Universal spin wavepacket transport in van der Waals antiferromagnets

Antiferromagnets (AFMs) are promising platforms for transduction and transmission of quantum information via magnons (the quanta of spin waves), offering advantages over ferromagnets with regard to dissipation, speed of response, and immunity to external fields. Recently transduction was shown in a van der Waals (vdW) AFM, where strong spin-exciton coupling enables readout of the amplitude and phase of coherent magnons by photons of visible light. This demonstration shifts the focus of research to transmission, specifically to understanding the nonlocal interactions that enable magnon wavepackets to propagate. Here we show, through theory and experiment, that propagation is mediated by the long-range dipole-dipole interaction. This coupling is an inevitable consequence of fundamental electrodynamics, and as such, will likely mediate the propagation of spin at long wavelengths in the entire class of vdW magnets currently under intense investigation. Successfully identifying the mechanism of spin propagation provides a set of optimization rules, as well as caveats, that are essential for any future applications of these promising systems.