Experimental Upper Limit on the Magnetic Monopole Moment of Electrons, Protons, and Neutrons, Utilizing a Superconducting Quantum Interferometer

A superconducting quantum interferometer (magnetic flux sensitivity {~}10^-17 Wb) is shown to be capable of directly detecting magnetic charge, in particular Dirac's quantized magnetic monopoles (magneticflux=h/e=4×10^-15 Wb). In samples free of Dirac monopoles, divB was determined to be less than 1.2×{}10^-14 Wb/kg, while the magnetic charge of the neutron and of the proton (relative to the electron) were found to be less than 10^-26 Dirac monopole per nucleon. The magnetic charge of the electron itself was measured to be less than 4×{}10^-24 Dirac monopole.