Question about Quantum Computers

[AustinLee]

Hello Everyone

I was hoping someone can answer how quantum computers work. I understand how digital logic works such as in AND gates and OR gates, but I am not sure how quantum physics wave functions can make this any faster. I was thinking maybe because whenever a piece of matter has the properties of wave, it can have a higher 'resolution' than a collapsed particle. What I mean by 'resolution' is that it has more divisibility, because a wave is analog making it much more detailed than a two state collapsed system. This extra resolution allows more info to be processed in a smaller space. Can you tell me if this is a correct way at looking at quantum computers.

 

[yoron]

The idea is one of superposition, or indeterminacy. You can have different states existing in a photon as long as you don't measure it. Quantum computing seems to be based on 'circular polarisation'. Using that as a 'definition/measurement' will allow you six states simultaneously, in one photon. Think of polarised sunglasses to see what it means.

"Photon polarization can be changed by directing a photon through quartz crystals or Scotch tape. All of these processes, regardless of the species of qubit or the type of change, have a simple interpretation on the single-qubit sphere: they act as rotations. These rotations are defined by a single axis, just like measurements. But, instead of projecting all possible states into two possible outcomes, they rotate all states on an axis."

And that is what you use to 'program' this quantum computer for your question.

Then you use entanglement(s) to ask your 'question'/measure an answer. When you do so your question/measurement will force the entanglement into a position, that then also should be the answer as I understands it.

It's also called a qubit, you can by introducing more qubits get more of those simultaneous states of superposition, and so increase the complexity of the 'question' you ask by measuring the entanglement.

Describing n qubits requires 2n numbers, with every added qubit doubling the amount of information needed to describe the quantum computer. 1 2 4 8 16 32 64 etc. It goes up in 'information value/complexity' very quickly. "Single-qubit states can be represented by a point inside a sphere in 3-dimensional space. Two-qubit states, in comparison, need to be represented as a point in 15-dimensional space."

There is a price to pay for increasing that complexity though, and that is called decoherence, as when one of those states gets 'disturbed', by 'outside interference'. That destroys all 'programming' you might have done on your system, and invalidates all answers. And how will you know that there hasn't been such a situation, remember that you only 'measure' once, and that you expect that measurement to be your 'answer'. There is also the point that everything defining a entanglement, as you setting the 'question', might interfere with the final answer. You interact by creating it, and that without you there would be no entanglement in this specific 'position'.

A tale of two qubits: how quantum computers work.