Half-solitons in a polariton quantum fluid behave like magnetic monopoles

Magnetic monopoles ¹ are point-like sources of magnetic field, never observed as fundamental particles. This has triggered the search for monopole analogues in the form of emergent particles in the solid state, with recent observations in spin-ice crystals ^2-4 and one-dimensional ferromagnetic nanowires ⁵ . Alternatively, topological excitations of spinor Bose-Einstein condensates have been predicted to demonstrate monopole textures ^6-8 . Here we show the formation of monopole analogues in an exciton-polariton spinor condensate hitting a defect ^9,10 potential in a semiconductor microcavity. Oblique dark solitons are nucleated in the wake of the defect in the presence of an effective magnetic field acting on the polariton pseudo-spin ¹¹ . The field splits the integer soliton into a pair of oblique half-solitons ¹² of opposite magnetic charge, subject to opposite effective magnetic forces. These mixed spin-phase excitations thus behave like one-dimensional monopoles ¹³ . Our results open the way to the generation of stable magnetic currents in photonic quantum fluids.