Linked by Thom Holwerda on Thu 23rd Feb 2006 18:09 UTC
Geek stuff, sci-fi... Ok, this one is just plain scary. "By combining quantum computation and quantum interrogation, scientists at the University of Illinois at Urbana-Champaign have found an exotic way of determining an answer to an algorithm - without ever running the algorithm. Using an optical-based quantum computer, a research team led by physicist Paul Kwiat has presented the first demonstration of 'counterfactual computation', inferring information about an answer, even though the computer did not run." The research team published their results in Nature.
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That's not so difficult...
by Gadget on Thu 23rd Feb 2006 18:36 UTC
Gadget
Member since:
2005-10-21

... this morning I was able to predict that I would be here at work right now even before I had actually left my home.

Amazing!!!!

Reply Score: 1

RE: That's not so difficult...
by someone on Fri 24th Feb 2006 13:55 UTC in reply to "That's not so difficult..."
someone Member since:
2006-01-12

Please don't bother commenting if you have no idea about the basic principles of Quantum Mechanics.

Quantum Mechanics is not an ivory-tower science as perceived by most laymen: without it, your classical computers (yes, the one you are staring at right now!) won't even run.

Reply Score: 1

Quantum physics vs. preconceived ideas
by KenJackson on Thu 23rd Feb 2006 19:01 UTC
KenJackson
Member since:
2005-07-18

“By placing our photon in a quantum superposition of running and not running the search algorithm, we obtained information about the answer even when the photon did not run the search algorithm,”

“That is at the heart of quantum interrogation schemes, and to my mind, quantum mechanics doesn’t get any more mysterious than this.”

Makes me appreciate my favorite quantum physics quote all over again:

"If one has to stick to this damned quantum jumping, then I regret ever having been involved in this thing."
Erwin Schrödinger

But it also reminds me that someone (forget who) prophesied that it would take at least a couple generations to take good advantage of quantum physics because earlier scientists had preconceived concepts of reality that inhibited them.

Reply Score: 2

transputer_guy Member since:
2005-07-08

Its right up there with string theory which at least sounds a bit more plausible otherwise they wouldn't put it on PBS Nova or BBC Horizon?. I've never seen this quantum stuff try to be explained to the layman on any of these shows yet, perhaps if it were, it would disappear.

Reply Score: 1

DigitalAxis Member since:
2005-08-28

Well, I don't pretend to understand it very well, but here goes. Please don't blame me if I'm wrong (and I probably am)

Basically, the problem with Quantum Mechanics as opposed to Regular Mechanics is that Quantum Mechanics is "non-deterministic". What non-deterministic means is that you can't actually just determine what state something is going to be in, just a probability.

One other bizarre related prediction of this theory is that observing something changes it.

It's Schrodingers' Cat: Imagine a cat in a box, with a bottle of poison and a radioactive source. If the radioactive source happens to decay (there's only a probability that it will or it won't, like flipping a coin), a hammer will smash the bottle and kill the poor kitty.
Until you open the box, you won't know if the radioactive source decayed and killed the cat. The catch is, according to Quantum Mechanics, the cat is BOTH until you check.
Once you check, the "waveform collapses" into one possibility at random. This has actually been tested and observed (the Stern-Gerlach device, I can't find a good explanation of that actual experiment online), fortunately not with cats (that thought experiment was invented to highlight how absurd the whole idea was)

Albert Einstein hated the idea ("God does not play dice with the universe"), and spent a large portion of his later years trying to prove, essentially, that you really could tell whether or not the cat was dead without checking. (The Einstein-Podolsky-Rosen Paradox) Unfortunately, he wasn't successful.

Reply Score: 4

John Nilsson Member since:
2005-07-06

I've never liked the Shrödingers cat example. It's to philosophical.

A more direct example of quantum mechanics is the experiment with lasers and mirrors.

The setup is a laser fireing into a semi-transparent mirror, where the resulting rays are later collected in another semi-transparent mirror with two detectors on either exit path.

As long as you don't measure anything except the detectors at the end both detectors get 50% of the laser photons.

The quantum weirdness starts when you block one of the two paths. Common sense dictates that 50% of the photons would be lost and the other 50% would be divided in by the last mirror so that the detectors get 25% of the photons each.

But what happends is that 100% of the photons goes to one of the detecors.


I don't know if quantom logic is involved but it certainly seems similar.

