Superconducting Quantum Interference Devices (specifically, dc-SQUIDs) are the world’s most sensitive sensors for magnetic flux. What lies at the heart of this quantum interference technology is the Josephson effect, which is a striking example of a macroscopic quantum phenomenon . When two macroscopically coherent quantum systems such as superconductors are weakly coupled together, through a thin insulating layer for example, a direct current can appear across the junction with no applied voltage. This is known as the dc Josephson effect. As if that were not strange enough, if one tries to put a constant voltage across the junction, the direct current disappears and counterintuitively an alternating current now appears. This is known as the ac Josephson effect. By setting up a loop of a quantum system interrupted by a pair of such junctions, we can exploit these phenomena for interferometry. Magnetic flux acting on the electrons through a superconducting loop causes a phase shift between electric currents that flow across the two junctions and this interference alters the amplitude of the overall oscillation. As a result, dc-SQUIDs find application as ultrasensitive magnetometers .To read more, click here.