We have demonstrated experimentally that magnetic-flux quantization in a superconducting circuit is obeyed in an absolute sense, and have ruled out the possibility of a fractional quantum of magnetic flux. We use an optically switched superconducting microbridge to destroy and reestablish the phase coherence of the order parameter around a superconducting circuit. Changes in the vector potential while the global phase coherence is destroyed are still observed when the coherence is reestablished, even though the magnetic field is inacessible to the superconductor.

• Received 28 November 1988

DOI:https://doi.org/10.1103/PhysRevLett.62.2040