The quantum tunneling effect is a quantum phenomenon which occurs when particles move through a barrier that, according to the theories of classical physics, should be impossible to move through. The barrier may be a physically impassable medium, such as an insulator or a vacuum, or a region of high potential energy.

In classical mechanics, when a particle has insufficient energy, it would not be able to overcome a potential barrier. In the quantum world, however, particles can often behave like waves. On encountering a barrier, a quantum wave will not end abruptly; rather, its amplitude will decrease exponentially. This drop in amplitude corresponds to a drop in the probability of finding a particle further into the barrier. If the barrier is thin enough, then the amplitude may be non-zero on the other side. This would imply that there is a finite probability that some of the particles will tunnel through the barrier. Source