Is it possible (in principle) to make a very thin phtotodetector that has the same sensitivity with respect to light coming from either side, so that we can see interference between two light waves arriving from opposite sides? That is, as we vary the relative phase shift between the right hand side and left hand side inputs, could we see the detection count increase and decrease? Of course, this would call for a detector thinner than a wavelength so that the phase difference would be nearly constant over the thickness. And we assume very good collimation with the beams perfectly normal to the detector surface. I'd like to know if there is a fundamental reason that would make this impossible. And if there is no such reason, what technologies currently exist.