" The principle underlying a SQUID is that variations in a magnetic field alter the phase difference between the quantum waves on either side of its two junctions, and therefore change the supercurrents tunneling through them. Just as ripples on a pond can either add up when they collide (if a crest meets a crest) or cancel each other out (if a crest meets a trough), the quantum waves in the two arms of a SQUID interfere in a way that depends sensitively on their phases, and hence on the amount of magnetic flux passing through the loop. In this way, a SQUID transforms tiny variations in magnetic flux into measurable changes in current and voltage across the device, allowing ultrafaint electromagnetic signals to be detected and quantified. "

― Steven H. Strogatz , Sync: The Emerging Science of Spontaneous Order