Observation of Dirac Monopoles in a Synthetic Magnetic Field

摘要

Magnetic monopoles --- particles that behave as isolated north or southmagnetic poles --- have been the subject of speculation since the firstdetailed observations of magnetism several hundred years ago. Numeroustheoretical investigations and hitherto unsuccessful experimental searches havefollowed Dirac's 1931 development of a theory of monopoles consistent with bothquantum mechanics and the gauge invariance of the electromagnetic field. Theexistence of even a single Dirac magnetic monopole would have far-reachingphysical consequences, most famously explaining the quantization of electriccharge. Although analogues of magnetic monopoles have been found in exoticspin-ices and other systems, there has been no direct experimental observationof Dirac monopoles within a medium described by a quantum field, such assuperfluid helium-3. Here we demonstrate the controlled creation of Diracmonopoles in the synthetic magnetic field produced by a spinor Bose-Einsteincondensate. Monopoles are identified, in both experiments and matchingnumerical simulations, at the termini of vortex lines within the condensate. Bydirectly imaging such a vortex line, the presence of a monopole may bediscerned from the experimental data alone. These real-space images provideconclusive and long-awaited experimental evidence of the existence of Diracmonopoles. Our result provides an unprecedented opportunity to observe andmanipulate these quantum-mechanical entities in a controlled environment.