The quantum halo state of the helium trimer

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Science  01 May 2015:
Vol. 348, Issue 6234, pp. 498-499
DOI: 10.1126/science.aaa9102

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The systems of two interacting particles are often simple enough to appear in textbooks. The situation is drastically different when three interacting particles are involved. General solutions for “the three-body problem” do not yet exist in classical or in quantum domains. A step forward was made in 1970 by Vitaly Efimov, who established that when two identical bosons are either very weakly bound or just unbound, an infinite series of three-body bound states emerges (1). The forces are sufficient to bind the trimer even when they are too weak to hold the particles in pairs, very much like Borromean rings. Efimov's original proposal concerned nuclear physics, but the helium trimer 4He3 was soon suggested as the most likely candidate for experiments (2). Now, after 38 years, Kunitski et al. (3) report on page 551 of this issue the detection of these very fragile clusters and image their structure.

  • * Max Born Institute, 12489 Berlin, Germany.