Editing Quantum tunnelling (section)

{{Short description|Quantum mechanical phenomenon}}
{{Use dmy dates|date=August 2019}}
{{Quantum mechanics|cTopic=Fundamental concepts}}

In physics, '''quantum tunnelling''', '''barrier penetration''', or simply '''tunnelling''' is a [[quantum mechanics|quantum mechanical]] phenomenon in which an object such as an electron or atom passes through a [[potential barrier|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.

Tunneling is a consequence of the [[Matter wave|wave nature of matter]], where the quantum [[wave function]] describes the state of a particle or other [[physical system]], and wave equations such as the [[Schrödinger equation]] describe their behavior. The probability of transmission of a [[wave packet]] through a barrier decreases exponentially with the barrier height, the barrier width, and the tunneling particle's mass, so tunneling is seen most prominently in low-mass particles such as [[electron]]s or [[proton]]s tunneling through microscopically narrow barriers. Tunneling is readily detectable with barriers of thickness about 1–3&nbsp;nm or smaller for electrons, and about 0.1&nbsp;nm or smaller for heavier particles such as protons or hydrogen atoms.<ref>{{cite book|last1=Lerner|url=https://archive.org/details/encyclopediaofph00lern/page/1308|title=Encyclopedia of Physics|last2=Trigg|publisher=VCH|year=1991|isbn=978-0-89573-752-6|edition=2nd|location=New York|page=[https://archive.org/details/encyclopediaofph00lern/page/1308 1308]}}</ref> Some sources describe the mere penetration of a wave function into the barrier, without transmission on the other side, as a tunneling effect, such as in tunneling into the walls of a [[finite potential well]].<ref name="Davies 2004">{{cite journal | last=Davies | first=P C W | author-link=Paul Davies | title=Quantum mechanics and the equivalence principle | journal=Classical and Quantum Gravity | volume=21 | issue=11 | date=2004-05-06 | issn=0264-9381 | doi=10.1088/0264-9381/21/11/017 | pages=2761–2772 | quote=But quantum particles are able to tunnel into the classically forbidden region ...| arxiv=quant-ph/0403027 | bibcode=2004CQGra..21.2761D }}</ref><ref>{{cite web |last1=Fowler |first1=Michael |title=Particle in a Finite Box and Tunneling |url=https://chem.libretexts.org/Courses/BethuneCookman_University/B-CU%3ACH-331_Physical_Chemistry_I/CH-331_Text/CH-331_Text/03._The_Schrodinger_Equation_and_a_Particle_In_a_Box/3.09%3A_Particle_in_a_Finite_Box_(and_Tunneling) |website=LibreTexts Chemistry |date=5 December 2019 |access-date=4 September 2023 |quote=Tunneling into the barrier (wall) is possible.}}</ref>

Tunneling plays an essential role in physical phenomena such as [[nuclear fusion]]<ref>{{cite book |title=College Physics |volume=2 |last1=Serway |last2=Vuille |year=2008 |publisher=Brooks/Cole |location=Belmont |isbn=978-0-495-55475-2 |edition=Eighth }}</ref> and [[Alpha decay|alpha radioactive decay]] of atomic nuclei. [[Quantum tunnelling#Applications|Tunneling applications]] include the [[tunnel diode]],<ref>{{cite book |last=Taylor |first=J. |title=Modern Physics for Scientists and Engineers |page=234 |publisher=Prentice Hall |year=2004 |isbn=978-0-13-805715-2 }}</ref> [[quantum computing]], [[flash memory]], and the [[scanning tunnelling microscope|scanning tunneling microscope]]. Tunneling limits the minimum size of devices used in [[microelectronics]] because electrons tunnel readily through insulating layers and [[transistor]]s that are thinner than about 1&nbsp;nm.<ref>{{Cite news|url=https://semiengineering.com/quantum-effects-at-7-5nm/|title=Quantum Effects At 7/5nm And Beyond|work=Semiconductor Engineering|access-date=2018-07-15|language=en-US}}</ref>

The effect was predicted in the early 20th century. Its acceptance as a general physical phenomenon came mid-century.<ref name="Razavy">{{cite book |first=Mohsen |last=Razavy |title=Quantum Theory of Tunneling |url=https://archive.org/details/quantumtheoryoft0000raza |url-access=registration |pages=[https://archive.org/details/quantumtheoryoft0000raza/page/4 4], 462 |publisher=World Scientific |year=2003 |isbn=978-9812564887 }}</ref>