The Josephson effect can be calculated using the laws of quantum mechanics. A diagram of a single Josephson junction is shown at right. Assume that superconductor A has Ginzburg–Landau order parameter ψ ψ A = n A e i ϕ ϕ A , and superconductor B ψ ψ B = n B e i ϕ ϕ B , which can be interpreted as the wave functions of Cooper pairs in the two superconductors. If the electric potential difference across the junction is V , then the energy difference between the two superconductors is 2 e V , since each Cooper pair has twice the charge of one electron. The Schrödinger equation for this two-state quantum system is therefore: