Attractive photons in a quantum nonlinear medium

By coupling light to strongly interacting atomic Rydberg states in a dispersive regime, it is possible to induce individual photons to travel as massive particles with strong mutual attraction, such that the propagation of photon pairs is dominated by a two-photon bound state. 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. The fundamental properties of light derive from its constituent particles—massless quanta (photons) that do not interact with one another 1 . However, it has long been known that the realization of coherent interactions between individual photons, akin to those associated with conventional massive particles, could enable a wide variety of novel scientific and engineering applications 2 , 3 . Here we demonstrate a quantum nonlinear medium inside which individual photons travel as massive particles with strong mutual attraction, such that the propagation of photon pairs is dominated by a two-photon bound state 4 , 5 , 6 , 7 . We achieve this through dispersive coupling of light to strongly interacting atoms in highly excited Rydberg states. We measure the dynamical evolution of the two-photon wavefunction using time-resolved quantum state tomography, and demonstrate