Quanta at the double

Two papers published in this issue of Nature show that the propagation of energy quanta in very different physical systems can exhibit the same, unusual dynamics, where bound pairs of quanta become dominant. Ofer Firstenberg et al. realize coherent interactions between individual photons — quanta of light — which are massless and do not usually interact. They achieve this using a quantum nonlinear medium inside which individual photons pair up and travel as massive particles with strong mutual attraction. Potential applications of this technique include all-optical switching, deterministic photonic quantum logic and the generation of strongly correlated states of light. The second paper deals with magnons, the quanta that carry energy in magnets. More than eighty years ago, Hans Bethe predicted the existence of bound states of elementary spin waves (magnons) in one-dimensional quantum magnets. Experimental observation of the phenomenon remained elusive, but now Takeshi Fukuhara et al. have observed two-magnon bound states in a system of ultracold bosonic atoms in an optical lattice. The results provide a new way of studying fundamental properties of quantum magnets. In an accompanying News and Views, Sougato Bose puts these two independent findings into a general context of quantum many-body dynamics.