Quantum tunnelling

In physics,quantum tunnelling,barrier penetration,or simplytunnellingis aquantum mechanicalphenomenon in which an object such as an electron or atom passes through apotential energy barrierthat, according toclassical mechanics,should not be passable due to the object not having sufficient energy to pass or surmount the barrier.

Tunneling is a consequence of thewave nature of matter,where the quantumwave functiondescribes the state of a particle or otherphysical system,and wave equations such as theSchrödinger equationdescribe their behavior. The probability of transmission of awave packetthrough 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 aselectronsorprotonstunneling through microscopically narrow barriers. Tunneling is readily detectable with barriers of thickness about 1–3 nm or smaller for electrons, and about 0.1 nm or smaller for heavier particles such as protons or hydrogen atoms.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 afinite potential well.

Tunneling plays an essential role in physical phenomena such asnuclear fusionandAlpha radioactive decayof atomic nuclei.Tunneling applicationsinclude thetunnel diode,quantum computing,flash memory,and thescanning tunneling microscope.Tunneling limits the minimum size of devices used inmicroelectronicsbecause electrons tunnel readily through insulating layers andtransistorsthat are thinner than about 1 nm.

The effect was predicted in the early 20th century. Its acceptance as a general physical phenomenon came mid-century.

Introduction to the concept

Animation showing the tunnel effect and its application to anSTM

Quantum tunnelling falls under the domain ofquantum mechanics.To understand thephenomenon,particles attempting to travel across apotential barriercan be compared to a ball trying to roll over a hill. Quantum mechanics and classical mechanics differ in their treatment of this scenario.

Classical mechanics predicts that particles that do not have enough energy to classically surmount a barrier cannot reach the other side. Thus, a ball without sufficient energy to surmount the hill would roll back down. In quantum mechanics, a particle can, with a small probability,tunnelto the other side, thus crossing the barrier. The reason for this difference comes from treating matter ashaving properties of waves and particles.

Tunnelling problem

A simulation of a wave packet incident on a potential barrier. In relative units, the barrier energy is 20, greater than the mean wave packet energy of 14. A portion of the wave packet passes through the barrier.

Thewave functionof aphysical systemof particles specifies everything that can be known about the system.Therefore, problems in quantum mechanics analyze the system's wave function. Using mathematical formulations, such as theSchrödinger equation,the time evolution of a known wave function can be deduced. The square of theabsolute valueof this wave function is directly related to the probability distribution of the particle positions, which describes the probability that the particles would be measured at those positions.

As shown in the animation, a wave packet impinges on the barrier, most of it is reflected and some is transmitted through the barrier. The wave packet becomes more de-localized: it is now on both sides of the barrier and lower in maximum amplitude, but equal in integrated square-magnitude, meaning that the probability the particle issomewhereremains unity. The wider the barrier and the higher the barrier energy, the lower the probability of tunneling.

Some models of a tunneling barrier, such as therectangular barriersshown, can be analysed and solved algebraically.Most problems do not have an algebraic solution, so numerical solutions are used. "Semiclassical methods"offer approximate solutions that are easier to compute, such as theWKB approximation.

History

The Schrödinger equation was published in 1926. The first person to apply the Schrödinger equation to a problem that involved tunneling between two classically allowed regions through a potential barrier wasFriedrich Hundin a series of articles published in 1927. He studied the solutions of adouble-well potentialand discussedmolecular spectra.Leonid MandelstamandMikhail Leontovichdiscovered tunneling independently and published their results in 1928.

In 1927,Lothar Nordheim,assisted byRalph Fowler,published a paper that discussedthermionic emissionand reflection of electrons from metals. He assumed a surface potential barrier that confines the electrons within the metal and showed that the electrons have a finite probability of tunneling through or reflecting from the surface barrier when their energies are close to the barrier energy. Classically, the electron would either transmit or reflect with 100% certainty, depending on its energy. In 1928J. Robert Oppenheimerpublished two papers onfield emission,i.e.the emission of electrons induced by strong electric fields. Nordheim and Fowler simplified Oppenheimer's derivation and found values for the emitted currents andwork functionsthat agreed with experiments.

