*"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.

* “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.

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.

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.

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*

> 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

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)

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*

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.

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.

*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

> 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

*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.

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*