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1800 - Frederick William Herschel, discovery of infra-red
Frederick William Herschel (1738-1822) was working in Bath as astronomer, when he started to measure the heating effect of different colors of light coming from the Sun, resolved through the refraction in th eprism. He was using his thermometer and was measuring relative the heating of the thermometer placed in different colors of the solar spectrum. When the thermometer was just beyond the red end of the spectrum, Herschel noticed that relative temperature of the thermometer was highest, indicating strong heating. He concluded that some invisible rays, just beyond the red part of the spectrum, are coming from the sun and causing heating.
Image credits: reproduced from William Herschel, Experiments on the Solar, and on the Terrestrial Rays that Occasion Heat; With a Comparative View of the Laws to Which Light and Heat, or Rather the Rays Which Occasion Them, are Subject, in Order to Determine Whether They are the Same, or Different. Part I, Philosophical Transactions of the Royal Society of London 90 293, (1800).
1801 - Young's double slit experiment
The experiment of Thomas Young's (1773 - 1829), English physician and physicist, established strong evidence for wave theory of light at the time when the majority of scientists thought that Newton's corpuscular theory was sufficient. Young used wave theory of light to explain color of thin films and he calculated wavelengths of the principal colours in the visible spectrum. Some years later, in 1817, Young proposed that light waves are transverse, not longitudinal as it was assumed, explaining polarisation of light.
Image credits: reproduced from Thomas Young, II. The Bakerian Lecture. On the theory of light and colours, Phil. Trans. R. Soc. 92 12 (1802)
1801 - Johann Wilhelm Ritter, discovery of UV
Following on Herschel's discovery of infrared radiation beyond the red part of the spectrum, Johann Wilhelm Ritter (1776-1810) was examining if there is light beyond the blue part of the spectrum. He used silver chloride, which is a chemical compound that decomposes when exposed to light, leaving darker silver visible. Ritter noticed that silver chloride doesn't change in color in red part of the spectrum, but becomes darker in the blue part of the spectrum. He then put the silver chloride beyond the blue part of the spectrum, where no light was visible, and discovered that darkening of silver chloride is even faster there. He concluded that there are additional invisible light - now called ultraviolet - in this part of the spectrum that was causing the reaction.
Plants can have ultraviolet patterns. These can be seen by bees, as well as some other insects, birds and reptiles. On the picture we see side by side Mimulus flower photographed in visible light (left) and ultraviolet light (right). Dark strip visible only in UV is so called nectar guide for bees.Credits: Plantsurfer, Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons.
1802 - Discovery of absorption lines in solar spectra
Joseph von Fraunhofer (1787–1826) started as a glassmaker, and developed new types of glasses, standardised glass production methods and thanks to this created scientific instruments with unmatched quality. He also used diamonds to create optical diffraction gratings with grooves spaced only 0.003 mm. Using spectrometers he build, he mapped more than 500 dark lines in solar spectrum, that now we understand originate from selective absorption of sun radiation by elements present in the solar atmosphere.
(a) The spectroscope developed by Fraunhofer; (b) The spectrum of sunlight drawn by Fraunhofer. Image credits: VIII. Bestimmung des Brechungs- und Farbenzerstreuungs-Vermögens verschiedener Glasarten, in Bezug auf die Vervollkommnung achromatischer Fernröhre. München : [Verlag nicht ermittelbar], 1817. ETH-Bibliothek Zürich, Rar 5099,Public Domain
1809 - Etienne-Louis Malus discovers polarization
Back in 1669. Danish physician, mathematician, and physicist, Erasmus Bartholin (1625—1698) discovered double refraction in Icelandic spar (calcite). Light beam passing through the crystal would be split into two, and upon rotation of the crystal, one of the beams would would remain stationary while the other would rotate with the crystal. These two beams were called ordinary and extraordinary respectively but the phenomena was not understood at the time.
Using two calcite crystals, with crystal principal axis set at variable angles, French physicist Étienne-Louis Malus (1775-1812) observed effect of the second crystal on ordinary and extraordinary ray transmitted through the first calcite. He deduced that the calcite changed some property of light. By looking through a crystal of icelandic spar at the sunset reflected in a window of the Palais du Luxembourg in Paris he found that the same property of light was changed also upon reflection from surfaces. Following corpuscular theory, he explained this by particles of light having two poles - hence introducing concept of polarization of light for the first time.