A great success of the tunnelling theory was the mathematical explanation forAlpha decay,which was developed in 1928 byGeorge Gamowand independently byRonald GurneyandEdward Condon.The latter researchers simultaneously solved the Schrödinger equation for a model nuclear potential and derived a relationship between thehalf-lifeof the particle and the energy of emission that depended directly on the mathematical probability of tunneling. All three researchers were familiar with the works on field emission,and Gamow was aware of Mandelstam and Leontovich's findings.

In the early days of quantum theory, the termtunnel effectwas not used, and the effect was instead referred to as penetration of, or leaking through, a barrier. The German termwellenmechanische Tunneleffektwas used in 1931 by Walter Schottky.The English termtunnel effectentered the language in 1932 when it was used by Yakov Frenkel in his textbook.

In 1957Leo Esakidemonstrated tunneling of electrons over a few nanometer wide barrier in asemiconductorstructure and developed adiodebased on tunnel effect.In 1960, following Esaki's work,Ivar Giaevershowed experimentally that tunnelling also took place insuperconductors.The tunnelling spectrum gave direct evidence of thesuperconducting energy gap.In 1962,Brian Josephsonpredicted the tunneling of superconductingCooper pairs.Esaki, Giaever and Josephson shared the 1973Nobel Prize in Physicsfor their works on quantum tunneling in solids.

In 1981,Gerd BinnigandHeinrich Rohrerdeveloped a new type of microscope, calledscanning tunneling microscope,which is based on tunnelling and is used for imagingsurfacesat theatomiclevel. Binnig and Rohrer were awarded the Nobel Prize in Physics in 1986 for their discovery.

Applications

Tunnelling is the cause of some important macroscopic physical phenomena.

Solid-state physics

Electronics

Tunnelling is a source of current leakage invery-large-scale integration(VLSI) electronics and results in a substantial power drain and heating effects that plague such devices. It is considered the lower limit on how microelectronic device elements can be made.Tunnelling is a fundamental technique used to program the floating gates offlash memory.

Cold emission

Main article:Field electron emission

Cold emission ofelectronsis relevant tosemiconductorsandsuperconductorphysics. It is similar tothermionic emission,where electrons randomly jump from the surface of a metal to follow a voltage bias because they statistically end up with more energy than the barrier, through random collisions with other particles. When the electric field is very large, the barrier becomes thin enough for electrons to tunnel out of the atomic state, leading to a current that varies approximately exponentially with the electric field.These materials are important for flash memory, vacuum tubes, and some electron microscopes.

Tunnel junction

Main article:Tunnel junction

A simple barrier can be created by separating two conductors with a very thininsulator.These are tunnel junctions, the study of which requires understanding quantum tunnelling.Josephson junctionstake advantage of quantum tunnelling and superconductivity to create theJosephson effect.This has applications in precision measurements of voltages andmagnetic fields,as well as themultijunction solar cell.

Tunnel diode

Main article:Tunnel diode

A working mechanism of aresonant tunnelling diodedevice, based on the phenomenon of quantum tunnelling through the potential barriers

Diodesare electricalsemiconductor devicesthat allowelectric currentflow in one direction more than the other. The device depends on adepletion layerbetweenN-typeandP-type semiconductorsto serve its purpose. When these are heavily doped the depletion layer can be thin enough for tunnelling. When a small forward bias is applied, the current due to tunnelling is significant. This has a maximum at the point where thevoltage biasis such that the energy level of the p and nconduction bandsare the same. As the voltage bias is increased, the two conduction bands no longer line up and the diode acts typically.

Because the tunnelling current drops off rapidly, tunnel diodes can be created that have a range of voltages for which current decreases as voltage increases. This peculiar property is used in some applications, such as high speed devices where the characteristic tunnelling probability changes as rapidly as the bias voltage.

