From Few to Many: Observing the Formation of a Fermi Sea One Atom at a Time

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Science  25 Oct 2013:
Vol. 342, Issue 6157, pp. 457-460
DOI: 10.1126/science.1240516

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Not Very Many

In physics, the behavior of a system sometimes becomes easier to grasp when the number of particles is large and statistics begin to matter, but knowing how large the system needs to be for that to happen is a challenging computational problem. Wenz et al. (p. 457) used a one-dimensional trapped gas of 6Li atoms to study this crossover from few to many. To simplify the problem, they worked with one “impurity” atom that was in a state unlike the other—“majority”—atoms. For weak and intermediate interactions, the system approached the many-body limit with as few as four majority atoms.


Knowing when a physical system has reached sufficient size for its macroscopic properties to be well described by many-body theory is difficult. We investigated the crossover from few- to many-body physics by studying quasi–one-dimensional systems of ultracold atoms consisting of a single impurity interacting with an increasing number of identical fermions. We measured the interaction energy of such a system as a function of the number of majority atoms for different strengths of the interparticle interaction. As we increased the number of majority atoms one by one, we observed fast convergence of the normalized interaction energy toward a many-body limit calculated for a single impurity immersed in a Fermi sea of majority particles.

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