Geometric scaling of Efimov states in a ⁶textrm{Li}-¹³³textrm{Cs} mixture

In few-body physics, Efimov states are an infinite series of three-body bound states that obey universal discrete scaling symmetry when pairwise interactions are resonantly enhanced. Despite abundant reports of Efimov states in recent cold atom experiments, direct observation of the discrete scaling symmetry remains an elusive goal. Here we report the observation of three consecutive Efimov resonances in a heteronuclear Li-Cs mixture near a broad interspecies Feshbach resonance. The positions of the resonances closely follow a geometric series $1$, $\lambda$, $\lambda^2$. The observed scaling constant $\lambda_{\rm exp} = 4.9(4)$ is in good agreement with the predicted value of 4.88.