Theresonant tunnelling diodemakes use of quantum tunnelling in a very different manner to achieve a similar result. This diode has a resonant voltage for which a current favors a particular voltage, achieved by placing two thin layers with a high energy conductance band near each other. This creates a quantumpotential wellthat has a discrete lowestenergy level.When this energy level is higher than that of the electrons, no tunnelling occurs and the diode is in reverse bias. Once the two voltage energies align, the electrons flow like an open wire. As the voltage further increases, tunnelling becomes improbable and the diode acts like a normal diode again before a second energy level becomes noticeable.

Tunnel field-effect transistors

Main article:Tunnel field-effect transistor

A European research project demonstratedfield effect transistorsin which the gate (channel) is controlled via quantum tunnelling rather than by thermal injection, reducing gate voltage from ≈1 volt to 0.2 volts and reducing power consumption by up to 100×. If these transistors can be scaled up intoVLSI chips,they would improve the performance per power ofintegrated circuits.

Conductivity of crystalline solids

While theDrude-Lorentz modelofelectrical conductivitymakes excellent predictions about the nature of electrons conducting in metals, it can be furthered by using quantum tunnelling to explain the nature of the electron's collisions.When a free electron wave packet encounters a long array of uniformly spacedbarriers,the reflected part of the wave packet interferes uniformly with the transmitted one between all barriers so that 100% transmission becomes possible. The theory predicts that if positively charged nuclei form a perfectly rectangular array, electrons will tunnel through the metal as free electrons, leading to extremely highconductance,and that impurities in the metal will disrupt it.

Scanning tunneling microscope

Main article:Scanning tunnelling microscope

The scanning tunnelling microscope (STM), invented byGerd BinnigandHeinrich Rohrer,may allow imaging of individual atoms on the surface of a material.It operates by taking advantage of the relationship between quantum tunnelling with distance. When the tip of the STM's needle is brought close to a conduction surface that has a voltage bias, measuring the current of electrons that are tunnelling between the needle and the surface reveals the distance between the needle and the surface. By usingpiezoelectric rodsthat change in size when voltage is applied, the height of the tip can be adjusted to keep the tunnelling current constant. The time-varying voltages that are applied to these rods can be recorded and used to image the surface of the conductor.STMs are accurate to 0.001 nm, or about 1% of atomic diameter.

Nuclear physics

Nuclear fusion

Main article:Nuclear fusion

Quantum tunnelling is an essential phenomenon for nuclear fusion. The temperature instellar coresis generally insufficient to allow atomic nuclei to overcome theCoulomb barrierand achievethermonuclear fusion.Quantum tunnelling increases the probability of penetrating this barrier. Though this probability is still low, the extremely large number of nuclei in the core of a star is sufficient to sustain a steady fusion reaction.

Radioactive decay

Main article:Radioactive decay

Radioactive decay is the process of emission of particles and energy from the unstable nucleus of an atom to form a stable product. This is done via the tunnelling of a particle out of the nucleus (an electron tunneling into the nucleus iselectron capture). This was the first application of quantum tunnelling. Radioactive decay is a relevant issue forastrobiologyas this consequence of quantum tunnelling creates a constant energy source over a large time interval for environments outside thecircumstellar habitable zonewhere insolation would not be possible (subsurface oceans) or effective.

Quantum tunnelling may be one of the mechanisms of hypotheticalproton decay.

Chemistry

Energetically forbidden reactions

Chemical reactions in theinterstellar mediumoccur at extremely low energies. Probably the most fundamental ion-molecule reaction involves hydrogen ions with hydrogen molecules. The quantum mechanical tunnelling rate for the same reaction using thehydrogenisotopedeuterium,D^-+ H₂→ H^-+ HD, has been measured experimentally in an ion trap. The deuterium was placed in anion trapand cooled. The trap was then filled with hydrogen. At the temperatures used in the experiment, the energy barrier for reaction would not allow the reaction to succeed with classical dynamics alone. Quantum tunneling allowed reactions to happen in rare collisions. It was calculated from the experimental data that collisions happened one in every hundred billion.

