Byers–Yang theorem

In quantum mechanics, the Byers–Yang theorem states that all physical properties of a doubly connected system (an annulus) enclosing a magnetic flux ${\displaystyle \Phi }$ through the opening are periodic in the flux with period ${\displaystyle \Phi _{0}=hc/e}$ (the magnetic flux quantum). The theorem was first stated and proven by Nina Byers and Chen-Ning Yang (1961), and further developed by Felix Bloch (1970).