Researchers have made an exotic quantum object called an Alice ring, which changes the properties of other quantum objects when they pass through it – or when they are simply viewed through it.
Researchers manipulated thousands of extremely cold atoms to make a ring-like defect that can change the properties of quantum objects that pass through it.
Scientists from Aalto University in Finland and Amherst College in Massachusetts created a bizarre quantum object known as an ‘Alice ring.’ An homage to Carroll’s titular character in Alice in Wonderland, the name is an apt one. This decayed monopole—a particle with only one magnetic pole—opens a “vortex ring” that flips the magnetic charge of any other monopole passing through its center, creating an “anti-monopole.”
Symmetry-breaking phase transitions are ubiquitous in physics1, appearing in contexts as diverse as the cooling of the early universe, the emergence of ferromagnetism, and the onset of superconductivity. As a phase transition proceeds, uncorrelated domains of the new phase grow and assume preferred field configurations where they meet. Topological defects appear where no uniform configuration can unite the domains, adopting the form of surfaces (walls), lines (strings and vortices), and singular points (monopoles). Strings and monopoles carry conserved topological charges which, depending on the physical properties of the system, can manifest as magnetic, electric, or even quark colour charges2.
The Alice string3,4 is unusual among topological defects, appearing in certain grand unified theories2,5,6 as an element that converts a monopole into an anti-monopole as it travels around the string7. This fascinating “looking-glass” property, which gives the excitation its name8, has a direct counterpart in condensed matter systems9,10,11, where Alice strings have been identified with half-quantum vortices in superfluids12,13 and π-disclinations in nematic liquid crystals11,14.
Alice strings are intimately tied to monopoles in a second surprising way. At the monopole singularity itself, the field has no single well-defined configuration and is therefore required to vanish. The energy cost associated with the recovery of the field determines the characteristic size of the region over which the system heals to the phase that supports the defect. However, within such depleted singular regions another phase with a different symmetry may be present, and consequently the system energy can be reduced by adopting more exotic topological configurations. A monopole can thus become energetically unstable against a deformation into a closed loop of Alice string, i.e., an Alice ring, which preserves the topology of the monopole field far from its core. Such monopole core deformations have been predicted in nematic liquid crystals15,16,17,18, in ’t Hooft–Polyakov monopoles19,20 within the field theory of Alice electrodynamics21,22, and more recently in spinor Bose–Einstein condensates (BECs)23,24.
In this work, we present experimental evidence of an Alice ring in a spin-1 Bose–Einstein condensate. The Alice ring appears during the time evolution of a topological monopole defect25 in the polar magnetic phase of the BEC, where it takes the form of a vortex ring filled with superfluid in the ferromagnetic phase23 (Fig. 1). Experimental images, in good agreement with numerical simulations, reveal that after 5 ms of evolution the initial monopole decays into an extended spin structure consistent with the analytical expectation for an Alice ring . Interestingly, both the experiment and the numerical simulations reveal that an initially off-centered monopole defect evolves into a tilted Alice ring , dramatically underscoring its presence by enhancing the visibility of its ferromagnetic core.