Kinetic isotope effect

Main article:Kinetic isotope effect

Inchemical kinetics,the substitution of a lightisotopeof an element with a heavier one typically results in a slower reaction rate. This is generally attributed to differences in the zero-point vibrational energies for chemical bonds containing the lighter and heavier isotopes and is generally modeled usingtransition state theory.However, in certain cases, large isotopic effects are observed that cannot be accounted for by a semi-classical treatment, and quantum tunnelling is required.R. P. Belldeveloped a modified treatment of Arrhenius kinetics that is commonly used to model this phenomenon.

Astrochemistry in interstellar clouds

By including quantum tunnelling, theastrochemicalsyntheses of various molecules ininterstellar cloudscan be explained, such as the synthesis ofmolecular hydrogen,water(ice) and theprebioticimportantformaldehyde.Tunnelling of molecular hydrogen has been observed in the lab.

Quantum biology

Quantum tunnelling is among the central non-trivial quantum effects inquantum biology.Here it is important both as electron tunnelling andproton tunnelling.Electron tunnelling is a key factor in many biochemicalredox reactions(photosynthesis,cellular respiration) as well as enzymatic catalysis. Proton tunnelling is a key factor in spontaneousDNAmutation.

Spontaneous mutation occurs when normal DNA replication takes place after a particularly significant proton has tunnelled.A hydrogen bond joins DNA base pairs. A double well potential along a hydrogen bond separates a potential energy barrier. It is believed that the double well potential is asymmetric, with one well deeper than the other such that the proton normally rests in the deeper well. For a mutation to occur, the proton must have tunnelled into the shallower well. The proton's movement from its regular position is called atautomeric transition.If DNA replication takes place in this state, the base pairing rule for DNA may be jeopardised, causing a mutation.Per-Olov Lowdinwas the first to develop this theory of spontaneous mutation within thedouble helix.Other instances of quantum tunnelling-induced mutations in biology are believed to be a cause of ageing and cancer.

Mathematical discussion

Quantum tunnelling through a barrier. The energy of the tunnelled particle is the same but the probability amplitude is decreased.

Schrödinger equation

Thetime-independent Schrödinger equationfor one particle in onedimensioncan be written as $$-{\frac {\hbar ^{2}}{2m}}{\frac {d^{2}}{dx^{2}}}\Psi (x)+V(x)\Psi (x)=E\Psi (x)$$ or $${\frac {d^{2}}{dx^{2}}}\Psi (x)={\frac {2m}{\hbar ^{2}}}\left(V(x)-E\right)\Psi (x)\equiv {\frac {2m}{\hbar ^{2}}}M(x)\Psi (x),$$ where

• $\hbar$is thereduced Planck constant,
• mis the particle mass,
• xrepresents distance measured in the direction of motion of the particle,
• Ψ is the Schrödinger wave function,
• Vis thepotential energyof the particle (measured relative to any convenient reference level),
• Eis the energy of the particle that is associated with motion in thex-axis (measured relative toV),
• M(x) is a quantity defined byV(x) −E,which has no accepted name in physics.

The solutions of the Schrödinger equation take different forms for different values ofx,depending on whetherM(x) is positive or negative. WhenM(x) is constant and negative, then the Schrödinger equation can be written in the form $${\frac {d^{2}}{dx^{2}}}\Psi (x)={\frac {2m}{\hbar ^{2}}}M(x)\Psi (x)=-k^{2}\Psi (x),\qquad {\text{where}}\quad k^{2}=-{\frac {2m}{\hbar ^{2}}}M.$$

The solutions of this equation represent travelling waves, with phase-constant +kor −k.Alternatively, ifM(x) is constant and positive, then the Schrödinger equation can be written in the form $${\frac {d^{2}}{dx^{2}}}\Psi (x)={\frac {2m}{\hbar ^{2}}}M(x)\Psi (x)={\kappa }^{2}\Psi (x),\qquad {\text{where}}\quad {\kappa }^{2}={\frac {2m}{\hbar ^{2}}}M.$$

The solutions of this equation are rising and falling exponentials in the form ofevanescent waves.WhenM(x) varies with position, the same difference in behaviour occurs, depending on whether M(x) is negative or positive. It follows that the sign ofM(x) determines the nature of the medium, with negativeM(x) corresponding to medium A and positiveM(x) corresponding to medium B. It thus follows that evanescent wave coupling can occur if a region of positiveM(x) is sandwiched between two regions of negativeM(x), hence creating a potential barrier.