In quantum logic there is a concept of a sqrt!() (that is square root of not) function with the properties that

sqrt!(x) == random(2) (where x is 0 or 1)
and
sqrt!(sqrt!(x)) == !x

This also shows why it's impossible to measure quantum systems. If the result of the inner sqrt!() was know the result of the outer sqrt!() would be random, and thus unknown.

Edited 2006-02-25 06:06

Reply Score: 1

Morin Member since:
2005-12-31

> I've never liked the Shrödingers cat example. It's to
> philosophical.

I don't really know, but isn't it actually incorrect? The whole example assumes that the box-cat-system doesn't 'collapse' into a specific state, but with macroscopic systems like a cat this is (practically) always the case.

If I remember correctly, then for the same reason quantum computers don't work yet. Their computation is based on superposition of states, but as soon as something interacts with this mess-of-states, it collapses (by biased random choice) into one of those states, destroying the computation (that's as if your computer's RAM lost all its data at random intervals).

- Morin

Reply Score: 1

someone Member since:
2006-01-12

That's one possible intuitive explanation of QM. However, not all interpretations require the wavefunction collapse.

Personally, I like to keep the intuitive part to a minimum and stick to the equations: What's wrong with probability patterns that just happen to match the wavefunction? We will probably need the String Theory to really explain what really is a photon/electron/etc. (In the mean time, I am hoping the LHC will be finished soon)

Reply Score: 1

me thinks
by transputer_guy on Thu 23rd Feb 2006 20:26 UTC
transputer_guy
Member since:
2005-07-08

Straight out of DrWho, when any of the Doctors said this sort of stuff, it was good fun, but when real physicists say it I dont' believe it till I see it.

Reply Score: 2

RE: me thinks
by puddleglum on Thu 23rd Feb 2006 20:54 UTC in reply to "me thinks"
puddleglum Member since:
2005-07-20

Yeah -- you think they started to inhale.

Reply Score: 1

My head hurts
by DigitalAxis on Thu 23rd Feb 2006 20:30 UTC
DigitalAxis
Member since:
2005-08-28

This sounds far too much like Douglas Adams' account of the creation of the Improbability Drive, in The Hitchhiker's Guide to the Galaxy.

You know, the intern working while sweeping up the lab, who figures out exactly how improbable the device would be, and it suddenly appears?

So... these people have managed to run calculations on a quantum computer they haven't even turned ON?

...I understand enough about quantum mechanics to see how that makes sense quantum mechanically, but it still blows my mind.

Edited 2006-02-23 20:36

Reply Score: 2

RE: My head hurts
by Antarius on Fri 24th Feb 2006 00:36 UTC in reply to "My head hurts"
Antarius Member since:
2006-02-24

I was thinking the same thing;

But then I realised: This makes "Deep Thought" look pathetic.

Deep Thought took millions of years to calculate an answer to a question that wasn't known.

This thing almost instantly gave the answer to a question that wasn't asked!

Reply Score: 3

Douglas Adams was right
by gustl on Fri 24th Feb 2006 13:38 UTC in reply to "RE: My head hurts"
gustl Member since:
2006-01-19

May be more right than any of us dares to fear ;)

Maybe one day a computer will find the question to an answer that will not have been given.

Methinks my brain windings are becoming straight.

Reply Score: 1

meh
by Zedicus on Thu 23rd Feb 2006 20:51 UTC
Zedicus
Member since:
2005-12-05

if you cant prove it cant be done, it usually happens...
and
if you cant prove something, prove its oposite.

quite over simplified rules, and everyone knows ever rule has an exception....

Reply Score: 1

I'm sorry...what?
by Yamin on Thu 23rd Feb 2006 20:59 UTC
Yamin
Member since:
2006-01-10

Maybe my brain is just fried right now. What is this article saying?

Reply Score: 1

RE: I'm sorry...what?
by astroraptor on Thu 23rd Feb 2006 21:10 UTC in reply to "I'm sorry...what?"
astroraptor Member since:
2005-07-22

You're not alone on this, you really aren't.

So the computer DID run but it didn't like a photo can be in multiple places at once even though it appears only in one place at a time? Phheww, Boeing!!

