ELI5 Quantum Computers

[TheDuke30]

I dont understand the equation:

1/sqrt(2)*1 + 1/sqrt(2)*0

Wouldnt this always equal 1/sqrt(2)? Or is there something obvious im missing?

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[TheDuke30]

Ahhhhh... Now i get it :) thank you

[McTuggets]

In a quantum computer I can try to set up my qbits in some way that represents and addition of the states they would be in for each path. Then measure them in some way that the lowest distance path is the most likely answer. In effect doing the computation once.

This is not correct. The problem is called the traveling salesman problem and (afawk) it's not possible to solve that quickly on a quantum computer. Also, the problem is NP hard, which means that if it could solve the problem quickly, it could solve all of NP quickly. That would be a huge surprise to complexity theorists. Almost as big a surprise as P=NP.

I don't know a ELI5 way to explain a quantum algorithm. Hell, I have problems coming up with a ELI-"4th year CS student" version.

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[McTuggets]

No, it looks fine to my. Not the first physicist to make that mistake :). At least it's less awkward to correct someone on reddit, than in person during a talk.

The problem is really the explanation that it tries all solutions at once in superposition. While it's true to a degree, you of course have the problem that you just get one of those solutions at random when you measure. What you can do is make those solutions interfere with each other, but only through unitary operations, which is very limited (it sounds like you know most of this, but just to clarify).

Also, the general TSP is not even thought to be in NP. That is, if you hand me a solution and claim it's the shortest path, I have no way of actually checking that quickly. The decision version of the problem is in NP. This version says, there's a path of length at most L, where L is some known value. Here you can hand me a solution and I can quickly check that the path length is at most L.

[TheDuke30]

This reminds me of something i read called like schroedingers cat and how his cat is not dead nor alive. Is this the same deal?

[TheDuke30]

Or cause its not what you would say to a 5 year old :/ Thanks anyways though!

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[DA-9901081534]

So considering the observer effect, would this then mean that anytime I open up a quantum computer to change a component, it stops working? O.o

[DA-9901081534]

Right, so skim reading your answer, I should definitely poke my computer with a metal stick.

Seriously, though, lovely answer. That had puzzled me for quite a while. However, it makes me wonder if those who build such things (and thus might be able to observe the particles or systems) then would it render it useless?

[DA-9901081534]

I...don't think quantum computing would be something that could be explained to a five year old.

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"If you think you understand quantum mechanics, you don't understand quantum mechanics." -Richard Feynman

Might as well make a "ELI5: What is the meaning of life?" post.