Erasmus Bartholin, Experimenta crystalli Islandici disdiaclastici quibus mira & insolita refractio detegitur, (Copenhagen, 1669)
E.-L. Malus, Sur une Proprietede la Lumiere Reflechie, Memoires de Physique et de Chimie de la Societe d 'Arcueil II, 143 (1809, Paris, Mad.Ve:Bernard,quai des Augustins)
E.-L. Malus, Memoire sur de Nouveaux Phenomenes d'optique, Moniteur No. 72, 277 (1811)
Image credits: 445 nm laser comes from left to calcite crystal, and shows clearly birefringence forming two rays at the output. Fluorescence within the crystal shows light paths in the crystal. Jan Pavelka, CC BY-SA 4.0, via Wikimedia Commons.
1842 - Photograph of the solar spectrum
Edmond Becquerel (1820 - 1891) photographed the solar spectrum including the UV region.
E. Becquerel, L'image photographique colorée du spectre solaire, Comptes Rendus 26, 181. Picture credits: Edmond Becquerel, Spectres solaires, 1848, images photochromatiques. Courtesy of musée Nicéphore Niépce, Ville de Chalon-sur-Saône.
1856 - Demonstration of the absorption of heat by CO2 and water vapor.
Eunice Newton Foote (1819-1888) discovered the principal cause of the global warming, by measuring heating in dry air, damp air and CO2 filled atmosphere. She concluded that 'An atmosphere of that gas [CO2] would give to our earth a high temperature; and if as some suppose, at one period of its history the air had mixed with it a larger proportion than at present, and increased temperature from its own action as well as from increased weight must have necessarily resulted.' [Eunice Foote, 'Circumstances Affecting the Heat of the Sun's Rays', American Journal of Science and Arts (1856)]. In addition to her scientific work, she was inventor with several patents and active in women's rights movement.
1909 - G. I. Taylor photographs interference pattern for weak light
G. I. Taylor (1886-1975), physicist and mathematician, imaged shadow of a needle illuminated with very low light intensity, trying to observe deviation from Huygens-style wave interference when only a few light quanta are present in the wave front. For the lowest illumination, exposure time was 2000 hours or about 3 months. That means that in an apparatus there is approximately one photon per every 100.000 transit times it takes a photon to travel from source to image plate. In spite of such low intensity, shar interferometric pattern was obtained on the photographic plate.
G. I. Taylor, “Interference fringes with weak light,”, Proc. Camb. Phil. Soc., 15, 114 (1909) Image: G.I. Taylor's demonstration of diffraction bands caused by a thin wire in feeble light. Two black and white photographs of diffraction bands caused by a thin wire in strong and feeble light. The exposures were 4 minutes (left) and 400 hours (right). Although the light photons only arrived one at a time, the characteristic diffraction pattern was observed. Credits: G.I. Taylor's demonstration of diffraction bands caused by a thin wire in feeble light (1909), Courtesy of and copyright the Cavendish Laboratory, University of Cambridge.
1956 - Hanbury Brown-Twiss experiment
R. Hanbury Brown (1916-2002) and R. Q. Twiss (1920-2005) were working in the field of Radio Astronomy looking at the ways to expand on limits of usual Michelson-interferometer arrays of radio telescopes, where signals recording both phase and intensity were combined (interfered) and used to form radio telescopes with better angular resolution. They suggested first using intensity correlations between the distant dishes to measure angular size of distant objects. Since only intensity had to be recorded, this was feasible back then even if the baseline (distance between individual telescopes) was large. They then applied the same idea to detection of optical radiation, applying their technique to measure the angular diameter of the Sirius star. Today their experimental configuration is often used for measuring correlations within single beam.
R. Hanbury Brown and R. Q. Twiss, A New Type of Interferometer for Use in Radio Astronomy, Philosophical Magazine 45, 663 (1954), R. Hanbury Brown, R. Q. Twiss, A test of a new type of stellar interferometer on Sirius, Nature 178, 1046 (1956), Image credits: Figure 2. from R. Hanbury Brown and R. Q. Twiss, Correlation between photons in two coherent beams of light, Nature 177, 27 (1956), reproduced with permission from Springer Nature, showing experimental apparatus for measurement of correlations.
