The RF-SQUID flux qubit is described by U(φ) = E_L(φ−2πf)²/2 − E_J·cos(φ), where φ is the loop flux and f=Φ_ext/Φ₀. At f=0.5, the potential is symmetric with a double well; tunneling through the barrier creates the qubit states |0⟩ and |1⟩ with an avoided crossing (tunnel splitting Δ).