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What is quantum entanglement?
Entanglement means several qubits share a joint quantum state, so their measurement results are correlated even when the qubits are far apart.
The state of entangled qubits cannot be written qubit by qubit; it can only be described as a whole. The best-known example is the Bell state: measure the two qubits and both give 0 or both give 1, yet which one is not decided until the measurement.
Entanglement does not allow sending information faster than light; results look random on each side and the correlation only appears when the results are compared. Even so, it underpins quantum teleportation, quantum key distribution and quantum error correction.
In a circuit, entanglement is usually created with a Hadamard gate followed by a CNOT gate.
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Related concepts
- Bell state · A Bell state is one of four special states in which two qubits are entangled in the simplest and strongest way; the best known gives only 00 or 11 when measured.
- CNOT gate · The CNOT (controlled-NOT) gate is a two-qubit gate that flips the target qubit when the control qubit is 1; it is the basic tool for creating entanglement.
- Quantum teleportation · Quantum teleportation is a protocol that transfers the unknown state of a qubit to another qubit using a shared entangled pair and two classical bits.