Quantum entanglement is a phenomenon where two particles become linked so that measuring one instantly determines the state of the other, regardless of distance. Einstein famously called it "spooky action at a distance."
How it works
When two particles are entangled, they share a quantum state. This means the properties of one particle are fundamentally connected to the properties of the other. Before measurement, each particle exists in a superposition of all possible states.
The key insight is that entangled particles don't carry hidden instructions — the outcome is genuinely undetermined until measurement, yet perfectly correlated between the pair.
Why it matters
Quantum entanglement is the foundation of several emerging technologies:
- **Quantum computing** — entangled qubits enable parallel computation across exponentially many states
- **Quantum cryptography** — any attempt to intercept entangled particles disturbs their state, making eavesdropping detectable
- **Quantum teleportation** — the state of one particle can be transferred to another using entanglement and classical communication
Common misconceptions
Entanglement does not allow faster-than-light communication. While the correlation is instantaneous, you cannot use it to send a message — the measurement results appear random to each observer independently. Only when they compare notes (via classical communication) do the correlations become apparent.
The phenomenon has been experimentally verified thousands of times, most decisively in the 2022 Nobel Prize-winning experiments by Aspect, Clauser, and Zeilinger, which closed the remaining loopholes in Bell test experiments.