Bell state

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In quantum information science, the Bell's states or EPR pairs are specific quantum states of two qubits that represent the simplest examples of quantum entanglement.^[1]: 25 The Bell's states are a form of entangled and normalized basis vectors. This normalization implies that the overall probability of the particles being in one of the mentioned states is 1: ${\displaystyle \langle \Phi |\Phi \rangle =1}$. Entanglement is a basis-independent result of superposition.^[2] Due to this superposition, measurement of the qubit will "collapse" it into one of its basis states with a given probability.^[1] Because of the entanglement, measurement of one qubit will "collapse" the other qubit to a state whose measurement will yield one of two possible values, where the value depends on which Bell's state the two qubits are in initially. Bell's states can be generalized to certain quantum states of multi-qubit systems, such as the GHZ state for three or more subsystems.

Understanding of Bell's states is useful in analysis of quantum communication, such as superdense coding and quantum teleportation.^[3] These mechanisms cannot transmit information faster than the speed of light, a result known as the no-communication theorem.^[4]: 100