Quotes from Art Hobson

The entire spread-out field comes through both slits and fills the region between source and screen, but...this field must deposit its quantum of energy all at once, in a single instant, because the field cannot carry some fraction of one quantum-it must always contain either exactly one or exactly zero quanta of energy. When the field deposits its quantum of energy on the viewing screen, the entire spread-out field must instantaneously lose this much energy.