What is quantum entanglement?

When particles such as photons or electrons interact in certain ways, they can become entangled, meaning their quantum states are correlated and described by a single shared wavefunction. If you measure a property like spin on one entangled particle, the corresponding property of its partner is instantly fixed, even if the two are separated by vast distances. Albert Einstein famously called this spooky action at a distance and doubted it, but decades of experiments have confirmed entanglement is real. Importantly, it does not allow faster-than-light communication, because the individual results appear random until the two parties compare notes through ordinary, slower-than-light channels. Entanglement is a cornerstone of emerging technologies: it underpins quantum computing, quantum cryptography, and quantum teleportation of information.