A mixture of Bose and Fermi superfluids

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Science  29 Aug 2014:
Vol. 345, Issue 6200, pp. 1035-1038
DOI: 10.1126/science.1255380

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Making a superfluid lithium mixture

At some of the coldest temperatures achieved in the laboratory, researchers can coax dilute gases of atoms into becoming a superfluid, with the whole gas behaving as one entity. Bosonic atoms, which like to congregate in one state, achieve this willingly. Fermions, which effectively repel each other, require more persuasion. Ferrier-Barbut et al. made a superfluid mixture of two gases, one made up of bosons and one of fermions. They used two isotopes of lithium, fermionic 6Li and bosonic 7Li. When they made the mixture oscillate, the two components took turns feeding energy into each other.

Science, this issue p. 1035


Superconductivity and superfluidity of fermionic and bosonic systems are remarkable many-body quantum phenomena. In liquid helium and dilute gases, Bose and Fermi superfluidity has been observed separately, but producing a mixture in which both the fermionic and the bosonic components are superfluid is challenging. Here we report on the observation of such a mixture with dilute gases of two lithium isotopes, lithium-6 and lithium-7. We probe the collective dynamics of this system by exciting center-of-mass oscillations that exhibit extremely low damping below a certain critical velocity. Using high-precision spectroscopy of these modes, we observe coherent energy exchange and measure the coupling between the two superfluids. Our observations can be captured theoretically using a sum-rule approach that we interpret in terms of two coupled oscillators.

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