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