# Quantum Gates and Circuits

1. Mar 22, 2005

### H-bar None

How do quantum gates work and how our they different than classical gates?

2. Mar 22, 2005

### Antiphon

Quantum gates would be the building blocks of quantum computers, and they could
theoretically calculate much faster than an ordinary computer for certain types of
arithmetic problems.

Crudely put, the idea is that a quantum system can achieve calculation with the
wavefunction in the "wave" mode rather than the "particle" mode. Wave phenomena
are inherently "parallel" when used as a computational tool, so you'd be doing lots
of "work" in a single computational step.

Imagine that you have a computer with a large number in it. You want to divide that
number by all the numbers from 1 to 1e100 to find the one that divides into it evenly.
In an ordinary computer, you would essentially try each divisor one after the next.

A quantum computer could in principle try all the divisors simultaneously. You would
make a quantum "measurment" of the result that had no remainder, forcing the one
calculation you wanted to see to become the manifested value.

It is much like an analog computer that solves a fluid dynamics problem by direct
simulation, but in the quantum computing case you still emply the methods of digital
computers and gain parallelism from the indeterminacy of the quantum system.

3. Mar 22, 2005

### marlon

Well, this is not really accurate. The biggest difference between a qubit and an ordinary bit is the fact that a bit is either 1 or 0. The qubit is a SUPERPOSITION of 1 and 0. So the qubit really is the 'combination' of the two possible bit-states.

The clue in QM-related calculations is the fact that you don't measure one specific qubit because all the information in this massive quantum paralellism would be gone (the superposition is broken). For example, you can 'calculate' a thousand values for any f(x) in just one step. Classically you would need 1000 calculations. Ofcourse you cannot just measure what outcome 926 is
( ie the term on the 926th position in the superposition of |x>|f(x)>).

Well, you can but then all other terms are lost and you have no benifits of the QM-approach compared to the classical one. What you can do is try to figure out mutual connections between the different terms in the superposition, like phase-differences or something like that.

Further info can be found on John Preskill's webpage, just google for his name;;;Also, look up the problem of Deutsch

marlon

4. Mar 23, 2005

### Antiphon

I did say it was crudely put.

5. Mar 25, 2005

### H-bar None

Thank you for the replies.

Is there a way to peform the operations without disturbing the system?

Does Deutch's algorithm solve this problem?

6. Nov 7, 2008

### sarapro

hello everyone
I have problem showing that the fredkin gate is reversible and also a universal classical gate.
is there anyone who can help me solve this problem?