1966 - Measurement of Photon Bunching in a Thermal Light Beam
Measuring distribution of successive photon arrival times from thermal source, B. L. Morgan and L. Mandel were able to observe for short delay times between photon detection that there is more photon detection events than what one would expect if the photons were completely randomly distributed in time - that is photons were bunched.
Image credits: Reprinted figures 1. and 2. with permission from B. L. Morgan and L. Mandel, Measurement of photon bunching in a thermal light beam, Physical Review Letters 16, 1012 (1966). Copyright 1966 by the American Physical Society.
1967 - Polarization Correlation of Photons Emitted in an Atomic Cascade
The first observation of quantum entanglement with polarization states of optical photons, that paved a way for later tests of Einstein-Rosen-Podolsky paradox and Bell's inequalities.
Image credits: Reprinted figure 1. with permission from Carl. A. Kocher and Eugene D. Commins, Polarization correlation of photons emitted in an atomic cascade, Physical Review Letters 18, 575 (1967). Copyright 1967 by the American Physical Society. The figure shows apparatus (a) for detection of photons emitted in cascade from the Ca beam excited with light from H2 arc lamp. The level scheme (b) shows initial excitation to 6^1P_1, and marks observed deexcitation cascade 6^1S_0 -> 4^1P_1 -> 4^1S_0.
1970 - Photon pairs produced by parametric down conversion
Using setup shown on image to the right, David C. Burnham and Donald L. Weinberg confirmed that in parametric fluorescence, initial photons from the source are converted into photon pairs that are (i) created simultaneously, (ii) conserve total energy and (iii) conserve total momentum. Relative polarisation of the photons is fixed by the phase matching conditions on efficient down-conversion in the non-linear crystal. This became a workforce method for production of entangled photons with solid-state components.
Image credits: Reprinted figure 1. with permission from David C. Burnham and Donald L. Weinberg, Observation of simultaneity in parametric production of optical photon pairs, Physical Review Letters 25, 84 (1970). Copyright 1970 by the American Physical Society. The figure shows experimental arrangement. 325-nm laser is being converted into two spatially separated beams in phase matched ADP (Ammonium Dihydrogen Phosphate) crystal using the crystal's birefringence. In the horizontal plane there are two directions where phase-matched photons should go. In the channel 1 arm narrow-band band-pass filter is used in front of the photomultiplier used in counting mode. In the channel 2 monochromator is used as tunable narrow-band band-pass filter.
1972 - Experimental Test of Local Hidden-Variable Theories
Using the polarisation entangled photons from Ca cascade first measured by Carl. A. Kocher and Eugene D. Commins in 1967, Stuart J. Freedman and John F. Clauser tested polarisation correlations between generated photon pairs. They obtained results in agreement with quantum mechanics, and provided strong evidence against local hidden-variable theories.
Image credits: Reprinted figures 1. and 3. with permission from Stuart J. Freedman and John F. Clauser, Physical Review Letters 28, 938 (1972). Copyright 1972 by the American Physical Society.
1977 - Photon Antibunching in Resonance Fluorescence
H. J. Kimble, M. Dagenais, and L. Mandel, observing fluorescence of continuously resonantly excited atom beam (using dye laser). Analysing two-time correlation function of photon fluorescence, they observed that upon detection of one photon, during a certain time interval (determined by strength of driving necessary to excite again deexcited atoms) probability of detecting another photon is reduced significantly below what one would expect if photons were completely uncorrelated. This so-called antibunching of photons is both illustration of quantum nature of light and an illustration if quantum jumps: while under coherent driving atom state continuously evolves, upon detection of atom emission its state is abruptly projected on the ground state (jump down).
Image credits: Reprinted figure 1. and 2. with permission from H.J. Kimble, M. Dagenais, and L. Mandel, Photon antibunching in reosnance fluorescence, Physical Review Letters 39, 691 (1977). Copyright 1977 by the American Physical Society.
1981 - Experimental Tests of Realistic Local Theories via Bell's Theorem
Using the polarisation entangled photons from Ca cascade first measured by Carl. A. Kocher and Eugene D. Commins in 1967, Alain Aspect, Philippe Grangier, and Gerard Roger measured correlations of the photons now with improved statistics thanks to the improved efficiency of the source (two-photon laser excited), strongly ruling out local realistic theories.
