Library of human understanding
Quantum Computing
Computing with the same rules that govern atoms.
In 1981 Richard Feynman pointed out that simulating even a modest molecule exactly on an ordinary computer is hopeless, because each added particle doubles the work. His suggestion was disarmingly simple: if nature computes with quantum rules, build the computer from quantum parts.
How quantum computing works
A qubit is a point on the Bloch sphere. Before measurement, it exists in a superposition of both |0⟩ and |1⟩ states simultaneously.
Quantum gates rotate the qubit on the Bloch sphere
Entanglement: qubits connected in superposition
Measuring Q1 instantly affects Q2, even if separated. Einstein called this “spooky action at a distance.”
Interference: paths combine and cancel
The qubit
The qubit · 1995
Gates and circuits
Two qubits, starting at |00⟩. Add gates, then measure. Try H on qubit 1, then CNOT, and measure a thousand times: only 00 and 11 appear, about half each, and never 01 or 10. That is an entangled pair.
circuit: (empty)
- |00⟩+1.000
- |01⟩+0.000
- |10⟩+0.000
- |11⟩+0.000
The CNOT gate · —
CNOT|a, b⟩ = |a, a ⊕ b⟩
Entanglement as a resource
An entangled pair (a Bell state) · 1935 / 1964
Algorithms are choreography of interference
Why it matters, and what it will not do
Error correction: the real bottleneck
The threshold theorem · 1996–1998
physical error rate p < p_threshold ⇒ logical errors fall as the code grows
Interactive Bloch sphere
Drag the point to see how superposition states collapse to |0⟩ or |1⟩ upon measurement.
Superposition is a core quantum principle: a quantum system can exist in multiple states simultaneously until measured. A qubit on the Bloch sphere occupies a point in superposition; measurement collapses it to |0⟩ or |1⟩. The double-slit experiment shows wave-particle duality: without detection, particles behave like waves and interfere with themselves; with detection determining which slit they pass through, they behave like particles with no interference. Superposition isn’t the system “trying all answers”—it’s a coherent state where all possibilities coexist as a single quantum state until observation.
Four approaches to quantum hardware
No one yet knows which will scale best. Each trades speed, accuracy and size differently.
Superconducting
Strength: Fast, scalable
Challenge: Needs extreme cold
Trapped ion
Strength: High fidelity, long coherence
Challenge: Slower gates
Photonic
Strength: Room temperature, integrated photonics
Challenge: Photon loss
Topological
Strength: Error resistant by design
Challenge: Early stage, unproven at scale
The people
From the first suggestion in 1980 to today’s experiments. Several of them, such as Feynman, Zurek and the Bell-test experimenters, also appear in the Quantum Mechanics section.
- Richard Feynman1918 New York – 1988 Los Angeles · United StatesReformulated quantum theory with sums over histories and pointed toward quantum computers.
- Wojciech Zurekborn 1951, Poland · Poland, working in the United StatesDeveloped the decoherence programme and co-proved the no-cloning theorem.
- John Clauser, Alain Aspect and Anton ZeilingerClauser b. 1942, United States · Aspect b. 1947, France · Zeilinger b. 1945, Austria · United States, France and AustriaTested Bell's inequality and showed the quantum world is not locally real; Nobel Prize in Physics 2022.
- Yuri Manin1937 Simferopol – 2023 Bonn · Soviet Union, later GermanyRaised the idea of quantum computing in 1980, a year before Feynman's talk.
- Paul Benioff1930 Pasadena – 2022 · United StatesShowed that a computer can be described entirely by quantum mechanics.
- David Deutschborn 1953, Haifa · Israel, working in the United KingdomDefined the universal quantum computer and the first quantum algorithms.
- Charles Bennett and Gilles BrassardBennett b. 1943, New York · Brassard b. 1955, Montréal · United States and CanadaInvented quantum cryptography and quantum teleportation.
- Artur Ekertborn 1961, Wrocław · Poland, working in the United Kingdom and SingaporeBased quantum cryptography on entanglement and Bell's theorem.
- Benjamin SchumacherUnited States · United StatesCoined the word 'qubit' and founded quantum information theory.
- Peter Shorborn 1959, New York · United StatesShowed that a quantum computer could break widely used encryption, and that quantum errors can be corrected.
- Lov Groverborn 1961, India · India, working in the United StatesFound the quantum search algorithm.
- Andrew Steaneactive from the 1990s · United KingdomCo-discovered quantum error-correcting codes.
- Alexei Kitaevborn 1963, Moscow · Soviet Union, working in the United StatesProposed topological quantum computing and the toric code.
- John Preskillborn 1953, United States · United StatesNamed the NISQ era and the idea of quantum supremacy; champion of fault tolerance.
- Ignacio Cirac and Peter ZollerCirac b. 1965, Manresa · Zoller b. 1952, Innsbruck · Spain and AustriaProposed the trapped-ion quantum computer in 1995.
- David Winelandborn 1944, Wauwatosa, Wisconsin · United StatesTrapped and controlled single ions; first ion-based logic gate.
- Yasunobu Nakamuraborn 1968, Japan · JapanBuilt the first superconducting qubit.
- John Clarke, Michel Devoret and John MartinisClarke b. 1942, Cambridge · Devoret b. 1953, Paris · Martinis b. 1958, United States · United Kingdom, France and the United StatesShowed quantum behaviour in a circuit big enough to hold in the hand; Nobel Prize in Physics 2025.
- Emanuel Knill, Raymond Laflamme and Gerard Milburnactive from the 1990s · working in the United States, Canada and AustraliaShowed that ordinary optics and photon detectors suffice for quantum computing.
- Jian-Wei Panborn 1970, China · ChinaLed experiments in quantum teleportation, satellite quantum communication and photonic quantum advantage.
- Mikhail Lukinactive from the 1990s · Russia, working in the United StatesA leader of neutral-atom quantum computing and quantum simulation.
Timeline
- 1980
Manin and Benioff each describe quantum mechanical computation. Paul Benioff
- 1981
Feynman asks for a quantum computer to simulate nature (paper 1982). Richard Feynman
- 1984
Bennett and Brassard publish BB84 quantum key distribution. Charles Bennett
- 1985
Deutsch defines the universal quantum computer. David Deutsch
- 1991
Ekert's entanglement-based cryptography. Artur Ekert
- 1993
Quantum teleportation protocol. Charles Bennett
- 1994
Shor's factoring algorithm. Peter Shor
- 1995
Schumacher names the qubit; first quantum error-correcting code; Cirac–Zoller ion proposal and first ion gate. Benjamin Schumacher
- 1996
Grover's search algorithm; Steane and Calderbank–Shor codes. Lov Grover
- 1997
Kitaev's toric code and topological quantum computation. Alexei Kitaev
- 1999
First superconducting qubit. Yasunobu Nakamura
- 2001
KLM linear-optics scheme; Shor's algorithm factors 15 on a 7-qubit device. Emanuel Knill, Raymond Laflamme
- 2012
Wineland shares the Nobel Prize; Preskill names 'quantum supremacy'. David Wineland
- 2019
Google's Sycamore supremacy claim, contested by classical-simulation rebuttals. John Clarke, Michel Devoret
- 2023
Neutral-atom logical-qubit processor; Turing Award to Bennett and Brassard (announced 2024). Mikhail Lukin
- 2024
Below-threshold surface code on Google's Willow chip; NIST publishes first post-quantum encryption standards. John Clarke, Michel Devoret
- 2025
Clarke, Devoret and Martinis receive the Nobel Prize in Physics. John Clarke, Michel Devoret