The mathematics of dealing with the situation whereM(x) varies withxis difficult, except in special cases that usually do not correspond to physical reality. A full mathematical treatment appears in the 1965 monograph by Fröman and Fröman. Their ideas have not been incorporated into physics textbooks, but their corrections have little quantitative effect.

WKB approximation

Main article:WKB approximation

The wave function is expressed as the exponential of a function: $$\Psi (x)=e^{\Phi (x)},$$where$$\Phi ''(x)+\Phi '(x)^{2}={\frac {2m}{\hbar ^{2}}}\left(V(x)-E\right).$$ $\Phi '(x)$is then separated into real and imaginary parts: $$\Phi '(x)=A(x)+iB(x),$$whereA(x) andB(x) are real-valued functions.

Substituting the second equation into the first and using the fact that the imaginary part needs to be 0 results in:

$$A'(x)+A(x)^{2}-B(x)^{2}={\frac {2m}{\hbar ^{2}}}\left(V(x)-E\right).$$

Quantum tunneling in thephase space formulationof quantum mechanics.Wigner functionfor tunneling through the potential barrier$U(x)=8e^{-0.25x^{2}}$in atomic units (a.u.). The solid lines represent thelevel setof theHamiltonian$H(x,p)=p^{2}/2+U(x)$.

To solve this equation using the semiclassical approximation, each function must be expanded as apower seriesin$\hbar$.From the equations, the power series must start with at least an order of$\hbar ^{-1}$to satisfy the real part of the equation; for a good classical limit starting with the highest power of thePlanck constantpossible is preferable, which leads to $$A(x)={\frac {1}{\hbar }}\sum _{k=0}^{\infty }\hbar ^{k}A_{k}(x)$$ and $$B(x)={\frac {1}{\hbar }}\sum _{k=0}^{\infty }\hbar ^{k}B_{k}(x),$$ with the following constraints on the lowest order terms, $$A_{0}(x)^{2}-B_{0}(x)^{2}=2m\left(V(x)-E\right)$$ and $$A_{0}(x)B_{0}(x)=0.$$

At this point two extreme cases can be considered.

Case 1

If the amplitude varies slowly as compared to the phase$A_{0}(x)=0$and $$B_{0}(x)=\pm {\sqrt {2m\left(E-V(x)\right)}}$$ which corresponds to classical motion. Resolving the next order of expansion yields $$\Psi (x)\approx C{\frac {e^{i\int dx{\sqrt {{\frac {2m}{\hbar ^{2}}}\left(E-V(x)\right)}}+\theta }}{\sqrt[{4}]{{\frac {2m}{\hbar ^{2}}}\left(E-V(x)\right)}}}$$

Case 2

If the phase varies slowly as compared to the amplitude,$B_{0}(x)=0$and $$A_{0}(x)=\pm {\sqrt {2m\left(V(x)-E\right)}}$$ which corresponds to tunneling. Resolving the next order of the expansion yields $$\Psi (x)\approx {\frac {C_{+}e^{+\int dx{\sqrt {{\frac {2m}{\hbar ^{2}}}\left(V(x)-E\right)}}}+C_{-}e^{-\int dx{\sqrt {{\frac {2m}{\hbar ^{2}}}\left(V(x)-E\right)}}}}{\sqrt[{4}]{{\frac {2m}{\hbar ^{2}}}\left(V(x)-E\right)}}}$$

In both cases it is apparent from the denominator that both these approximate solutions are bad near the classical turning points$E=V(x)$.Away from the potential hill, the particle acts similar to a free and oscillating wave; beneath the potential hill, the particle undergoes exponential changes in amplitude. By considering the behaviour at these limits and classical turning points a global solution can be made.

