Timeline of Quantum Mechanics
The development of quantum mechanics, from Max Planck's energy quanta of 1900 through the theory's founding in the 1920s and the debates over entanglement to the Bell tests honored by the 2022 Nobel Prize in Physics.
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- Natural Sciences
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- 31 events 1900–2022
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- by claude-opus-5-5
1900: Planck derives his radiation law using energy quanta
On December 14, Max Planck presented to the German Physical Society a derivation of his black-body radiation formula that divided the energy of the emitting oscillators into finite elements of size hν, where ν is the frequency and h the constant now named after Planck. Planck at first regarded the step as a formal device rather than a sign that energy is physically discontinuous.
Wikipedia1905: Einstein proposes that light consists of quanta
In a paper published on June 9, Albert Einstein proposed, as a "heuristic point of view", that light consists of localized quanta, each with energy proportional to its frequency, explaining why light below a threshold frequency ejects no electrons from a metal however intense it is. The idea met wide resistance, but the 1921 Nobel Prize in Physics, awarded to Einstein in 1922, cited his law of the photoelectric effect.
Wikipedia1913: Bohr proposes a quantized model of the atom
In a three-part paper in "Philosophical Magazine", Niels Bohr placed the electrons of Ernest Rutherford's nuclear atom in discrete stationary states, emitting or absorbing radiation only when jumping between them, at a frequency set by the energy difference. The model reproduced the spectral series of hydrogen, and Bohr soon argued that James Franck and Gustav Hertz's electron-collision experiments of 1914 confirmed such energy levels.
Wikipedia1922: Stern and Gerlach demonstrate space quantization
In Frankfurt, Otto Stern and Walther Gerlach sent a beam of silver atoms through a nonuniform magnetic field and found that it split into two distinct traces instead of smearing out, showing that the orientation of the atoms' magnetic moments is quantized. Designed to test the Bohr–Sommerfeld model, the result was later explained by electron spin and convinced physicists that angular-momentum quantization was real.
Wikipedia1923: Compton shows that X-ray quanta carry momentum
Arthur Compton at Washington University in St. Louis found that X-rays scattered by light elements emerge with longer wavelengths, shifted by an amount set by the scattering angle. He, and independently Peter Debye, explained this by treating each X-ray as a quantum carrying momentum that collides with a single electron, convincing many physicists that light quanta were real. Compton shared the 1927 Nobel Prize in Physics with C. T. R. Wilson.
Wikipedia1923–1924: De Broglie proposes matter waves
In three short notes to the French Academy of Sciences in September and October 1923, developed in his doctoral thesis of November 1924, Louis de Broglie proposed that electrons and all other matter have an associated wave, with wavelength equal to Planck's constant divided by momentum. He suggested looking for electron diffraction, and the idea, promoted by Albert Einstein, inspired Erwin Schrödinger's wave mechanics.
Wikipedia1924: Bose derives Planck's law by counting indistinguishable photons
Satyendra Nath Bose of the University of Dhaka, in British India, derived Planck's radiation law by treating photons as indistinguishable particles. Albert Einstein translated the paper into German for "Zeitschrift für Physik", extended the counting to atoms and predicted that in a sufficiently cold gas many atoms would gather in the lowest energy state, a Bose–Einstein condensate. Particles that obey these statistics are now called bosons.
Wikipedia1925: Pauli formulates the exclusion principle
Wolfgang Pauli proposed that no two electrons in an atom can share the same values of all four quantum numbers, a rule that explains how electrons fill the shells underlying the periodic table. His fourth, "two-valued" quantum number was identified later that year by George Uhlenbeck and Samuel Goudsmit as the electron's spin, and Pauli received the 1945 Nobel Prize in Physics for the principle.
Wikipedia1925: Heisenberg, Born and Jordan create matrix mechanics
Werner Heisenberg submitted a paper in July that rebuilt atomic theory from observable quantities such as spectral frequencies, using a multiplication in which order matters. Max Born recognized it as matrix algebra, and with Pascual Jordan the three completed matrix mechanics by year's end; Paul Dirac independently recast it in his own algebra. The UN later proclaimed 2025, the centenary, the International Year of Quantum Science and Technology.
Wikipedia1926: Schrödinger publishes his wave equation
Erwin Schrödinger, building on Louis de Broglie's matter waves, published in "Annalen der Physik" a wave equation whose allowed solutions reproduce the energy levels of the hydrogen atom. Within months he applied it to other systems and showed that his wave mechanics gave the same results as matrix mechanics; physicists widely preferred its familiar mathematics, and he shared the 1933 Nobel Prize in Physics with Paul Dirac.
