Two quantum skyrmions form a magnon Josephson junction

Two quantum skyrmions in a frustrated spin-$1\mathrm{/}2$ magnet bind into a molecule, and the molecule acts as a magnon Josephson junction: the relative helicity $\theta$ and the number $n$ of magnons transferred between them obey $H=E_C{n}^2-W\cos \theta$. DMRG on the $J_1$–$J_2$ triangular lattice gives $E_C$ as the curvature of the skyrmion energy at its natural size of seven flipped spins, and the measured helicity coherence fixes $W\simeq 0.02J_1$, half the skyrmion–antiskyrmion tunnel splitting and independent of the anisotropy. Two helicity qubits side by side are thus strongly coupled by their own interaction.