Quantum entanglement occurs when two or more qubits become correlated such that the quantum state of each qubit cannot be described independently of the others — even when separated by large distances. When qubits are entangled, measuring one immediately determines the state of the other. For example, in a Bell state |Φ+⟩ = (|00⟩ + |11⟩)/√2, measuring the first qubit as 0 instantly guarantees the second will also be 0. Entanglement is created using two-qubit gates like CNOT. It is a key resource for quantum computing, quantum teleportation, and quantum cryptography. Entanglement does not allow faster-than-light communication because the measurement outcomes are random and only correlated when compared classically.
Related Terms
Bell State
FundamentalsOne of four maximally entangled two-qubit states — the simplest example of quantum entanglement.
CNOT Gate
GatesControlled-NOT — a two-qubit gate that flips the target qubit when the control qubit is |1⟩.
Quantum Gate
GatesA unitary operation that transforms the state of one or more qubits.
Measurement
FundamentalsThe act of observing a qubit's state, which collapses the superposition to a definite 0 or 1.