Evidence of Dirac Monopoles in a Spin-1 Bose-Einstein Condensate

Isolated magnetic poles (monopoles) have not yet been observed, although there are good theoretical reasons for thinking that they may exist --- and profound implications if they do. The first successful theoretical description of a magnetic monopole consistent with quantum mechanics was formulated by Dirac [1], but may be applied more generally to quantum-mechanical systems in the presence of gauge potentials. We describe the successful experimental creation of Dirac monopoles in a \emph{synthetic} magnetic field in the context of a dilute-gas Bose-Einstein condensate. The existence of a monopole is inferred from direct observations of a vortex line that terminates inside the condensate, which evidence is supported by excellent agreement between experiment and numerical simulations. [1] P.A.M. Dirac, Proc. R. Soc. Lond. A \textbf{133}, 60 (1932).