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Quantum tunnelling is transmission through a potential barrier even when the particle energy is below the barrier height.
The wavefunction penetrates the classically inaccessible region; matching it to an outgoing scattering state on the far side gives a nonzero transmission probability. For a real, time-independent potential barrier, the transmitted energy equals the incident energy: quantum tunnelling does not require a temporary violation of conservation of energy. A tunnel junction turns this small transmission probability into a measurable electric current, whose sensitivity to separation makes the scanning tunnelling microscope possible.
Examples:
- flash memory uses quantum tunneling as the basis for setting and resetting bits
- alpha decay is understood as a quantum tunneling effect in the nucleus