Image credits: Reprinted figures 2. and 4. with permission from Alain Aspect, Philippe Grangier, and Gerard Roger, Experimental Tests of Realistic Local Theories via Bell's Theorem, Physical Review Letters 47, 460 (1981). Copyright 1981 by the American Physical Society.
1987 - Hong-Ou-Mandel interference
If two exactly identical photons (frequency, pulse width), whose modes are exactly spatially overlapped at 50:50 beam splitter, arrive at exactly the same time, then completely destructive interference in amplitudes between both photons being reflected and both photons being transmitted means that at the two outputs of the beam splitter the photons will always be found in pairs. In other words, the coincidence counts of two photons, arriving in two different beam splitter outputs, will drop to zero. This has been observed for the first time by C. K. Hong, Z. Y. Ou, and L. Mandel, and this effect, called after their surnames Hong-Ou-Mandel interference, can be used for measuring very small offsets between the photons, either in time, or in their properties. This is often used for example in characterising nowadays how identical are 'identical' single photons produced from single photon sources.
Reprinted figures 1. and 2. with permission from C. K. Hong, Z. Y. Ou, and L. Mandel, Measurement of subpicosecond time intervals between two photons by interference, Physical Review Letters 59, 2044 (1987). Copyright 1987 by the American Physical Society.
1900 - Blackbody spectra, Max Planck
In order to explain black body spectra, Max Planck (1858-1947) introduces idea that oscillators in the black body can only have discrete number of energy elements, each containing energy proportional to the frequency of oscillations, with the constant of proportionality now known as Planck's constant.
Image credits: Reprinted excerpt from The old Quantum Theory, D. Ter Haar, page 84, translation of Max Planck, On the Theory of the Energy Distribution Law of the Normal Spectrum, copyright Pergamon press (1967), originally published as Max Planck, Verh. Dtsch. Phys. Ges. Berlin 2, 237 (1900)
1905 - Einstein introduces the concept of photon
In his 1905. paper, Albert Einstein (1879-1955) acknowledges tremendous success of wave theory of light, carefully highlights that it has been chiefly tested in observations of time averages rather than instantaneous values. Then he focuses attention to transient phenomena involved in transformation of light, and uses probabilistic interpretation of entropy to argue for introduction of energy quanta that cannot be divided, and can be produced and absorbed only as complete units - that is a concept of photon.
Albert Einstein, Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt, Ann. Physik 17, 132 (1905)Image credits: Reprinted excerpt from A. B. Arons and M. B. Peppard, Einstein's Proposal of the Photon Concept - a translation of the Annalen der Physik paper of 1905, American Journal of Physics 33, 367 (1965), with the permission of AIP Publishing.
1963 - The quantum theory of optical coherence
Roy J. Glauber (1925 - 2018) introduces quantum theory of arbitrary order correlations of the light field, and defines coherent states as a fields whose higher-order correlation functions factorise into first order coherence functions.
Image credits: Reprinted excerpt with permissoin from Roy J. Glauber, The Quantum Theory of Optical Coherence, Physical Review 130, 2529 (1963). Copyright 1963 by the American Physical Society.
1935 - Thin coatings
Katharine Burr Blodgett (1898–1979) extended research of Irving Langmuir (1881–1957) on single-molecule thin film creation on water surface, creating methods for depositing stacks of monomolecular coatings, allowing creation of thin film coatings for glass and metal that are of precisely controlled thickness. In this way she made a coating that make a glass more than 99% transmissive - thanks to destructive interference of reflections from air-coating and coating-glass interface - that had influence on all optical devices, from cinematography, submarine periscopes to optical assemblies in research. She also patented a color gauge for measuring thickness of the thin films.
Image credits: Kathrine Burr Blodgett at General Electric Research Laboratories, Smithsonian Institution Archives, Science Service Records, 1902-1965 (Record Unit 7091). Courtesy of Smithsonian Institution Archives.
