This article is part of a special series celebrating the 100th anniversary of the birth of quantum theory. Read more here
The seeds of quantum theory were sown by Albert Einstein and others as early as 1905. But the theory came together properly 100 years ago in 1925 鈥� and has exerted its influence ever since, as this timeline shows.
> 1905 Riffing on earlier work by Max Planck, Albert Einstein suggests that light is made of particles with certain energies. These 鈥渜uanta of light鈥� were an early step on the road to quantum theory.
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> 1913 Niels Bohr produces a quantum description of the atom in which electrons can only exist in certain orbits with fixed energies.
> 1919 Physicist Hendrika Johanna van Leeuwen writes a thesis proposing that magnetism is also a quantum mechanical phenomenon.
> 1925 On the windswept island of Helgoland, Werner Heisenberg carries out a calculation that treats the electron鈥檚 characteristics not as single values, but as tables of values. In this, his supervisor, Max Born, spots a key truth of quantum mechanics (see 鈥淐arlo Rovelli on what we get wrong about the origins of quantum theory鈥�).
> 1926 Erwin Schr枚dinger develops an alternative quantum framework that paints electrons as waves using a mathematical construct called the wave function.
> 1935 Schr枚dinger devises a thought experiment in which a cat in a closed box may be considered both alive and dead while it is unobserved. Einstein, Nathan Rosen and Boris Podolsky write a paper on quantum entanglement, which links two particles even when separated by vast distances. They argue that entanglement implies quantum mechanics is incomplete.
>1938 Using ideas from quantum theory, Lise Meitner and Otto Hahn discover nuclear fission, the process that would undergird the development of nuclear power 鈥� and nuclear bombs.
> 1950 Julian Schwinger, Richard Feynman, Freeman Dyson and Shinichiro Tomonaga develop the modern form of quantum electrodynamics, explaining how light and matter interact. It forms the basis of modern particle physics.
> 1957 Hugh Everett introduces an idea that later becomes known as the many-worlds interpretation. It suggests that all possible outcomes of a quantum process are real across multiple parallel universes.
> 1961 Eugene Wigner proposes a more involved version of the 1935 thought experiment Schr枚dinger鈥檚 cat. Known as Wigner鈥檚 friend, it shows up the weirdness of quantum theory in new ways.
> 1964 John Stewart Bell produces an expression that defines whether the behaviour of entangled particles can be explained by the 鈥渉idden variables鈥� Einstein wanted. His own and later experiments show they can鈥檛, and quantum theory really is as strange as it seems.
> 1994 Carlo Rovelli and Lee Smolin publish a founding paper on loop quantum gravity, one of several frameworks that attempt to describe space-time itself as quantised 鈥� or made of infinitesimally tiny 鈥済rains鈥�.
> 1998 The first experimental quantum computer is reported 鈥� it has just two quantum bits.
> 2016 China launches the Micius satellite, designed to distribute quantum encryption keys and so enable long-distance communication that is, in principle, unhackable.
> 2019 Google uses a quantum computer with 53 qubits to claim it has reached 鈥渜uantum supremacy鈥� 鈥� that is, solving a computational problem that no classical computer can feasibly solve. (Advanced non-quantum computers have since pulled off the same feat.)
> 2023 Start-up Atom Computing unveils the first quantum computer with more than 1000 qubits.