To start, a classical turning point,$x_{1}$is chosen and${\frac {2m}{\hbar ^{2}}}\left(V(x)-E\right)$is expanded in a power series about$x_{1}$: $${\frac {2m}{\hbar ^{2}}}\left(V(x)-E\right)=v_{1}(x-x_{1})+v_{2}(x-x_{1})^{2}+\cdots$$

Keeping only the first order term ensures linearity: $${\frac {2m}{\hbar ^{2}}}\left(V(x)-E\right)=v_{1}(x-x_{1}).$$

Using this approximation, the equation near$x_{1}$becomes adifferential equation: $${\frac {d^{2}}{dx^{2}}}\Psi (x)=v_{1}(x-x_{1})\Psi (x).$$

This can be solved usingAiry functionsas solutions. $$\Psi (x)=C_{A}Ai\left({\sqrt[{3}]{v_{1}}}(x-x_{1})\right)+C_{B}Bi\left({\sqrt[{3}]{v_{1}}}(x-x_{1})\right)$$

Taking these solutions for all classical turning points, a global solution can be formed that links the limiting solutions. Given the two coefficients on one side of a classical turning point, the two coefficients on the other side of a classical turning point can be determined by using this local solution to connect them.

Hence, the Airy function solutions will asymptote into sine, cosine and exponential functions in the proper limits. The relationships between$C,\theta$and$C_{+},C_{-}$are $$C_{+}={\frac {1}{2}}C\cos {\left(\theta -{\frac {\pi }{4}}\right)}$$ and

Quantum tunnelling through a barrier. At the origin (x= 0), there is a very high, but narrow potential barrier. A significant tunnelling effect can be seen.

$C_{-}=-C\sin {\left(\theta -{\frac {\pi }{4}}\right)}$

With the coefficients found, the global solution can be found. Therefore, thetransmission coefficientfor a particle tunneling through a single potential barrier is $$T(E)=e^{-2\int _{x_{1}}^{x_{2}}dx{\sqrt {{\frac {2m}{\hbar ^{2}}}\left[V(x)-E\right]}}},$$ where$x_{1},x_{2}$are the two classical turning points for the potential barrier.

For a rectangular barrier, this expression simplifies to: $$T(E)=e^{-2{\sqrt {{\frac {2m}{\hbar ^{2}}}(V_{0}-E)}}(x_{2}-x_{1})}.$$

Faster than light

See also:Faster-than-light

Some physicists have claimed that it is possible for spin-zero particles to travel faster than thespeed of lightwhen tunnelling.This appears to violate the principle ofcausality,since aframe of referencethen exists in which the particle arrives before it has left. In 1998,Francis E. Lowreviewed briefly the phenomenon of zero-time tunnelling.More recently, experimental tunnelling time data ofphonons,photons,andelectronswas published byGünter Nimtz.Another experiment overseen byA. M. Steinberg,seems to indicate that particles could tunnel at apparent speeds faster than light.

Other physicists, such asHerbert Winful,disputed these claims. Winful argued that the wave packet of a tunnelling particle propagates locally, so a particle can't tunnel through the barrier non-locally. Winful also argued that the experiments that are purported to show non-local propagation have been misinterpreted. In particular, the group velocity of a wave packet does not measure its speed, but is related to the amount of time the wave packet is stored in the barrier. Moreover, if quantum tunneling is modeled with the relativisticDirac equation,well established mathematical theorems imply that the process is completely subluminal.

Dynamical tunneling

Quantum tunneling oscillations of probability in an integrable double well of potential, seen in phase space

The concept of quantum tunneling 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. This phenomenon is known as dynamical tunnelling.

Tunnelling in phase space

The concept of dynamical tunnelling is particularly suited to address the problem of quantum tunnelling in high dimensions (d>1). In the case of anintegrable system,where bounded classical trajectories are confined ontotoriinphase space,tunnelling can be understood as the quantum transport between semi-classical states built on two distinct but symmetric tori.

