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What is a 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.
The best-known Bell state is (|00⟩+|11⟩)/√2. It is prepared with an H gate followed by a CNOT. Measurement gives 00 with 50% probability and 11 with 50%; 01 and 10 never appear.
Bell states are building blocks of quantum teleportation, superdense coding and entanglement-based quantum key distribution. They are also usually the first circuit people write when they start quantum programming.
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Related concepts
- Quantum entanglement · Entanglement means several qubits share a joint quantum state, so their measurement results are correlated even when the qubits are far apart.
- 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.