If the bomb is a dud (fixed mirror), then this is an ordinary interferometer. At the second beamsplitter, the two beam wavefunctions (representing a single photon) interfere with themselves to produce a state with 100% probability of being detected at B and 0% probability of being detected at A. (You can see this from symmetry: the beam enters the system horizontally and has to come out horizontally too since the apparatus is symmetric.) This is precisely what you would expect classically with wave interference, by the way. Nothing too odd.
However, suppose that the bomb isn't a dud. Then the the impact of a photon on the mirror is free to move the trigger and set off the bomb, so the bomb serves as a measuring device! If it a photon hits it, the bomb will explode, so you have definitely measured that the photon took the lower path. What happens here? Like I said, there's a 50% probability that a measuring device (such as the bomb) inserted into the lower beam will measure a photon. If that happens, the bomb will explode. But what if the photon is measured to not take that path (by virtue of the bomb not exploding)? Then with 100% probability it took the upper path. When it hits the second beamsplitter, it's just as if it hit the first beamsplitter, since there's no interference from the other beam -- we know that nothing went that way. So with 50% probability it goes to detector A, and with 50% it goes to detector B.
The upshot: if the bomb is a dud, then you will get a photon at B with 100% probability. If the bomb is good, then you get an explosion with 50% probability, a photon at A with 25% probability, and a photon at B with 25% probability.
The point: if you get any photons at A, then you know for sure that the bomb was good. But you didn't actually ever send a photon to the bomb to find out! This is a "counterfactual" -- you are obtaining information about something that never happened (a photon hitting the bomb), but could have!!
Of course, you don't have an infallible scheme. Half the time you have a good bomb your test blows it up, and half the time it doesn't you can't tell whether it was good or not (because you got a photon at B). But 25% of the good bombs are provably good (without blowing them up). It turns out you can cascade this process to make the probability of detecting a good bomb as high as you want.
This effect is known as "quantum non-demolition" and has been experimentally verified (not with real bombs of course). You can use it to measure things using photons without destroying the photons (normally a photon is destroyed whenever it interacts with something -- it is absorbed).
Questions (Score:3)
But what about:
-energy required to set up initial values so that the answer is all zeroes
-energy required for cooling or otherwise insulating/maintaining computer during the time it will be computing and simultaneously off
-who's going to police those qubits and tell them not to cannabilize energy from that environment (and presumably return it.. oops don't want to go there)
-would you need a reservoir of energy attached to the thing so that it would be theoretically possible for the system to go up the energy hump if calculation (in other universe required it)?
-do you ever even turn a quantum computer on anyway? seems a delicate enough computer could get by without humans intentionally putting power to it
-(back to the question of energy required to set up the computer): is there not a law which requires energy to create or change information, or is this a silly misconception. If so, are they not just taking care of energy expenditures before getting to the calculation stage, leaving a lot of the actual energy goings on in the unfathomable, unaccountable finite time span of computation? Or is there something else going on?
** It seems there is something more interesting going on, but there is neither mathematical meat for the professional nor a real explanation for the layman. The Royal Society Proceedings are too briefly abstracted to get anywhere. Could someone paraphrase or post the text of those proceedings or is this going to be an exercise in frustration?
In particular what exactly are the limits to the kind of probing you can accomplish which was mentioned in that abstract?
Re:"switches off" doesn't mean computer is off. (Score:3)
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Re:The true effect of quantum computers (Score:3)
Quantum entanglement is nothing more than the superposition of multiparticle states that aren't simple tensor products of the individual particle states. That's it. In fact your description is just as inaccurate as his. If two particles are entangled it doesn't mean that knowing the state of one tells you the state of the other. Things can be slightly entangled in that you might be able to glean some information about one by observing the other.
--"switches off" doesn't mean computer is off. (Score:3)
Just because your register is zeroed doesn't mean the chip is off.
I'll accept that the computer is off if it's not using any energy.
Cheerio,
Link.
Re:Yep (Score:3)
Quantum Bomb testing - thought experiment (Score:3)
It took a bit of head scratching and squinting, but I did finally manage to figure out how the thing works.
It uses some of the same theory as I would expect the "quantum computer" to use. In this case, they use a bomb which has a trigger which is sensative to a single photon of light. A dud bomb will pass the photon, but a live bomb will stop it. The experiment shows how it is possible to detect, using quantum mechanics, whether a bomb is live or a dud, without actually exposing it to the photon of light, and thereby exploding it.
I'm sure this is a much simplified version of what they're planning for the computer, but it's described in terms that pretty much anyone can understand, even though it's not obvious at first.
Already done. (Score:4)
Re:The true effect of quantum computers (Score:4)
"Quantum computers are based on the concept of quantum entanglement, the ability of a quantum state to exist in a superposition of all of its mutually exclusive states"
and stopped reading. What he describes is just called superposition; quantum entanglement is when two particles' quantum states depend on each other in such a way that once you know the state of one, you also know the state of the other. To read more about it, look up the EPR paradox.
Don't spout off BS, and please, moderators, don't moderate it up!
Re:I really don't grok this at all. (Score:4)
It's not so much that it picks a *right* answer out of all possible answers, but rather, the impossible states collapse and you are left with a quantum superposition of all the possible states. (Or, to try to put it more simply, those answers that cannot exist cease to exist, leaving only those answers that can exist.)
Whose policy are you smoking? (Score:4)
The White House said [go.com]: White House spokesman Ari Fleischer was adamant Monday when asked whether the president would ask Americans to stop using so much energy.
"The president believes that it's an American way of life, that it should be the goal of policy-makers to protect the American way of life. The American way of life is a blessed one."
I say: Iron Monkey, what are you smoking? It's UNamerican to conserve. It is my patriotic duty to be an energy hog. Who needs these new-fangled 'putin machines work'in without 'tricity?
Quantum Girlfriend? (Score:4)
Re:Quantum Computing explained... (Score:4)
A Zen machine.
Quantum Computer Questions (Score:4)
And if you watch the screen while it's running, will this collapse the computer's state and break it?
Re:I really don't grok this at all. (Score:5)
They've tried that before, the answer was 42.
But what was the question again?
Just a thought... (Score:5)
Does this machine run on software that doesn't have to be written?
That's nothing new! (Score:5)
Problem solved.
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And that's why I hate reading things on "Nature" (Score:5)
Schrödinger's computer (Score:5)