Observation of the Efimov state of the helium trimer

Understand

Quantum theory dictates that upon weakening the two-body interaction in a three-body system, an infinite number of three-body bound states of a huge spatial extent emerge just before these three-body states become unbound.

• Three helium atoms have been predicted to form a molecular system that manifests this peculiarity under natural conditions without artificial tuning of the attraction between particles by an external field.
• Here we report experimental observation of this long predicted but experimentally elusive Efimov state of $^{4}$He$_{3}$ by means of Coulomb explosion imaging.
• We show spatial images of an Efimov state, confirming the predicted size and a typical structure where two atoms are close to each other while the third is far away.