Evidence for Efimov quantum states in an ultracold gas of caesium atoms

Abstract

Systems of three interacting particles are notorious for their complex physical behaviour. A landmark theoretical result in few-body quantum physics is Efimov's prediction of a universal set of bound trimer states appearing for three identical bosons with a resonant two-body interaction. Counterintuitively, these states even exist in the absence of a corresponding two-body bound state. Since the formulation of Efimov's problem in the context of nuclear physics 35 years ago, it has attracted great interest in many areas of physics. However, the observation of Efimov quantum states has remained an elusive goal. Here we report the observation of an Efimov resonance in an ultracold gas of caesium atoms. The resonance occurs in the range of large negative two-body scattering lengths, arising from the coupling of three free atoms to an Efimov trimer. Experimentally, we observe its signature as a giant three-body recombination loss when the strength of the two-body interaction is varied. We also detect a minimum in the recombination loss for positive scattering lengths, indicating destructive interference of decay pathways. Our results confirm central theoretical predictions of Efimov physics and represent a starting point with which to explore the universal properties of resonantly interacting few-body systems. While Feshbach resonances have provided the key to control quantum-mechanical interactions on the two-body level, Efimov resonances connect ultracold matter to the world of few-body quantum phenomena.

Potential energy surface of the 12A′ Li2 + Li doublet ground state

“…In these few-body systems, experimental interest in three body effects range from loss rate predictions to probing few-body quantum effects such as Efimov states (Esry and co-workers [6] and Grimm and co-workers [7] for example) due to the strong non-additive effects seen in alkali systems. To date, ab initio calculations for the sodium trimer have been done by several groups [8,9,10] as well as the potassium trimer [8,11,12].…”

Approaching Universality in Weakly Bound Three-Body Systems

“…Cx, Among the most striking results of recent experimentation with supercooled atomic gases is the demonstration that trap loss rates are extraordinarily sensitive to few-body interactions within a trapped many-body system. The experiments of many groups [1] show signatures of few-body correlations within a trapped ensemble of Bose alkali vapors at nano-Kelvin temperatures. This discovery has stimulated a large number of theoretical and experimental investigations, particularly in the unitarity limit of a divergent two-body scattering length, where the system has no natural length scale beyond that of the trapping potential.…”

Ideal n-body correlations with massive particles

“…g (2) (Δz) g (4) (Δz) g (6) (Δz) (Δz) g (5) (Δz) Figure 2 | Many-body correlation functions along the longitudinal direction (temporal dimension on the detector). The data is averaged over the x and y transverse directions using ∼1 cm × 1 cm spatial bins.…”

“…On the other hand, when one of the particles is taken to large separation (that is, z i ∼ > 2 mm in our case), one recovers the two-body correlation function from this plot, along the remaining dimension. The complete fourth-, fifth-, and sixth-order correlation functions, g (4) ( z 1 , z 2 , z 3 ), g (5) ( z 1 , z 2 , z 3 , z 4 ), g (6) ( z 1 , z 2 , z 3 , z 4, , z 5 ) require four-, five-and six-dimensional plots respectively, so for clarity we have plotted these in Fig. 2c-e similarly to g (3) , that is, for equal spatial separations for all particles.…”