DiscoverScroll article

In physics, quantum tunnelling, barrier penetration, or simply tunnelling is a quantum mechanical phenomenon in which an object such as an electron or atom passes through a potential energy barrier that, according to classical mechanics, should not be passable due to the object not having sufficient energy to pass or surmount the barrier.

Tunnelling is a consequence of the wave nature of matter and quantum indeterminacy. The quantum wave function describes the states of a particle or other physical system and wave equations such as the Schrödinger equation describe their evolution. In a system with a short, narrow potential barrier, a small part of wavefunction can appear outside of the barrier representing a probability for tunnelling through the barrier. Since the probability of transmission of a wave packet through a barrier decreases exponentially with the barrier height, the barrier width, and the tunnelling particle's mass, tunnelling is seen most prominently in low-mass particles such as electrons tunnelling through atomically narrow barriers. However tunnelling has been observed with protons and even atoms and tunnelling has been used to explain physical effects with particles this large. Tunnelling plays an essential role in physical phenomena such as nuclear fusion and alpha radioactive decay of atomic nuclei. Tunnelling applications include the tunnel diode, quantum computing, flash memory, and the scanning tunnelling microscope. Tunnelling limits the minimum size of devices used in microelectronics because electrons tunnel readily through insulating layers and transistors that are thinner than about 1 nm.

Quantum tunnelling falls under the domain of quantum mechanics. In quantum mechanics, a particle can, with a small probability, tunnel to the other side, thus crossing the barrier. Some sources describe the mere penetration of a wave function into the barrier, without transmission on the other side, as a tunnelling effect, such as in tunnelling into the walls of a finite potential well.

The wider the barrier and the higher the barrier energy, the lower the probability of tunnelling.

He assumed a surface potential barrier that confines the electrons within the metal and showed that the electrons have a finite probability of tunnelling through or reflecting from the surface barrier when their energies are close to the barrier energy. The latter researchers simultaneously solved the Schrödinger equation for a model nuclear potential and derived a relationship between the half-life of the particle and the energy of emission that depended directly on the mathematical probability of tunnelling. Esaki, Giaever and Josephson shared the 1973 Nobel Prize in Physics for their works on quantum tunnelling in solids. In 1981, Gerd Binnig and Heinrich Rohrer developed a new type of microscope, called scanning tunnelling microscope, which is based on tunnelling and is used for imaging surfaces at the atomic level. Devoret received the Nobel Prize in physics for experiments done in 1984 and 1985 that demonstrated how quantum tunnelling can be observed on a macroscopic scale, involving many particles.

The resonant tunnelling diode makes use of quantum tunnelling in a very different manner to achieve a similar result.

Quantum tunnelling is an essential phenomenon for nuclear fusion. Quantum tunnelling increases the probability of penetrating this barrier.

This is done via the tunnelling of a particle out of the nucleus (an electron tunnelling into the nucleus is electron capture). This was the first application of quantum tunnelling.

The quantum mechanical tunnelling rate for the same reaction using the hydrogen isotope deuterium,

Quantum tunnelling is among the central non-trivial quantum effects in quantum biology. Here it is important both as electron tunnelling and proton tunnelling.

The concept of quantum tunnelling can be extended to situations where there exists a quantum transport between regions that are classically not connected even if there is no associated potential barrier.

The concept of dynamical tunnelling is particularly suited to address the problem of quantum tunnelling in high dimensions (

Quick Facts

• Since the probability of transmission of a wave packet through a barrier decreases exponentially with the barrier height, the barrier width, and the tunnelling particle's mass, tunnelling is seen most prominently in low-mass particles such as electrons tunnelling through atomically narrow barriers.
• Tunnelling applications include the tunnel diode, quantum computing, flash memory, and the scanning tunnelling microscope.
• Quantum tunnelling is among the central non-trivial quantum effects in quantum biology.
• However tunnelling has been observed with protons and even atoms and tunnelling has been used to explain physical effects with particles this large.
• Quantum tunnelling falls under the domain of quantum mechanics.

Source material: Wikipedia - "Quantum tunnelling". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.