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 Φ {\displaystyle \Phi } through the opening are periodic in the flux with period Φ 0 = h c / e {\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).