# Quantum computing

1. Oct 5, 2008

### cam875

this seems like a very interesting thing but what I dont understand is what is representing the quantum bit? the atom itself? the particles in the atom? an ion? or what. I am a little confused with it all. And what is representing the value of that particle or whatever. Is it its spin or charge. Any help is appreciated.

2. Oct 5, 2008

### atyy

The qbit can be realised in several ways. |0> and |1> could be two different states of an atom, two different polarizations of a photon, two different spin states of a nucleus etc. It is the same as classical computing, you can do addition with different hardware systems like an abacus, a calculator or a personal computer.

3. Oct 5, 2008

### cam875

so if they used spin states of a nucleus or something, would the superposition be represented by the nucleus having the two opposite spins at the same time? this is hurting my brain lol.

4. Oct 5, 2008

### f95toli

Something like that. There are also more "macroscopic" versions used in solid state qubits. In e.g. a flux qubit the two states corresponds to a current flowing in two different directions (clockwise or anti-clockwise in a loop), in a charge qubit the states corresponds to there being 0 or 1 extra electrons on a small metallic island etc

5. Oct 10, 2008

### LaserMind

It does hurt the brain to try to think of an atom spinning both ways at once, and it is similar to trying to think of an atom in two places at once. But the atom would only actually have a position if it were 'observed' and that would be the result of a probabilty from the density function -the 'observable'. Same with the spin, it only gets an actual spin (in our world) when we measure it. When we are not measuring it, then its back to probabilities again (it could have either spin)
Is that any clearer or have I confused it more? :)

6. Oct 10, 2008

### cam875

sorta, so are you saying that because we are not observing something we just have to use math to come up with a probability of what state it is in such as a 1 or a 0. But if we observe it than we can obviously tell what it is in reality? but just because we cant see something it still has a definite spin like up or down doesnt it? we just dont no the for sure answer. Therefore how can they calculate with a 1 and a 0 at the same time.

7. Oct 10, 2008

### Crosson

This is not true in quantum mechanics, we can do experiments that exclude this possibility. It is counterintuitive, but with the right mathematics it makes sense.

8. Oct 10, 2008

### cam875

so your saying human beings eyes directly affect it where it is or it doenst take on a spin until we decide to observe it? lol i think im way off now.

9. Oct 10, 2008

### LaserMind

No, it has both spins at once except when we observe a spin then it gives us one chosen by probability.

I suppose a very very very rough picture would be to think of a violin string vibrating. You may ask where exactly is the wire when its looks like it could be anywhere (in a short range). Then if you were to grab it instantly, it would get 'caught' somewhere, but before that is was sort of 'everywhere' or nowhere!
You need to look at the wave equation to get a better opinion about superposotion etc. At the moment you seem to be stuck on a ball like object spinning both ways at once. Not nice!
The violin string is only a bad analogy, but brings you on a little. Let me know how you get on and any revelations you get.

10. Oct 10, 2008

### Crosson

That is correct, but human eyes don't directly observe quantum systems, the system obtains a definite spin as soon as it comes into contact with a macroscopic measuring device.

11. Oct 10, 2008

### cam875

so is there any good articles or things to get me started with a little info on the wave function or something cuz it seems quite interesting. So this basically throws classical physics on its head doesnt it, I mean i would have thought that the electron is moving the way it is due to whatever circumstances and doesnt give a crap about us. But your saying that it doesnt get a definite spin until its measured? or is it that it has a definite spin already we just dont no what it is until we observe it and therefore it is no longer a guess but a known fact. So is it safe to say that technically it is either up or down spin we just dont no until we observe it and therefore it is both.

12. Oct 10, 2008

### Crosson

It is not really worthwhile to learn quantum mechanics without a mathematical background in differential equations and linear algebra, as an absolute minimum. I have never seen a non-mathematical description of quantum mechanics that made any sense!

The best introductory book would be the one by Grifiths, very popular.

I completely understand what you are asking, and as strange as it is I promise you that there is no definite spin until a measurement takes place, at least according to the standard theory of quantum mechanics.

13. Oct 11, 2008

### cam875

ok, I think its time i hit the math books first though before trying to understand quantum mechanics. So that I can actually understand everything in it. Thanks for all this help.

14. Oct 12, 2008

### cam875

is there an actual chance that it could end up being that a lot of quantum stuff was way off and quantum computers will be impossible or something. I mean do they even have a quantum chip that works properly built? I am not doubting them just posing a question of curiousity.

15. Oct 14, 2008

### cam875

one more question, since we can't actually work with a superposition number because it hasnt been defined yet, is all the computer memory stored going to be similar to a classical computer architecture where it is just pure binary because I can see how usiing QUbits to compute logic can make things faster, but u cant really store that as data without making it into classical binary right?