Efimov states are a remarkable quantum phenomenon in which three particles can form a stable bound state—even though none of the two-particle combinations are bound. This counterintuitive effect arises purely from quantum mechanical interactions and was first predicted by Russian physicist Vitaly Efimov in 1970.
In classical physics or even in standard quantum scenarios, if two particles cannot bind together, it would seem impossible for three to do so. However, under certain conditions—typically involving resonant interactions and ultra-low temperatures—quantum mechanics allows the formation of a loosely bound three-body system. These Efimov states have very large spatial extent and extremely weak binding energies, and they exhibit a universal scaling pattern: each successive Efimov state is larger and more weakly bound than the previous one by a constant factor.
Efimov states have been observed in ultracold atomic gases, where experimentalists can fine-tune interactions using Feshbach resonances. Their discovery deepens our understanding of few-body quantum systems and plays a significant role in nuclear physics, atomic physics, and the study of universality in quantum mechanics.