Not sure what you mean, since it is clearly explained that such a device is mathematically impossible. A naive way to it would be with a Michelson interferometer where the two beams are displaced exactly by (2n+1)*pi radians and thus interfere so that the target detector sees nothing. Most physics undergraduates do this experiment at least once during their lab courses. But again, if you account for everything, you'll find that the "missing" energy at the detector just went back to the source via the mirror.