DC SQUID ($I_c(\Phi )=2I_0|\cos (\pi \Phi /{\Phi }_0)|$)

A SQUID with two Josephson junctions in a superconducting loop. For equal junctions, negligible self-inductance and no intrinsic phase offset, adding their sinusoidal currents gives the displayed flux-dependent critical current. An rf SQUID instead has one junction and uses loop inductance.

For an ideal symmetric two-junction interferometer with negligible self-flux, the stable zero-current phase is zero or pi according to the sign of the flux cosine. Linearization about that stable phase gives positive inductance. The stronger small-current condition is $|I|\ll 2I_C|\cos (\pi \Phi /{\Phi }_0)|$, and fails near a half-integer flux quantum where the ideal critical current vanishes.

The 1964 report established quantum interference in superconducting tunneling. It is an experimental counterpart to the phase-sensitive current predicted for coupled superconductors.^[ref]