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Is it possible to construct an arrangement of optical devices (lasers, mirrors, slits, splitters) such that the construction could carry out a single quantum "computation"?

I understand that such a setup for arbitrary computations is quite difficult and is being worked on, but from the perspective of a fun demo or simple project could you construct a setup where the input and resulting operations (maybe a CNOT or n few bits of addition) are uniquely determined by the arrangement of slits and the output can be read by measuring the resulting interference pattern?

If not are there any other simplifying assumptions one could make in order to carry out something which would still qualify as a single quantum "computation", even in a trivial manner for a DIY demo?

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Since you can't generate an interference pattern with single photons, this isn't really a "quantum" computer. Yes, you can create interference patterns and change them by adjusting slit spacings and whatnot, but you need enough photons to build a statistically valid diffraction pattern or you can't "read" the result. – Carl Witthoft Jul 2 '14 at 15:01
Yea, upon thinking about it further I realized that this would essentially devolve into a classical scheme which exploits wave mechanics in order to accomplish some computational task. – user52600 Jul 3 '14 at 0:18

Yes it can be done using two double slits. Place the first slit in front of the laser the second slit beyond the first and off set so that only 1 slit is illuminated by the interference pattern created by the first double slit. This will give you an AND GATE on the output. If you cover one of the slits in the first double slit the out put from the second double slit will be an interference pattern instead of just a single line of light. This idea was patented in the 1970's and it has the limitation of no gain so it does not make an effective optical transistor. It is also large in comparison to electrical transistors. This is not quantum computing though.

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