Entanglement

Definition:
Entanglement is a uniquely quantum phenomenon where the properties of two or more particles become so deeply correlated that the state of each particle cannot be described independently of the others. Instead, the system as a whole is described by a single joint wavefunction.

Scientific context:
In classical physics, objects can be correlated: knowing something about one gives you information about the other. In quantum mechanics, entangled particles share correlations that are stronger and more fundamental than any classical correlation. Even when the individual properties of each particle (such as spin or position) are uncertain, joint measurements can produce perfectly predictable outcomes due to their entangled nature.

Mathematically, an entangled state is a superposition of multiple quantum particles, such that the full system cannot be written as a product of individual states. Entanglement underlies many key quantum technologies, including quantum teleportation, quantum cryptography, and quantum computing.

Example in practice:

• Two electrons are prepared in a spin-entangled state. Although each electron’s spin is uncertain, if one is measured as “up,” the other will instantly be “down” even if the particles are separated by hundreds of kilometres.

• In quantum key distribution, entangled photons are used to ensure that any eavesdropping attempt is detectable by disturbing the expected correlations.

Media:

Source: Institute of Quantum Computing  – University of Waterloo

Did you know?
Entanglement has been observed over distances greater than 1,000 km, including between a satellite and ground stations. This confirms that quantum correlations can persist across vast separations without loss of integrity.