Quantum entanglement is a relationship between quantum systems in which their joint state cannot be described as independent states for each part, producing correlations with no classical equivalent.
Entanglement is a property of a joint quantum state
In classical physics, two objects can be correlated because they share a history or because one influenced the other. Quantum entanglement is stronger: the mathematical state of the whole system contains information that cannot be assigned separately to each component.
When measurements are made on entangled systems, their outcomes can show correlations that violate limits obeyed by local classical models. Experiments testing Bell inequalities have repeatedly demonstrated this distinctly quantum behavior.
What measurement does—and does not—mean
Measuring one member of an entangled pair changes what can be predicted about measurements on the other member. This does not provide a way to send useful information faster than light. Individual measurement outcomes are not controllable messages, and ordinary communication is still needed to compare results.
That distinction matters because popular explanations sometimes describe entanglement as instant communication. The experimentally observed correlations are real, but relativity’s prohibition on faster-than-light signaling remains intact.
How entanglement is created
Particles can become entangled through interactions or through processes that create them in a shared quantum state. Photons, trapped ions, superconducting circuits and other platforms can all be engineered to produce entanglement.
Entanglement is fragile. Interaction with an uncontrolled environment can spread quantum information into many degrees of freedom, a process related to decoherence, making the useful entangled state difficult to preserve.
Why entanglement matters
Entanglement is a resource in quantum information science. Quantum computers use entangled multi-qubit states in many algorithms and error-correction schemes. Quantum networks and cryptographic protocols can also use entanglement to perform tasks that have no direct classical analogue.
It is also a foundational test of what quantum mechanics says about reality, which is why entanglement remains both a practical technology and a subject of fundamental physics.
