Quantum interference is the term used in quantum mechanics to describe the phenomenon whereby two or more identical particles (or wave-like packets) collide and produce an interference pattern. This can happen either when the particles are scattered off of each other, or when they pass through each other. The latter case is known as Young’s double-slit experiment, and was first performed in 1801 by Thomas Young.
In classical physics, waves can interfere with each other to create patterns of constructive and destructive interference. However, according to the principles of quantum mechanics, particles should not be able to interfere with each other because they do not have a definite position until they are observed. The wave-like nature of matter allows for the possibility of particles passing through each other without interacting; this is known as the principle of superposition. Interference patterns occur when there is a phase difference between the two waves; that is, when their crests (or troughs) line up perfectly, constructive interference occurs and peaks appear in the resulting pattern. When crests and troughs align perfectly out of phase, destructive interference occurs and valleys form in the resulting pattern.
The key to understanding how quantum interference works is understanding that particles do not have a definite position until they are observed. In essence, measurement creates reality. Until a particle is measured, it exists as a wave-like packet spread out over all possible positions; this is known as its wave function. It is only when a measurement is made that the wave function “collapses” into a single point at some specific location; this postulate forms the basis for Heisenberg’s uncertainty principle.
It should be noted that while measurements play an important role in determining reality on subatomic scales, classical physics still applies on larger scales where objects have well-defined positions even if they are not being observed (for example, you always know exactly where your car is). Quantum interference only becomes significant on very small scales where objects no longer have well-defined positions; at these scales, multiple paths become equally likely and strange effects like superposition and tunneling become common place.