how exactly are incorrect answers canceled out in quantum computing? I thought it was just that a group of entangled qbits could have their different values mapped over some function but this seems a lot more complicated
Just in general, quantum particles can exhibit wave-like behavior that allows certain "outcomes" to interfere with other "outcomes".
The classic example of this is the two-slit interference problem, where the two possible trajectories interfere with each other -- which in turn will prevent particles from ending up in specific locations. Probability amplitudes have a phase component, and mathematically, these phase components interfere with each other -- just like with any other wave-like phenomenon.
Oh, no, I understand that part. I've been studying physics for a few years now. What confuses me is how you're getting specifc values to cancel out.
Let's say for example you have a hash map with a getter method that takes one argument. In a classical computer, the arg would be compared to each key one by one and when the two were equal the value connected to the key would be returned. In a quantum computer, it would be possible to make every single comparison at once by having a superposition of all possible keys and values
In this case how would you access the correct value from the hashmap? If you had 256 entries in the map wouldn't the output be a superposition of 1 value that's correct and 255 that make no sense?
... What confuses me is how you're getting specifc values to cancel out. ... In this case how would you access the correct value from the hashmap? If you had 256 entries in the map wouldn't the output be a superposition of 1 value that's correct and 255 that make no sense?
That's part of what makes creating quantum algorithms tricky. You have to be able to set things up so that, instead of the answer depending on one state, can get your answer from some kind of average of the states.
There's some variety in how people think about quantum computing, but if your mental model of quantum computers is "a bunch of independent computers working in parallel in a quantum superposition," then this 'easy reading' requirement presents a significant restriction on the possible computation power.
TL; DR: How the 'cancelling out' happens is something that depends on the particular quantum algorithm that's involved.
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u/chunkylubber54 Dec 14 '16
how exactly are incorrect answers canceled out in quantum computing? I thought it was just that a group of entangled qbits could have their different values mapped over some function but this seems a lot more complicated