Quote from: mlgblockchain on December 28, 2017, 07:31:34 PM

I just read about Quantum Computer on google. But honestly, I didn't get a clear picture. Can anybody help me out here with some simple words? I read the replies on this thread as well. But everybody is using "fake science" or similar words. What is the reality here?

The word "computer" has been changed by the transistor, integrated circuits, personal computers and the Internet. 50 years ago, the word "computer" had a lot more mystique and referred to a much broader class of systems. Today, the word "computer" refers to a very definite kind of system, the kind you're using to view this post. Prior to the rise of the digital computer, there were many types of computers, including mechanical computers and analog electronic computers. Analog computers are very efficient for specialized problem solving.

Quantum computers are best thought of as a very noisy analog computer. On a digital computer, we ask a question just once, and it either calculates the answer or hangs. On an analog computer, we pose a problem within the computer's domain and then we set it solving. Usually, the analog computation proceeds "directly" to the solution, that is, with a minimum of wait time. But the results of the analog computation are subject to limits of precision imposed by physical measurement - to get more digits of accuracy, you require finer measurement and more lossless action within the mechanism or circuit. For a quantum computer, we have the same measurement problem as with analog computers, plus the solutions it gives do not have the property that digital computers have - either correct or it hangs. Rather, the quantum computer will return a "distribution" of answers over repeated computations that hopefully clusters tightly around an average value, which we take to be the solution.

Imagine you're a physicist running simulations on some difficult problem of physics. Your options:

- Build a digital simulation (requires a lot of programming). Once finished, set the simulator running and walk away. When you come back, either the simulator will have solved your problem (exactly) or it will have hung.

- Build an analog simulator. You have to build a new simulator for each problem domain you want to solve. It also requires high-precision parts and cannot perform simulations to the level of precision of a digital computer. But, once built, it's much faster than a digital computer.

- Build a quantum computer. This also requires a lot of programming but you don't have to build a new quantum simulator for each problem domain since quantum computers are "general-purpose". It requires high-precision parts and other exotic measures to prevent "decoherence" (a problem that other computers do not have). It can, in principle, perform simulations to the level of precision of a digital computer but each "run" of a quantum computer is random, meaning, you don't get "the answer" on any particular run of the computer, you must run the problem repeatedly and take the average on the output. This makes quantum computers quite different from either analog computers or digital computers.