Chaos-assisted tunnelling

Chaos-assisted tunnelling oscillations between two regular tori embedded in a chaotic sea, seen in phase space

In real life, most systems are not integrable and display various degrees of chaos. Classical dynamics is then said to be mixed and the system phase space is typically composed of islands of regular orbits surrounded by a large sea of chaotic orbits. The existence of the chaotic sea, where transport is classically allowed, between the two symmetric tori then assists the quantum tunnelling between them. This phenomenon is referred as chaos-assisted tunnelling.and is characterized by sharp resonances of the tunnelling rate when varying any system parameter.

Resonance-assisted tunnelling

When$\hbar$is small in front of the size of the regular islands, the fine structure of the classical phase space plays a key role in tunnelling. In particular the two symmetric tori are coupled "via a succession of classically forbidden transitions across nonlinear resonances" surrounding the two islands.

Related phenomena

Several phenomena have the same behavior as quantum tunnelling. Two examples areevanescent wave coupling(the application ofMaxwell's wave-equationtolight) and the application of thenon-dispersive wave-equationfromacousticsapplied to"waves on strings".

These effects are modeled similarly to therectangular potential barrier.In these cases, onetransmission mediumthrough which thewave propagatesthat is the same or nearly the same throughout, and a second medium through which the wave travels differently. This can be described as a thin region of medium B between two regions of medium A. The analysis of a rectangular barrier by means of the Schrödinger equation can be adapted to these other effects provided that the wave equation hastravelling wavesolutions in medium A but realexponentialsolutions in medium B.

Inoptics,medium A is a vacuum while medium B is glass. In acoustics, medium A may be a liquid or gas and medium B a solid. For both cases, medium A is a region of space where the particle'stotal energyis greater than itspotential energyand medium B is the potential barrier. These have an incoming wave and resultant waves in both directions. There can be more mediums and barriers, and the barriers need not be discrete. Approximations are useful in this case.

A classical wave-particle association was originally analyzed as analogous to quantum tunneling,but subsequent analysis found a fluid dynamics cause related to the vertical momentum imparted to particles near the barrier.

See also

• Dielectric barrier discharge
• Field electron emission
• Holstein–Herring method
• Proton tunneling
• Quantum cloning
• Superconducting tunnel junction
• Tunnel diode
• Tunnel junction
• White hole

Further reading

• Binney, James; Skinner, David (2010).The physics of quantum mechanics(3. ed.). Great Malvern: Cappella Archive.ISBN978-1-902918-51-8.
• Fröman, Nanny; Fröman, Per Olof (1965).JWKB Approximation: Contributions to the Theory.Amsterdam: North-Holland.ISBN978-0-7204-0085-4.
• Griffiths, David J. (2004).Introduction to electrodynamics(3. ed.). Upper Saddle River, NJ: Prentice Hall.ISBN978-0-13-805326-0.
• Liboff, Richard L. (2002).Introductory quantum mechanics(4th ed.). San Francisco: Addison-Wesley.ISBN978-0-8053-8714-8.
• Muller-kirsten, Harald J. W. (2012).Introduction To Quantum Mechanics: Schrodinger Equation And Path Integral(2nd ed.). Singapore: World Scientific Publishing Company.ISBN978-981-4397-76-6.
• Razavy, Mohsen (2003).Quantum theory of tunneling.River Edge, NJ: World Scientific.ISBN978-981-238-019-7.OCLC52498470.
• Hong, Jooyoo; Vilenkin, Alexander; Winitzki, Serge (2003). "Particle creation in a tunneling universe".Physical Review D.68(2): 023520.arXiv:gr-qc/0210034.Bibcode:2003PhRvD..68b3520H.doi:10.1103/PhysRevD.68.023520.ISSN0556-2821.S2CID118969589.
• Wolf, E. L. (2012).Principles of electron tunneling spectroscopy.International series of monographs on physics (2nd ed.). Oxford; New York: Oxford University Press.ISBN978-0-19-958949-4.OCLC768067375.

External links

• Animation, applications and research linked to tunnel effect and other quantum phenomena(Université Paris Sud)
• Animated illustration of quantum tunneling
• Animated illustration of quantum tunneling in a RTD device
• Interactive Solution of Schrodinger Tunnel Equation