# Quantum computing (Why is it important?)

1. Nov 30, 2009

### Lamented_Soul

Alright, I'm just getting my feet wet in the world of physics, but, I've taken a particular intrest in quantum mechanics/physics. And, my first question is, what is the point of quantum computing? It's my understanding (Though it's probably seriously flawed) that quantum computations are used to study extremely small units of matter (I.E Quarks).

2. Nov 30, 2009

### JK423

You have misunderstood it. Quantum computing takes advantage of the quantum property "superposition" in order to make calculations faster and faster in contrast to classical computing with bits.
So, quantum computation doesnt "study extremely small units of matter (I.E Quarks)." in the sense you say it.
Look it up in wikipedia

3. Nov 30, 2009

### Lamented_Soul

So, using Quantum comptuing, would it essentially be possible to compute the entire atomic make-up of a human being without using a massive amount of space to store the data? I've heard that the individual paths that electrons, protons, and neutrons take when revolving around the Nuclei, is almost an inordinate amount of data by even todays standards.

4. Nov 30, 2009

### JK423

No.
Quantum computing is just another way to compute things! What you can compute with a classical computer (classical computing) you can also cumpute with a quantum computer (quantum computing). The only difference is that quantum computing is much faster than classical computing. What you are computing is irrelevant. You can compute anything you want, from 3+4 and differential equations to whatever you like. Its just a matter of how fast youre going to do that

5. Nov 30, 2009

### Lamented_Soul

Meh, alright, nvm then. Then if that's all it is used for (In general), then why is it important? I mean computing equations at faster speeds doesn't exactly seem all that important to me. (But, I'm still a noob when it comes to physics, so yeah.)

6. Nov 30, 2009

### JK423

Its very important.
When it comes to modern theories like quantum chromodynamics (the theory that describes quarks) the only way to get a solution from the equations is by using your computer. But its so demanding in computational power that physicists use supercomputers in order to get a solution "in this century" :P
A quantum computer would compute such things a looooooot faster, so we would be able to compute things that we werent able before. Thats important, isnt it?

7. Nov 30, 2009

### TMFKAN64

In computer science, one of the classic problems is called P vs. NP. The basic idea is that P is the class of problems where the answer can be computed in time that is less than some polynomial of the length of the input, and NP is the class of problems where an answer can be guessed and verified in polynomial time. (It is not known if these classes are identical or not.)

For example, suppose we wanted to know if the minimum of a list is less than a constant C. We could simply scan the list, and report if we find an item less than C or not. This requires looking at each item once, so the time is linear in the length of the list, so this problem is in P.

Similarly, suppose we are given a list of cities and distances between them, and want to know if there is a path that visits each city exactly once that has a total length less than C. If we make a lucky guess of a path, it is a simple matter to add up the total length of the path and verify that it is less than C. The number of additions is linear in the number of cities, so this "travelling salesman" problem is in NP.

If you want to solve one of the hardest problems in NP with a computer program, there are few options besides guessing each possible answer in turn and then attempting to verify it. Since the number of possible answers is exponential in the length of the input, this procedure will be exponential in the input, not polynomial.

However, the hope is that by using superposition, we can use superposition to do a calculation on *all* of the possible guesses at the same time, and as a result, we would have a method of solving NP-hard problems in polynomial time.

8. Nov 30, 2009

### f95toli

That is not quite correct, a quantum computer is only faster than a classical computer for some problems, not all.
So far only a handful of algorithms have been developed that benefit from being run on a quantum computer. the most famous being Shor's algorithm for factorizing numbers.
Hence, it is extremely unlikely that quantum computers will ever be more than a compliment to classical computers. That said. more algorithms are being developed all the time (there was a new one in PRL a couple of weeks ago) so this might of course change in the future.

9. Dec 2, 2009

### juzzy

A quantum computer (or at least a computer using quantum logic) should be faster at any calculation that can be done by a classical computer. Using entangled states and applying schrodingers equation to determine how the system evolves, you should be able to disregard one bit of information going through a logic gate, whereas with a classical computer you have to perform an action on 2 bits.