| quantum tunneling | |
|---|---|
| Name | Quantum tunneling |
| Caption | Schematic of a particle tunneling through a potential barrier |
| Field | Quantum mechanics |
| Introduced | Early 20th century |
| Introduced by | George Gamow; Ronald Gurney and Edward Condon |
| Related | Wave–particle duality, Schrödinger equation, Quantum field theory |
quantum tunneling
Quantum tunneling is a quantum mechanical phenomenon in which a particle or excitation traverses a classically forbidden energy barrier due to the probabilistic nature of its wavefunction. It underlies a range of processes in condensed matter physics, nuclear physics, and physical chemistry, and is essential for technologies such as the tunnel diode and the scanning tunneling microscope.
Quantum tunneling allows systems to access states that would be inaccessible under classical mechanics because the barrier energy exceeds the particle's kinetic energy. The effect arises from the nonzero amplitude of the particle's wave function on the far side of a barrier, producing a finite transmission probability. In nuclear physics, tunneling explains alpha decay as first quantitatively described by George Gamow and by Ronald Gurney and Edward Condon. In solid state physics, tunneling drives transport in Josephson junctions and influences carrier dynamics in semiconductor devices. In chemistry and biology, tunneling can modify reaction rates via proton or electron tunneling in enzymes and molecular chemistry.
The conceptual basis rests on principles of quantum mechanics including wave–particle duality and the linearity of the Schrödinger equation. A localized quantum state is represented by a wavefunction whose amplitude penetrates classically forbidden regions; continuity and boundary conditions determine reflection and transmission coefficients. The phenomenon is consistent with energy conservation and the time-dependent formalism of quantum theory, and extends into quantum field theory where tunneling of fields underlies phenomena such as instantons and false vacuum decay in cosmology. Historical contributors include Max Born (probabilistic interpretation) and the early 20th-century pioneers who applied quantum ideas to radioactive decay.
Analytic solutions exist for idealized potentials such as the rectangular barrier, the potential well, and the delta function potential. For a rectangular barrier of height V0 and width a, the transmission coefficient T can be expressed via matching conditions on the Schrödinger equation solutions and decays exponentially with barrier width when particle energy E < V0. The WKB approximation (Wentzel–Kramers–Brillouin) provides semiclassical estimates of tunneling rates for smoothly varying potentials and is widely used in nuclear physics and quantum chemistry. In multidimensional systems, tunneling pathways are analyzed with instanton methods, path integral techniques developed by Richard Feynman, and reaction-rate theories such as transition state theory augmented by quantum corrections. Numerical approaches include finite-difference solutions, density functional theory for electronic tunneling in materials, and matrix-product methods for many-body tunneling.
Early confirmation came from measurements of radioactive decay rates consistent with Gamow's theory. In condensed matter, the advent of the scanning tunneling microscope (STM), invented by Gerd Binnig and Heinrich Rohrer at IBM Zurich Research Laboratory, provided atomic-scale evidence by measuring tunneling currents between a tip and a sample surface. Josephson tunneling in superconducting junctions, predicted by Brian Josephson, produces measurable supercurrents and led to devices such as superconducting qubits used by groups at IBM, Google and academic laboratories. Low-temperature experiments probe macroscopic quantum tunneling in SQUIDs and phase slips in nanowires. Time-resolved measurements of electron tunneling employ ultrafast spectroscopy and pump–probe techniques; scanning probe and single-electron transistor setups enable measurement of tunneling at the single-charge level in quantum dots.
Tunneling is exploited in electronic components such as the tunnel diode (Esaki diode), resonant tunneling diodes used in high-frequency electronics, and flash memory where tunneling enables charge storage and removal through thin oxide layers. The STM is a cornerstone tool in surface science and nanotechnology for imaging and manipulating individual atoms. In quantum computing, tunneling dynamics are central to the operation of certain qubit architectures, including flux qubits and quantum annealers by companies like D-Wave Systems that use tunneling in optimization protocols. In chemistry, proton and electron tunneling affect reaction kinetics, isotope effects, and enzyme catalysis; theoretical and experimental studies connect tunneling to rate enhancements in cold and room-temperature reactions. In astrophysics and stellar nucleosynthesis, tunneling governs fusion cross sections at stellar energies, impacting models of energy generation in stars and the work of laboratories such as CERN and accelerator facilities measuring low-energy nuclear reactions.
Tunneling closely relates to phenomena that exploit quantum coherence and barrier penetration: quantum reflection, Andreev reflection at superconductor–normal interfaces, and resonant tunneling in heterostructures. Extensions include many-body tunneling processes like cotunneling in quantum dots, Landau–Zener transitions in driven two-level systems, and topological tunneling events in quantum field theory such as instantons and sphalerons. Tunneling also interfaces with foundational questions in quantum measurement and decoherence; experimental platforms investigating these issues include ultracold atoms in optical lattices (groups at MIT and Max Planck Institute for Quantum Optics) and superconducting circuits developed in academic and industrial labs. Further theoretical developments connect tunneling to quantum thermodynamics and non-equilibrium statistical mechanics.
Category:Quantum mechanics Category:Nuclear physics Category:Condensed matter physics