EfimovStates 2009-04-15 10:42:33 2009-04-15 10:42:33 Definition stable bound quantum states 3-body state \section{Efimov states} The {\em Efimov states} were initially proposed in 1970 as hypothetical, stable quantum states of coupled three-body systems such as three coupled bosons in ultracold `experiments'. A most interesting feature of such ultracold 3-boson systems was the presence of an infinite number of Efimov states when the boson coupling was weak but finite when the coupling became strong. Subsequently, in 2006, there have been published claims of experiments indicating the presence of such ``Efimov trimers''. References.