Quantum tunnelling or tunneling (see spelling differences) is the quantum mechanical phenomenon where a particle passes through a potential barrier that it classically cannot surmount. This plays an essential role in several physical phenomena, such as the nuclear fusion that occurs in main sequence stars like the Sun. It has important applications to modern devices such as the tunnel diode, quantum computing, and the scanning tunnelling microscope. The effect was predicted in the early 20th century, and its acceptance as a general physical phenomenon came mid-century.
Fundamental quantum mechanical concepts are central to this phenomenon, which makes quantum tunnelling one of the novel implications of quantum mechanics. Quantum tunneling is projected to create physical limits to how small transistors can get, due to electrons being able to tunnel past them if they are too small.
Tunnelling is often explained in terms of the Heisenberg uncertainty principle and the premise that the quantum object has more than one fixed state (not a wave nor a particle) in general.
https://en.wikipedia.org/wiki/Quantum_tunnelling