What it means for two qubits to share a single quantum state, and why this is the resource most quantum algorithms depend on.
Two entangled qubits are described by a single quantum state, not two separate ones. Measure one, and the other instantly has a definite value too — even if it's on the other side of the lab, or the other side of the planet. This correlation is stronger than anything classical physics allows, and it's been verified experimentally since the 1970s.
The phrase "spooky action at a distance" comes from Einstein, who spent decades arguing entanglement meant quantum mechanics was incomplete. He was wrong. John Bell's 1964 paper provided a way to test the question, and every experiment since — most conclusively the 2015 loophole-free Bell tests — has confirmed that entanglement is real and not just hidden classical variables.
The simplest entangled state is the Bell state: two qubits whose joint state is (|00⟩ + |11⟩) / √2. If you measure the first qubit and get 0, the second is guaranteed to be 0. If you get 1, the second is guaranteed to be 1. The probabilities are 50/50, but the outcomes are perfectly correlated.
Creating a Bell state requires a two-qubit gate, typically a CNOT (controlled-NOT). After applying a CNOT to two qubits in superposition, the qubits are entangled. From then on, manipulating one qubit's state changes what the other qubit will measure to — not because of a physical signal, but because the joint state was never two states to begin with.
Classical correlations work too. If you put a red marble and a blue marble in two boxes and send one to a friend, opening your box tells you what your friend has. The difference: the marbles had definite colors the whole time. Entangled qubits do not have definite values until measured. Bell test experiments rule out the "hidden values" explanation by showing that the correlations exceed what any classical strategy could produce.
Most quantum speedups depend on entanglement, not just superposition. Shor's algorithm creates a deeply entangled state across many qubits during the modular exponentiation step. Without entanglement, the speedup disappears. This is why classical computers can simulate small quantum systems with modest overhead — up to about 50 qubits, the entanglement is limited enough to track. Beyond that, classical simulation becomes exponentially expensive.
Quantum teleportation transfers an unknown qubit state from Alice to Bob using one shared entangled pair and two classical bits of communication. The original qubit is destroyed in the process. The "teleportation" name comes from the fact that the state appears at Bob's location without traveling through the intervening space. It's the basis of quantum repeaters and would-be quantum internet protocols.
BB84 and E91 (the entanglement-based version) use entangled photon pairs to detect eavesdropping. Any attempt to intercept the quantum channel disturbs the entanglement, which Alice and Bob can detect by comparing a subset of their measurements. The result is a cryptographic key whose security rests on physics rather than computational hardness.
The leading error correction schemes, like the surface code, encode a logical qubit across a 2D grid of physical qubits using entanglement. The logical qubit's information is distributed across many physical qubits in such a way that single-qubit errors can be detected and corrected without disturbing the encoded state.
Photons have been entangled over more than 1,200 km, demonstrated by China's Micius satellite in 2017. The entanglement survives space travel because the photons travel through vacuum with little decoherence. The record for fiber-based entanglement is around 100 km without a quantum repeater — beyond that, photon loss in the fiber destroys the entanglement before it can be used.
For solid-state qubits (superconducting circuits, trapped ions), entanglement is usually demonstrated within a single chip or trap. Distributing entanglement between separate quantum processors is an active research area. Companies like IBM and PsiQuantum are betting on modular architectures where multiple small quantum chips are linked by optical interconnects.