A 2-qubit Bell state, also known as an EPR (Einstein-Podolsky-Rosen) pair, is one of the four specific maximally entangled quantum states of two qubits. These states are named after John Bell, who formulated Bell's theorem to test the principles of quantum mechanics.
The four Bell states are:
|\Phi^+\rangle = \frac{1}{\sqrt{2}} (|00\rangle + |11\rangle)
|\Phi^-\rangle = \frac{1}{\sqrt{2}} (|00\rangle - |11\rangle)
|\Psi^+\rangle = \frac{1}{\sqrt{2}} (|01\rangle + |10\rangle)
|\Psi^-\rangle = \frac{1}{\sqrt{2}} (|01\rangle - |10\rangle)These states are used extensively in quantum information theory and quantum computing because of their entanglement properties.