The neutrons enter the interferometer and reach a beam splitter. Each neutron that passes through, enters into a quantum superposition state of two different paths, namely A and B. This initial state is referred to as the preselected state. As the neutrons travel the different paths, their wave functions reunites at a second beam splitter, causing interference. If there is nothing in the path of the neutrons, every neutron exits to the interferometer moving to the right and activates the detector. No neutron escapes upwards into the dark port due to destructive interference.