Quantum entanglement

Quantum entanglement is a property of two or more quantum systems whose combined state cannot be written as a product of independent states for each part. Measurements on entangled systems remain correlated even when the systems are separated by large distances, in ways that no classical description of the individual parts can reproduce. The correlations are nonlocal, yet they cannot be used to send information faster than light.

#The EPR paper and its challenge

The concept was sharpened in 1935 by Albert Einstein, Boris Podolsky, and Nathan Rosen, whose paper asked whether the quantum-mechanical description of reality is complete (Einstein, Podolsky, and Rosen 1935). They considered two particles prepared so that measuring one seems to fix a property of the other instantly, and argued that either quantum mechanics is incomplete or nature permits what Einstein later called "spooky action at a distance." Erwin Schrodinger, responding the same year, introduced the term entanglement and called it the characteristic trait of quantum mechanics. For decades the EPR argument was treated as a philosophical dispute rather than a testable question.

#Correlations without a classical explanation

Consider a pair of particles prepared in an entangled state of their Spin such that the total spin is zero. Neither particle has a definite spin direction on its own; the pair is in a Superposition of joint possibilities. When one particle is measured along some axis and found to point up, the other is instantly found to point down along that axis, and this holds for whichever axis the experimenters choose. Classically one might explain such matching by assuming each particle carried a hidden instruction set from the start. Bell's theorem showed in 1964 that no such local hidden-variable account can reproduce all the quantum predictions (Bell 1964), turning the interpretive debate into an experiment.

#No faster-than-light signaling

The correlations are nonlocal, but they do not carry usable information across the gap. An experimenter looking only at one particle sees random outcomes regardless of what is done to the distant partner; the correlation is only visible after the two sets of results are brought together and compared, which requires a classical channel no faster than light. This no-signaling property is why entanglement is consistent with special relativity (Stanford Encyclopedia of Philosophy). Entanglement is a resource that must be shared in advance and is consumed when used.

#Experimental confirmation

Beginning in the 1970s, experiments tested the Bell inequalities directly. John Clauser and Stuart Freedman reported an early violation in 1972, and Alain Aspect's group in the early 1980s closed important loopholes by switching measurement settings while the particles were in flight. Anton Zeilinger's group extended entanglement experiments over long distances and to multi-particle states. The three shared the 2022 Nobel Prize in Physics for these tests of Bell inequalities and pioneering quantum information science (Nobel Foundation 2022). Loophole-free experiments reported in 2015 closed the remaining gaps simultaneously.

#How entangled states arise

Entanglement is produced whenever two systems interact and are then separated without fully recording the result, or when a single process emits correlated particles. A common laboratory source is spontaneous parametric down-conversion, in which a laser photon passing through a suitable crystal splits into two lower-energy photons whose polarizations are entangled. Trapped ions and superconducting circuits are entangled by controlled interactions in a quantum processor, and the decay of certain particles yields entangled products. Entanglement is fragile: contact with an uncontrolled environment couples the systems to their surroundings and degrades the correlation, the process of decoherence, which is why maintaining it over distance or time is a central experimental challenge.

Entanglement can also be quantified and is subject to strict rules. It cannot be increased by acting on the parts separately with only classical communication between them, and it obeys a monogamy constraint: a system that is maximally entangled with one partner cannot at the same time be entangled with a third. These features are what make entanglement a well-defined physical resource rather than a vague notion of correlation.

#Applications

Entanglement is a central resource in quantum information. It is the mechanism behind quantum teleportation, which transfers an unknown quantum state using a shared entangled pair and classical communication. Entangling operations between qubits are essential to quantum computing, where algorithms such as Shor's algorithm depend on correlations that classical machines cannot cheaply mimic. Entanglement also underlies device-independent forms of quantum key distribution, whose security can be tied to observed Bell violations.

#Significance

Entanglement is the feature of quantum mechanics that most clearly departs from a classical picture of separable, locally defined objects. It sits at the center of the measurement problem and of ongoing work on the foundations of physics, while also serving as a practical tool in the technologies of quantum communication and computing.