# Quantum computers, quick question

1. Jul 29, 2008

### Nick89

Hi,

This is probably a very basic question but it's just something I don't seem to understand...

I'm just reading up about quantum computers and how they are supposed to work. I don't know all that much about quantum physics so my ideas might be off, but I might know enough to understand the basics...

While reading about quantum computers I found that they are supposed to be so powerful because their qubits can take not just the values 0 and 1 (like ordinary bits) but also any value in between (a mixture of 0 and 1). This is supposedly what allows them to hold much more data and compute much faster.

Now my question is, how the hell does a computer deal with "a mixture between 0 and 1"?
I can understand how computers use 0's and 1's to calculate things, and for some reason I can't find anything (at least nothing I can understand) about how quantum computers compute with this mixture between 0 and 1.

I also read that the state of the qubit, when measured, collapses into 0 or 1, and nothing else. So if the qubit is still only 0 or 1 after being measured, do quantum computers somehow use the value of the qubit before measuring it..?! I don't seem to understand this... If they must measure the value before they can use it, how can quantum computers be more powerfull? How is a qubit then different from a normal bit, since after measurement it's still only 0 or 1??

There's probably a huge misunderstanding on my part, I would appreciate it if someone can point it out and possibly explain what I'm not getting... Thanks!

2. Jul 30, 2008

### mrandersdk

Quantum computers are very advanced to understand, but to try to make it simple. The computer as you say can calculate a lot of data on the same time because the qubits are in a superposition state, but as you also say when you measure it it collapses, and this is a problem in general. But some clever people have designed algorithms that is so smart that when calculations are made the probability for measuering the right answer get bigger and bigger. So if you get the right answer 95 % of the time and you run it alot of times you have a great chance of getting the right answer.

Also these computers is often used (in theory that is) to solve problems where you know if the answer it gives you are right. Fx. if you want to find the prime factors of some number they can be hard to find, but when you get an answer it is easy to check if they are the prime factors, also if you search in an unsorted database it is easy to check if the computer found the right thing afterwards.

You can try to take a look at

http://en.wikipedia.org/wiki/Grover's_algorithm

3. Jul 30, 2008

### Hertyque

This would be my take... First yes they callapse into 0 or 1... However a quantum computer wouldn't just be measuring one of these. There would be a whole block or wall of them(picture an entire RAM chip all gig or so of one's and zero's). All in this superpositional state between zero and one.

Then aply the question or calculation to be made to this block..The coresponding cascade of collapsing superpositions would spit out a string of zero's and one's and that would be the answer.

Basically, One superpostioned qubit would collapse and affect the one's around it in such a way as to yeild the answer in the wall of qubits. So it'dd be like forcing the wall to show a picture of the answer.

Just one idea on how to utilize the concept of quantum computing.

Ender