Now, we have previously created a hologram for the left side. Let's 
    make one for the right side. 
 
     If we simply produce a hologram directly from the flipped wavefront, 
    putting them together would simply make a "zone plate" "magnifying 
    glass" (composite hologram.) All of the input light would simply be 
    focused onto that point. 
 
      In order to make a photonic transistor, we must be able to cause 
    the photons of the two images projected from each of the holograms  
    to react so as to produce an entirely different image when both are 
    on, as opposed to when only one side is on. (Since the moot case of 
    having both input beams off always produces an off output.) 

      There are many ways to cause the individual images to interact 
    with each other. (That's why the actual development package has so 
    many programs for working with these images.)  For this simple 
    demonstration, we can produce a hologram that causes its light at 
    same spot to be 180 degrees out of phase.  Then, when both input beams 
    are on at the same instant, destructive interference will occur at the 
    place where the two beams combine, rather than the constructive  
    interference that occurs when only one hologram is lit.