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Quantum entanglement
1935· Idea
ProvenTested many times; part of working physics.
Two or more particles can share a single quantum state, so measuring one instantly tells you about the other, however far apart.
See it on the Science map →How it works
Entangled particles have no individual properties of their own, only joint ones. Two photons can be prepared so their polarizations always match, though each alone is perfectly random. Measure one in Paris and the matching result in Tokyo is fixed. The correlations are stronger than any pre-arranged 'instruction sheet' could produce, as Bell tests prove. Yet entanglement cannot be used to send messages.
Why it matters
It is the most distinctly quantum phenomenon, a resource for quantum computing and encryption, and a challenge to the idea that distant objects are independent.
Think of it like
Like co-tenants on a joint-and-several lease: there is one obligation, not two, and neither tenant's position can be fully described alone.
How we know
Bell tests since 1972, loophole-free in 2015 (Delft, Vienna, Boulder). China's Micius satellite distributed entangled photons over 1,200 km in 2017. Nobel Prize 2022 to Aspect, Clauser and Zeilinger.
Still a mystery
What entanglement means for space and reality is debated; some theorists speculate space itself emerges from entanglement, an untested idea.
Connections (8)
▸Superposition: many states at once shared between particles becomes Quantum entanglementProven
The relationship
When two particles interact, their joint state can become a superposition of combinations, such as 'first up, second down' plus 'first down, second up'. Neither particle then has a state of its own; only the pair does. That shared superposition is entanglement, and its correlations persist after the particles separate.
How the link was found
Schrödinger named it 'Verschränkung', entanglement, in 1935, calling it the characteristic trait of quantum mechanics.
Why it's proven
Created and measured directly in labs since the 1970s with photons, atoms, ions and superconducting circuits; Bell tests confirm the correlations are genuinely quantum.
Open questions and mysteries
How entanglement fades into ordinary correlation as systems grow, and its possible role in building space itself, are active research.
▸Quantum entanglement with the environment causes Decoherence: why big things look normalProven
The relationship
When a quantum system bumps into air molecules or light, it becomes entangled with them, sharing its superposition with countless environmental particles. Since nobody can track all of them, the system on its own no longer shows interference. Decoherence is entanglement with the environment, seen from the system's side.
How the link was found
H. Dieter Zeh (1970) and Wojciech Zurek (1981–1982) recognized that entanglement with the environment explains the loss of quantum behavior.
Why it's proven
Experiments that control environmental contact, such as gas pressure around interfering molecules (Vienna, 2003), show interference fading as predicted.
Open questions and mysteries
Is decoherence enough to explain the classical world entirely? Zurek's 'quantum Darwinism' proposes how shared records arise, but it is still being tested.
▸The EPR paradox: "spooky action at a distance" first described Quantum entanglementProven
The relationship
EPR's thought experiment used two particles whose properties were linked after interacting, so measuring one fixed the other. That linkage is entanglement. EPR meant it as evidence the theory was incomplete; instead it identified the theory's most distinctive and useful feature.
How the link was found
Einstein, Podolsky and Rosen in May 1935; Schrödinger coined the word 'entanglement' months later in response.
Why it's proven
The correlations EPR described have been produced and measured directly since the 1970s, exactly as quantum mechanics predicts.
Open questions and mysteries
Whether entanglement reveals non-local connections or the absence of definite properties before measurement is still debated.
▸Bell tests: nature really is non-local (Nobel 2022) confirmed Quantum entanglementProven
The relationship
Bell tests confirmed that entangled particles share correlations no pre-arranged plan could explain. That moved entanglement from a debated theoretical oddity to a confirmed physical fact, and then to a working tool in quantum encryption, teleportation of quantum states and quantum computers.
How the link was found
Clauser in 1972, Aspect in 1982, Zeilinger from the 1990s, sharing the 2022 Nobel Prize. Loophole-free tests in 2015 by Hensen, Giustina, Shalm and colleagues.
Why it's proven
Consistent results across decades, technologies and continents, including entanglement distributed by the Micius satellite over 1,200 km in 2017.
Open questions and mysteries
The tests confirm entanglement but do not say what mechanism, if any, underlies the correlations.
▸Quantum entanglement is still bound by No faster-than-light messagesProven
The relationship
Measuring one entangled particle reveals the correlation, but each side's results look completely random on their own. Only by comparing results through an ordinary channel does the pattern appear. Since no choice on one side changes the odds seen on the other, entanglement carries no usable information faster than light.
How the link was found
Philippe Eberhard (1978) and Ghirardi, Rimini and Weber (1980) proved this 'no-communication theorem' from quantum theory.
Why it's proven
It is a mathematical theorem within quantum mechanics, and every experiment agrees, including teleportation demonstrations, which always require a normal signal.
Open questions and mysteries
Why nature permits correlations stronger than classical but not strong enough to signal is a puzzle; Popescu and Rohrlich showed in 1994 that stronger ones would not break relativity.
▸Quantum entanglement powers Quantum computingWell supported
The relationship
Entangled qubits cannot be described one at a time; their joint state spans combinations whose number doubles with each added qubit. Algorithms exploit these correlations for problems that would overwhelm ordinary computers. Entanglement is also needed for quantum error correction, which spreads one qubit's information across many.
How the link was found
Feynman (1981) and Deutsch (1985) conceived quantum computing; Peter Shor (1995) and Andrew Steane (1996) showed entanglement-based error correction was possible.
Is it proven?
Computations with too little entanglement can be efficiently simulated on ordinary computers, so for standard quantum circuits it is proven necessary for speedups. Labs routinely entangle dozens to hundreds of qubits.
Open questions and mysteries
Entanglement is necessary but not sufficient; exactly what gives quantum computers their power is still being studied.
▸ER = EPR: entanglement may build space ties wormholes to Quantum entanglementSpeculative
The relationship
ER = EPR claims entanglement has a geometric side. Two entangled black holes are linked by a real wormhole; for ordinary entangled particles, the link is a faint quantum version. This would explain why entanglement cannot send signals: the wormholes involved pinch off before anything can cross.
How the link was found
Juan Maldacena and Leonard Susskind, 2013, building on Einstein's 1935 papers on wormholes and on entanglement.
Is it proven?
Entanglement itself is proven, with a 2022 Nobel Prize for experiments. Its link to wormholes holds only in model calculations; untested.
Open questions and mysteries
If entanglement builds geometry, does every entangled pair in a lab share a tiny wormhole? No one knows how to test that.
▸Quantum entanglement may be the thread that weaves Space and time may be emergentSpeculative
The relationship
In holographic models, how strongly two regions of the boundary are entangled sets how close the matching regions of space are. Reduce the entanglement and the space between them stretches and eventually splits. So entanglement acts like stitching: the pattern of quantum links may decide which places count as near.
How the link was found
Mark Van Raamsdonk, 2010. Brian Swingle (2009-12) linked it to tensor networks, and Almheiri, Dong and Harlow (2015) showed space behaves like a quantum error-correcting code.
Is it proven?
Holds in mathematical models. Entanglement itself is proven; its role in building space has no experimental test yet.
Open questions and mysteries
Whether real space in our universe is woven this way, and what plays the role of the boundary, are unknown.