Quantum tunnelling is the quantum process in which a particle passes through an energy barrier it classically cannot cross.
In quantum mechanics, a particle is described by a wavefunction rather than by a single classical trajectory. When the wavefunction reaches a barrier, it can extend into and beyond that barrier, giving a nonzero probability of detecting the particle on the far side. The probability falls rapidly as the barrier becomes higher or wider.
Tunnelling is essential to radioactive alpha decay and helps power the Sun through stellar fusion. It also operates in scanning tunnelling microscopes, tunnel diodes and Josephson junctions. Tunnelling does not allow unlimited energy conservation violations: the particle’s total energy remains consistent with quantum dynamics.