# Quantum computation

1. Jan 24, 2009

### cam875

I have 2 questions about quantum computation that I find interesting.

1. I understand that because the qubits are in a superposition and therefore do not have defined values that they perform all the possible calculations at once therefore making it the most powerful and parallel processor possible. So lets say for example that we have a 1 qubit computer so in one process or sweep it will do all three possible additions simultaneously
0 + 0
0 + 1
1 + 1

so do they need to store all the results and than sift through them to find the answer there looking for or what, it doesnt seem very practical to have all the possible answers but not know which one is correct.

2. If another theory proves to be right in the next 50 or so years such as string theory would that mean that quantum computing is infact impossible since it relies on the principles of quantum physics.

any help with these questions is appreciated. thanks in advance.

2. Jan 24, 2009

### The Dagda

A computer has: 8,16,32,64 bits according to the processor, these make up 1 number in binary let's say 11111111=256 or 8^2 possible combinations.

Now let's say each of those 1s and 0s can make up 1,0 or 1 and 0. This means you have instead of say 8^2 combinations, 8^4 different combinations. In theory you have multiplied the processing capability by a power of 2.

As for string theory, it is compatible with QM, in fact it actually tries to bring QM and general relativity together.

3. Jan 24, 2009

### Hurkyl

Staff Emeritus
Was there a question in there? Anyways, part of this type of algorithm includes post-processing to make the desired answer(s) stand out from the rest, so when we "look" at the result of the computation, it's very likely the answer we were looking for.

No. Small computations with quantum computers have actually been done. A new physical theory cannot change that fact -- at worst, it can merely put practical (or theoretical) limitations on how 'big' or 'efficient' a quantum computer can be.

(Well, to be precise, a new physical theory could predict quantum computing is impossible, but it would be automatically falsified in this domain)

4. Jan 24, 2009

### cam875

so which companies are currently leading this type of research, I would like to see what they have so far?

and I dont understand what you mean by the answer being able to stick out from the rest.

5. Jan 24, 2009

### Hurkyl

Staff Emeritus
Exactly what I said next -- to make it so that the result of observation is most likely to be the one we want. (i.e. the amplitudes of wanted results are made large, while the amplitudes of unwanted results are made small)

6. Jan 24, 2009

### cam875

I might be asking too much on this here but how do they actually instigate an OR or an AND logic gate to take place with whatever they are using between two qubits and than physically figure out which result is sticking out the most. Ive heard of ion traps and quantum dots but am not quite sure which one is a good example to learn.

7. Jan 24, 2009

### Hurkyl

Staff Emeritus
(As far as I know) quantum programming is all about finite-dimensional linear algebra: you only need the physics for engineering purposes (e.g. how to actually build one, what gates are possible, etc). Compare with the fact you don't need to know any electrical engineering at all in order to program an ordinary computer.

Here is Grover's algorithm, one of the fundamental algorithms of quantum computing.

8. Jan 25, 2009

### cam875

but do you understand how a quantum dot works or something? like the hardware of this stuff. and thanks for that link.

9. Jan 26, 2009

### QuantumBend

Quantum dot use coherent light not computer. No good for big computer.