1935 - EPR paradox
Einstein, Rosen and Podolsky paradox highlighted one key property of newly developed quantum theory: importance of measurement for establishing reality. Science up to that point has been based on the idea that the world around us has certain properties regardless if it is observed or not. These properties one can call 'real' or as EPR trio put it 'If, without in any way disturbing a system, we can predict with certainty (i.e. with probability equal to unity) the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity'. However, by examining the case of measuring non-commuting variables on two entangled systems, the trio showed that reality of one part of the systems seems to change without direct interaction with that part of the system. This opened a question: can it be that somehow quantum theory can be expanded with some additional variables that determine outcome of the sequential measurement on two parts of the system without requiring this abrupt change in 'reality' of the physical property of the system. Three decades later, J.S. Bell in 1964 ruled out the existence of such local, hidden variable theory, based on an experimentally measurable scheme. This firmly established that the importance of measurement and lack of 'reality' of entangled system properties before such measurements is genuine property of nature.
Image credits: Reprinted excerpt with permission from D. Bohm and Y. Aharonov, Discussion of Experimental Proof for the Paradox of Einstein, Rosen, and Podolsky, Physical Review 108, 1070 (1957). Copyright 1957 by the American Physical Society. It introduced two decades after the original EPR paper a more experimentally accessible version of the original gedanken experiment, this time using polarised photons.
1931 - Prediction of two-photon absorption
Maria Goeppert-Mayer (1906–1972) in her doctoral thesis discussed two-photon (de)excitation. This was experimentally verified only in 1961. when new high intensity laser light sources were invented. This phenomena later gave rise to non-linear microscopy. She got the Nobel Prize in 1963 for her work on a mathematical model that defined the structure of an atom’s nucleus. She worked on unpaid positions in academia for a long time, being granted full-time salaried academic position only when she was 53 years old.
Image credits: Reprinted figure 4-1 with permission from Göppert-Mayer, M. (1931). Über Elementarakte mit zwei Quantensprüngen. Ann. Phys (Leipzig), 9: 273-294. Copyright 1931 by the John Wiley and Sons.
1985 - Chirped pulse amplification
Intensity of pulsed lasers was limited due to onset of self-focusing in amplification of ultra-short pulses. Donna Strickland and Gerard Mourou found solution for this, taking inspiration from microwave radar circuits: they stretched a short pulse through a positively dispersive medium (single mode optical fiber), amplified low-intensity stretched pulse, before compressing it back using a negatively dispersive element, using a double diffraction grating setup. For this discovery that allowed new frontier in research exploiting high-intensity ultra-short pulses, they shared a Nobel Prize in Physics in 2018.
Image credits: Reprinted figure 1. from D. Strickland and G. Mourou, Compression of amplified chirped optical pulses, Optics Communications 56, 219 (1985), with permission from Elsevier. The figure shows setup for streaching, amplifying and compressing the pulse.
1964 - Bell's theorem
Einstein, Podolsky and Rosen argument from 1935 suggested that maybe additional variables should be introduced to quantum mechanics to make a theory where the state of the system at every time will be uniquely determined and dependent only on the local parameters. J. S. Bell (1928-1990) showed that no local, hidden variable theory can give measurement predictions consistent with all quantum-mechanical predictions for two entangled particles. See J. S. Bell, On the Einstein Podolsky Rosen Paradox, Physics 1, 195 (1964).
1976 - Prediction of antibunched light from resonance fluorescence
Solving dynamics for ensemble of driven two-level atoms, one in general obtains that the ensemble dynamics will saturate at some steady state. Yet, if one looks at individual atoms, each time atom decays emitting fluorescence, it will be starting from ground state, at that point incapable of emitting another photon. That atom will then oscillate and reach a steady state. The incapability of single two-level atom to emit second photon results in photon anti-bunching in resonant fluorescence, and the transient dynamics is imprinted on correlations between successive photon emissions, and it it is responsible for side peaks in resonance fluorescence spectrum, and together with the central resonant peak it gives rise to so-called Mollow triplet.
Image credits: Reprinted excerpt with permission from H. J. Carmichael, D. F. Walls, Proposal for the measurement of the resonant Stark effect by photon correlation techniques, J. Phys. B: Atom. Molec. Phys. 9, L43 (1976). Copyright 1979 by the IOP Publishing.
