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Comment by habosa | Hacker News Reader

This is a naive question, but how can we simulate a quantum computer on a classical computer? Isn't the whole point of developing quantum computers that they enable computations that are totally outside the capabilities of a classical computer? I'm sure I'm missing something simple here.

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Every computation that can be performed on a quantum computer can be performed on a classical computer, albeit sometimes with exponential slow-down.

Right, so the value of the simulation is somewhat questionable. Exponential slowdown is enough to kill all intuition about what works and what doesn't. You only get some correctness gurantee and that would be for very small problems / test cases.

I see the value: you get to debug real, working algorithms based on a quantum computer so that once they're available, you already know many of the fundamentals and pitfalls of programming it.

I was thinking myself about the problem of 'slowdown' of this type of approach -- I think it would be interesting if the simulated quantum computer could also give you performance metrics that say, 'If this were run on a real quantum computer, it could have been done in 100 cycles as opposed to 2000.'

So you could make an algorithm in normal computing, and one on a simulated quantum computer, and compare the running time of the normal computation with the projected running time of the simulated quantum algorithm (obviously the actual running time of the simulation would be incomparable to the projected running time). I think that would be quite useful.

Quantum computers aren't magic. Their computing primitives - qubits - just have some special properties that can be used to make algorithms that are much faster than classical computers for certain problems. A qubit is basically a bit with a certain probability that it will "reduce" to 0 or 1 when read. While this is non-deterministic, it can easily be modelled, with a slowdown.

Here is a talk which may explain it better https://www.youtube.com/watch?v=8bLXHvH9s1A (The stuff about qubits and quantum computers starts at 30 minutes, but you may find the lead-up interesting/informative).

“Anyone who considers arithmetical methods of producing random qubits is, of course, in a state of sin” John von Neumann

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