If it were as easy as cutting slits into metal foil, or even doing photolithography at the sub-10nm regime, then it would have been done by now, but that might not be a satisfactory answer. It's a good question and should not be dismissed.
The question is similar to "what is stopping us from achieving a computational speedup by running Shor's algorithm by merely cutting a bunch of slits into a metal foil, and looking at the interference patterns when light is shown through?" .
Indeed, Shor has referred to Shor's algorithm as a "computational interferometer." For example, one thing that quantum computers can do, and that diffraction gratings can do, is perform Fourier transforms on large data sets.
But diffraction gratings don't have much in the way of adaptive control, and you have to spend exponential resources before-hand in order to leverage the constructive and destructive interference of the photons.
For example, you could cut slits in your foil in a manner where the spacing is $a^x\bmod N$. Shining light through such a diffraction grating, even a single photon of light, will perform the quantum Fourier transform.
However, in this case you had to cut your diffraction grating a-priori into $a^x\bmod N$; that is, you had to perform an exponential number of cuts in the first place.
It's not clear how such non-adaptivity is still powerful enough to solve Shor's algorithm, or whether you would always need to pre-cut your grating with an exponential number of cuts in the first place.