Reply Score: 1

physics is mad science these days...
by koen on Thu 23rd Feb 2006 22:58 UTC
koen
Member since:
2005-11-15

it's mindblowingingly funny that they pulled this off.. they fired entangled photons to some reflective plates to 'query a database', but now interfered the photon directly after launch, and 'read' its state.. but the path was set and the 'ripple' continued and they got an answer that is, to a predictable (?) certainty, correct.. this is what you gotta love about contemporary science.

Reply Score: 1

Mmhh am I too dumb or...?
by pfortuny on Fri 24th Feb 2006 08:31 UTC
pfortuny
Member since:
2006-02-05

Do not try to understand "quantum algorithms" as algorithms. They are mere quantum **systems** which, as such **quantum** systems behave in **unpredictable** ways.

They have built up a quite clever experiment which **when interpreted as an algorithm** behaves like stated.

I would not draw any more complicated conclussions from that. Quantum **systems** are just that: complicated systems with random behaviour (though statistically sound, usually, in the long run).

The summary, though, is quite bad:

Although a photon can occupy multiple places simultaneously, it can only make an actual appearance at one location.

Typicall mistake. It is a quantum object; it is not in multiple places but everywhere as a function wave. And in "all the states" at once (ibid.)

We are still to have a Shor's factoring machine. That will be a great day.

Reply Score: 1

Not surprising...
by foljs on Fri 24th Feb 2006 13:37 UTC
foljs
Member since:
2006-01-09

Quantum Computer Solves Problem, Without Running

What's new here?

Not running Windows also solves many problems.

Reply Score: 5

RE: Not surprising...
by someone on Fri 24th Feb 2006 13:58 UTC in reply to "Not surprising..."
someone Member since:
2006-01-12

Again, please don't bother commenting if you don't have a basic understanding of Quantum Mechanics... It will only show your ignorance.

Reply Score: 1

42
by weirdnut on Fri 24th Feb 2006 14:50 UTC
weirdnut
Member since:
2006-01-19

Hell, I'm stupid and my brains hurt after reading that article.

Can't wait to see Windows run on any of these future quantum systems (no, I will not make fun of it).

Reply Score: 1

RE: 42
by Bit_Rapist on Fri 24th Feb 2006 18:12 UTC in reply to "42"
Bit_Rapist Member since:
2005-11-13

Can't wait to see Windows run on any of these future quantum systems (no, I will not make fun of it).

Its already been done. ;)

http://atomchip.com/_wsn/page4.html

Reply Score: 1

RE[2]: 42
by Morin on Fri 24th Feb 2006 18:41 UTC in reply to "RE: 42"
Morin Member since:
2005-12-31

> Its already been done. ;)
>
> http://atomchip.com/_wsn/page4.html

This is simply incorrect. What is described behind this link is not quantum computer. It doesn't utilize superposition and entanglement, which means it's power is just that of a classical computer.

- Morin

Reply Score: 1

RE[3]: 42
by Bit_Rapist on Fri 24th Feb 2006 18:49 UTC in reply to "RE[2]: 42"
Bit_Rapist Member since:
2005-11-13

This is simply incorrect. What is described behind this link is not quantum computer. It doesn't utilize superposition and entanglement, which means it's power is just that of a classical computer.

- Morin


It was meant to be a JOKE as we've had many long discussions on these forums about the AtomChip.

Reply Score: 1

RE[4]: 42
by Morin on Fri 24th Feb 2006 19:28 UTC in reply to "RE[3]: 42"
Morin Member since:
2005-12-31

> It was meant to be a JOKE as we've had many long
> discussions on these forums about the AtomChip.

Oops... sorry then!

- Morin

Reply Score: 1

RE[5]: 42
by Bit_Rapist on Fri 24th Feb 2006 20:20 UTC in reply to "RE[4]: 42"
Bit_Rapist Member since:
2005-11-13

Oops... sorry then!

- Morin


No biggie dude. its all good ;)

Reply Score: 1

Quantum computer
by aaronb on Fri 24th Feb 2006 16:58 UTC
aaronb
Member since:
2005-07-06

Not easy to understand but an intresgting subject.

Reading this and related text on Wiki, gives a good insite to quantun computers and science.

http://en.wikipedia.org/wiki/Quantum_computer

Edited 2006-02-24 16:58

Reply Score: 1

It appears ....
by Archamian on Fri 24th Feb 2006 19:47 UTC
Archamian
Member since:
2006-02-24

...that this system does not use the algorithm to determine the outcome, rather it is merely describing it. The setup itself seems to determine the potential outcomes.

Reply Score: 1