1981 - Proposal for using squeezed light to improve interferometer sensitivity
With discreteness of photons what came also is inevitable counting noise, that in principle can limit sensitivity precise optical measurement schemes. Coherent states, as those produced by single mode lasers, represent the minimum uncertainty states, yet their uncertainty area is equally distributed in amplitude and phase. While area of this uncertainty 'island' cannot be reduced, using optical non-linearities it can be shaped such that it is squeezed for example along the amplitude axis, reducing the amplitude uncertainty, or similarly phase uncertainty if the state is squeezed along the orthogonal axis. Such states were proposed by Carlton Caves to improve sensitivity of interferometers used for gravity wave detection These squeezed states are used precisely in this way in the currently operating gravity interferometers, improving their sensitivity, or equivalently, allowing detection of signals from larger volume of the universe surrounding the Earth
Early discussion of minimum-uncertainty states and their unitary equivalence with coherent states is presented in David Stoler, Equivalence Classes of Minimum Uncertainty Packets, Physical Review D, 1, 3217 (1970).Image credits: Reprinted figure 2. with permission from Carlton M. Caves, Quantum-mechanical noise in an interferometer, Physical Review D 23, 1693 (1981). Copyright 1981 by the American Physical Society.Left panels show electric field versus time for (a) coherent state, and two different squeezed states (b and c). Shading represents uncertainty in the electric field.Right panels show corresponding 'error-boxes' in the complex-amplitude plane
1993 - Theory of cascaded quantum systems
In two individual works, Howard Carmichael and Crispin Gardiner independently presented a theory that accounts for driving quantum systems with non-classical light of quantum sources.
H. J. Carmichael, Quantum trajectory theory for cascaded open systems, Physical Review Letters 70, 2273 (1993) and C. W. Gardiner, Driving a quantum system with the output field from naother driven quantum system, Physical Review Letters 70, 2269 (1993)Image credits: Reprinted figure 1. with permission from H. J. Carmichael, Quantum trajectory theory for cascaded open systems, Physical Review Letters 70, 2273 (1993). Copyright 1993 by the American Physical Society. The figure shows open quantum system B cascaded with a quantum source A.
1815 - Mathematical description of wave theory of light
In the period from 1815 to 1825 Augustin-Jean Fresnel (1788 – 1827), French civil engineer and physicists, establishes mathematical basis for wave theory of light, and realises that light is polarised in transverse direction to the direction of the propagation. Fresnel has numerous contributions including birefringence, diffraction , Fresnel–Arago laws, Fresnel equations, Fresnel integrals, Fresnel lens, Fresnel number, Fresnel rhomb, Fresnel zone, Huygens–Fresnel principle, Phasor representation.
Image credits: p. 773 in Oeuvres complètes d'Augustin Fresnel. Tome 1 / publiées par MM. Henri de Senarmont, Emile Verdet et Léonor Fresnel (1866), showing reprint of work A. Fresnel, Mémoire sur la loi des modifications que la réflexion imprime à la lumière polarisée (1823), showing derivation of Fresnel's sine law for field amplitude reflection coefficient of s-polarised light. Source gallica.bnf.fr / Bibliothèque nationale de France.
1873 - J. C. Maxwell publishes A Treatise on Electricity and Magnetism
Scottish physicists James Clerk Maxwell (1831-1879) provided a unifying description of all observed electric and magnetic phenomena to date in his monumental work 'A treatise on electricity and magnetism'. Crucially, he introduced another source of magnetic field: the electric field change that according to his mathematical argument based on consistency of theory, should have the same effect on production of magnetic field as current. This enabled him to explain light as electromagnetic phenomena, i.e. coupled oscillations of electric and magnetic field.
Image credits: reproduction of p. 233 from James Clerk Maxwell, A treatise on electricity and magnetism, 3rd edition (Oxford: Clarendon Press, 1881) showing Ampère's circuital law, and arguing for Maxwell's addition, namely that change in electric field should be taken as a source of magnetic field just as current is taken as source of magnetic field by Ampère, and highlighting importance of this addition for electromagnetic theory of light. Source: Internet archive, digitalised collection of University of California Libraries
1927 - P. A. M. Dirac, The Quantum Theory of the Emission and Absorption of Radiation
P. A. M. Dirac (1902–1984) introduces quantisation of the electromagnetic field, resolving apparent conflict between corpuscular and wave theories.