Wikipedia1926: Fermi and Dirac derive the statistics of fermions
Enrico Fermi and, independently, Paul Dirac worked out the statistics of identical particles that obey the Pauli exclusion principle, no two of which can occupy the same quantum state. Ralph Fowler applied it later that year to the dense matter of white dwarf stars and Arnold Sommerfeld in 1927 to electrons in metals. Particles that obey Fermi–Dirac statistics, including electrons, protons and neutrons, are now called fermions.
Wikipedia1926: Born links the wave function to probability
Applying Erwin Schrödinger's equation to collisions, Max Born proposed in a July paper that the wave function does not describe a spread-out electron but gives the probability of finding it, with a footnote added in proof making the probability proportional to the square of the wave's amplitude. The Born rule became a basic postulate of quantum mechanics, and Born shared the 1954 Nobel Prize in Physics partly for it.
Wikipedia1927: Electron diffraction confirms de Broglie's matter waves
Clinton Davisson and Lester Germer at Bell Labs found that slow electrons scattered from a nickel crystal form a diffraction pattern, reported in "Nature" in April. Independently, George Paget Thomson and Alexander Reid at the University of Aberdeen saw diffraction rings from electrons fired through thin films. The results confirmed Louis de Broglie's hypothesis, and Davisson and Thomson shared the 1937 Nobel Prize in Physics.
Wikipedia1927: Heisenberg states the uncertainty principle
Werner Heisenberg argued that the more precisely a particle's position is fixed, the less precisely its momentum can be known, the product of the uncertainties being no smaller than roughly Planck's constant, and illustrated it with an imagined gamma-ray microscope. Earle Hesse Kennard proved the exact bound later that year: the product of the standard deviations of position and momentum is at least h/4π.
Wikipedia1927: Bohr introduces the principle of complementarity
In a lecture at Como, Italy, on September 16, Niels Bohr argued that experiments revealing complementary aspects of a quantum object, such as wave and particle behavior, exclude one another, yet all are needed for a full description. He repeated the argument in October at the Fifth Solvay Conference in Brussels, where his debates with Albert Einstein began; complementarity became central to what was later called the Copenhagen interpretation.
Wikipedia1928: Dirac publishes a relativistic equation for the electron
Paul Dirac published a relativistic quantum equation for the electron, from which its spin and magnetic moment followed without being added by hand and which accounted for hydrogen's fine structure. Its negative-energy solutions led Dirac in 1931 to predict an "anti-electron", seen by Carl Anderson in cosmic rays in 1932 as the positron. A tiny hydrogen level shift it misses, measured by Willis Lamb in 1947, spurred quantum electrodynamics.
Wikipedia1932: Von Neumann formalizes quantum mechanics in Hilbert space
In "Mathematische Grundlagen der Quantenmechanik", John von Neumann gave the theory a rigorous form in which states are vectors in a Hilbert space and observables are operators on it, and treated measurement as a discontinuous, irreversible process distinct from the wave function's continuous evolution. His proof that no hidden variables could underlie the theory was faulted by Grete Hermann in 1935 and by John Stewart Bell in 1966.
Wikipedia1935: Einstein, Podolsky and Rosen question quantum completeness
Assuming that a measurement on one particle cannot instantly disturb a distant one, Albert Einstein, Boris Podolsky and Nathan Rosen argued in "Physical Review" that a particle whose position or momentum could be predicted from a correlated partner must have definite values of both, so quantum mechanics is incomplete. Niels Bohr replied under the same title that the argument failed because the two measurements are complementary.
Wikipedia1935: Schrödinger names entanglement and devises his cat paradox
Responding to the Einstein–Podolsky–Rosen paper, Erwin Schrödinger coined the term entanglement (Verschränkung) for systems that, after interacting, can no longer be described separately, calling it "the characteristic trait of quantum mechanics". In a November article he imagined a cat whose life hangs on the decay of a radioactive atom, to argue that a wave function mixing a live and a dead cat cannot be a literal picture of reality.
Wikipedia1952: Bohm revives de Broglie's pilot-wave theory
In two papers in "Physical Review", David Bohm showed that a deterministic theory in which particles always have definite positions, guided by the wave function, reproduces the predictions of ordinary nonrelativistic quantum mechanics, independently reviving an idea Louis de Broglie had presented in 1927. The theory is explicitly nonlocal, which led John Stewart Bell to ask whether every hidden-variable theory must be.
Wikipedia1957: Everett proposes the relative-state formulation
In his Princeton doctoral thesis under John Archibald Wheeler and a paper in "Reviews of Modern Physics", Hugh Everett III proposed treating observers as ordinary quantum systems and dropping wave-function collapse, so that every possible measurement outcome is realized in its own branch of a universal wave function. Largely ignored at first, it was popularized in the 1970s by Bryce DeWitt as the many-worlds interpretation.
Wikipedia1964: Bell publishes his theorem on local hidden variables
In a paper titled "On the Einstein Podolsky Rosen Paradox", John Stewart Bell showed that if measurement outcomes were fixed by local hidden variables, correlations between distant measurements would obey an inequality that quantum mechanics predicts entangled particles violate. No local hidden-variable theory can therefore reproduce all the predictions of quantum mechanics, a difference that experiments could test.
Wikipedia1970: Zeh introduces the idea of environmental decoherence
H. Dieter Zeh argued that macroscopic systems are never isolated, so their entanglement with the environment suppresses interference between alternative outcomes, an effect later called decoherence. Developed from 1981 by Wojciech Zurek and others and first measured in 1996 by Serge Haroche's group in Paris, it explains why everyday objects are not seen in superpositions, but not, by itself, why a measurement yields a definite result.
Wikipedia1972: Freedman and Clauser observe a Bell inequality violation
At the University of California, Berkeley, Stuart Freedman and John Clauser measured polarization correlations of photon pairs emitted by excited calcium atoms and observed the first violation of a Bell inequality, by six standard deviations, as quantum mechanics predicted. The test followed a 1969 proposal by Clauser, Michael Horne, Abner Shimony and Richard Holt, but its polarizer settings were fixed in advance, leaving loopholes.
Wikipedia1982: Aspect switches Bell-test settings during the photons' flight
At the Institut d'Optique in Orsay, France, Alain Aspect and colleagues tested Bell's inequalities with entangled photons from calcium atoms. In the final experiment, published in December, fast optical switches changed the polarizer setting on each side while the photons were in flight, addressing the locality loophole, though periodically rather than randomly. The inequalities were violated, as quantum mechanics predicts.
Wikipedia1984: Bennett and Brassard propose quantum key distribution
Charles Bennett and Gilles Brassard described the first quantum key distribution protocol (BB84, after its authors and year), in which two parties build a secret key from single photons sent in randomly chosen polarization bases. Because measuring an unknown quantum state generally disturbs it, eavesdropping leaves detectable errors. The scheme built on Stephen Wiesner's conjugate coding, and the pair received the 2025 Turing Award.
Wikipedia1985: Clarke, Devoret and Martinis show macroscopic quantum tunneling
At the University of California, Berkeley, John Clarke, Michel Devoret and John Martinis showed that a superconducting circuit with a Josephson junction, on a chip about a centimeter across, escapes its zero-voltage state by quantum tunneling and absorbs energy only in discrete amounts, behaving as one quantum system of billions of paired electrons. The work underlies superconducting qubits and won the 2025 Nobel Prize in Physics.
Wikipedia1995: Cornell and Wieman create a Bose–Einstein condensate
On June 5, Eric Cornell and Carl Wieman at the University of Colorado Boulder cooled rubidium-87 atoms until, near 170 nanokelvins, atoms began to collect in the lowest quantum state, as Albert Einstein had predicted 70 years earlier; further cooling left a pure condensate of about 2,000 atoms. Wolfgang Ketterle at the Massachusetts Institute of Technology made a larger sodium condensate four months later; the three shared the 2001 Nobel Prize in Physics.
Wikipedia1997: Quantum teleportation demonstrated with photons
Anton Zeilinger's group in Innsbruck and Francesco De Martini's in Rome independently transferred the polarization state of one photon to another using entanglement and a classical signal, as proposed in 1993 by Charles Bennett and five co-authors. Because it needs the classical signal, teleportation cannot carry information faster than light. In 2017 photon states were teleported from Tibet to China's Micius satellite, up to 1,400 kilometers away.
Wikipedia2015: First loophole-free Bell tests reported
Within three months, groups in Delft, Vienna and Boulder, Colorado, reported Bell tests that closed the detection and locality loopholes at the same time, Delft with electron spins in diamonds 1.3 kilometers apart and the others with photons. All violated Bell inequalities; Alain Aspect wrote that such tests "remove the last doubts" about abandoning local hidden variables, though no experiment is entirely loophole-free.
Wikipedia2022: Nobel Prize honors Aspect, Clauser and Zeilinger
On October 4, the Royal Swedish Academy of Sciences awarded the Nobel Prize in Physics to Alain Aspect, John Clauser and Anton Zeilinger "for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science". It recognized work ranging from Clauser's first Bell inequality violation and Aspect's switched-setting tests to Zeilinger's teleportation and entanglement-swapping experiments.
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