Aharonov-Bohm: fringe shift from a flux line

What you are seeing: a solenoid (orange dot) sits between the two paths of a double-slit interferometer. Even though $\mathbf B = 0$ outside the solenoid, the electron wavefunction picks up a phase $\delta \phi = e \Phi / \hbar$ around the closed loop, shifting the fringes. The shift is exactly one fringe per flux quantum $\Phi_0 = h/e$.