Image credits: reproduced from P. A. M. Dirac, The Quantum Theory of the Emissoin and Absorption of Radiation, Proc. R. Soc. Lond. A 114, 243 (1927)
1917 - Einstein's introduction of stimulated emission
Rederiving Planck's blackbody radiation law from radiative equilibrium (along the lines of Boltzmann's derivation of equilibrium distribution from the collision equations), Albert Einstein (1879-1955) finds based on energy balance that there exists process of stimulated emission, and from momentum balance, he finds that photons in stimulated emission must be emitted in the same direction as radiation causing it.
Image credits: Reprinted exceprt from The old Quantum Theory, D. Ter Haar, page 182, translation of Albert Einstein, On the Quantum Theory of Radiation, copyright Pergamon press (1967), originally published as Albert Einstein, Physikalische Zeitschrift 18, 121 (1917). See also derivation from present point of view in R. Friedberg, Einstein and stimulated emission: A completely corpuscular treatment of momentum balance, American Journal of Physics 62, 26 (1993)
1924 - Indistinguishable particles and Bose-Einstein statistics
In 1924, Satyendra Nath Bose (1894 - 1974), Indian mathematician and physicists, rederived Planck's black body spectrum purely from maximisation of entropy, assuming quantum nature of light, and - crucially - fundamental indistinguishability of photons when counting possible micro realizations of the states. Albert Einstein then published in 1925 statistics of ideal (Bose) quantum gas, predicting Bose-Einstein condensation at low temperatures. Although indistinguishability of the particles was implicitly in statistical theory for several decades - see Gibb's paradox and N! factors - this derivation brought clear focus to the assumption that photons are fundamentally indistinguishable, and was met initially with resistance as it seemed to imply '...a mutual influence of the particles on each other of a kind which is at this time still completely mysterious.' as Einstein put it, leading to condensation and reduced pressure under dense, cold conditions of quantum gas. Yet this was real phenomena, and later Bose and Fermi statistics was incorporated in treatment of all many-body quantum problems.
1982 - No-cloning theorem
Formal proof that one cannot construct a protocol to make a copy of an unknown quantum state.Originally created as rebuttal of some of the faster-of-the-light communication proposals no-cloning theorem is underlying element that makes current quantum communication protocols for quantum cryptography safe.W. K. Wootters and W. H. Zureck, A single quantum cannot be cloned, Nature 299, 802 (1982); D. Dieks, Communication by EPR devices, Physics Letters 92A, 271 (1982)
1996 - Quantum error correction
Quantum operations are not perfect - dynamics is continous and every pulse is usually ever so slightly off because of various reasons. As in analog commuincations, these continuous errors accumulate. In classical computers - in particular in communications and memory storage - errors can occur too, even if we discarded slow degradation of analog signals used in the past by switching to discrete states. Yet for these erros, there exists classical error-correcting codes, that spot and fix common errors. Dynamics of quantum operations, being continuous, looks more like analog problem, and poses question if any quantum-correction code can be applied. Even more so: can such error-correciton code be efficient enough, such that it does not kill speed-up promised by quantum algorithms? The first affirmative answer to this was published in Peter W. Shor, Fault-Tolerant Quantum Computation, Proceedings of 37th Conference on Foundations of Computer Science 0272-5428 (1996)
2001 - Is Entanglement Needed for Quantum Computation?
What is the mechanism that allows quantum algorithms to achieve speed-up? Is it purely that the possible state space of few physical qubits is huge? Or do they require also entanglement in the states of the qubits to perform faster? This question risen initially with advancement of NMR quantum computers that could perform high-fidelity single qubit gates, but had a hard time maintaining multi-qubit entanglement. The papers of (i) Noah Linden and Sandu Popescu, Good Dynamics versus Bad Kinematics: Is Entanglement Needed for Quantum Computation?, Physical Review Letters 87, 047901 (2001) and (ii) Richard Jozsa and Noah Linden, On the role of entanglement in quantum-computational speed-up, Proceedings: Mathematical, Physical and Engineering Sciences 459, 2011 (2003) point to conclusion that entanglement is necessary for quantum algorithms to achieve exponential speed-up.
