We have produced a macroscopic quantum system in which a 6Li Fermi sea coexists with a large and stable 23Na Bose-Einstein condensate. This was accomplished using interspecies sympathetic cooling of fermionic 6Li in a thermal bath of bosonic 23Na. The system features rapid thermalization and long lifetimes.

Radio-frequency techniques were used to study ultracold fermions. We observed the absence of mean-field "clock" shifts, the dominant source of systematic error in current atomic clocks based on bosonic atoms. This absence is a direct consequence of fermionic antisymmetry. Resonance shifts proportional to interaction strengths were observed in a three-level system. However, in the strongly interacting regime, these shifts became very small, reflecting the quantum unitarity limit and many-body effects. This insight into an interacting Fermi gas is relevant for the quest to observe superfluidity in this system.

We report on the generation of a quantum degenerate Fermi-Fermi mixture of two different atomic species. The quantum degenerate mixture is realized employing sympathetic cooling of fermionic 6 Li and 40 K gases by an evaporatively cooled bosonic 87 Rb gas. We describe the combination of trapping and cooling methods that proved crucial to successfully cool the mixture. In particular, we study the last part of the cooling process and show that the efficiency of sympathetic cooling of the 6 Li gas by 87 Rb is increased by the presence of 40 K through catalytic cooling. Due to the differing physical properties of the two components, the quantum degenerate 6 Li-40 K Fermi-Fermi mixture is an excellent candidate for a stable, heteronuclear system allowing to study several so far unexplored types of quantum matter. [3], which allowed to study the crossover regime between a molecular Bose-Einstein condensate (BEC) and a BardeenCooper-Schrieffer (BCS) like gas of paired fermions [4]. Current research aims at simulating correlated manybody quantum systems with ultracold gases. A particularly intriguing goal is the realization of a fermionic quantum gas with two different atomic species, which is a well controllable system and is predicted to be stable [5]. Due to the mass difference, it offers a variety of analogies to other many-body systems, in particular to a spatially inhomogeneous superfluid phase predicted to occur in certain types of high temperature superconductors [6]. Further, a transition to a cristalline phase in the bulk gas [7] and the possibility to simulate baryonic phases of QCD [8] have been theoretically proposed. Moreover, the mixture bears the prospect to create heteronuclear ground state molecules [9], in this way realizing a quantum gas with a particularly large dipolar interaction [10]. Finally, a two-species mixture offers the additional possibility to tune interactions and to conveniently apply componentselective methods. The main result reported in this letter is the first production of such a quantum degenerate twospecies Fermi-Fermi mixture opening the door to aforementioned unexplored types of quantum matter. This goal was attained by achieving efficient sympathetic cooling of fermionic 6 Li and 40 K by an evaporatively cooled bosonic 87 Rb gas. Moreover, we have also realized the first triple quantum degenerate mixture (see fig.1), and therefore will be able to compare quantum properties of Fermi-Fermi and Bose-Fermi mixtures directly.The basic idea of our experimental strategy is to sym- pathetically cool the fermions by a large rubidium cloud. In this way, the atom numbers of the fermions are in principle not reduced by evaporation and the initial fermion clouds can be loaded with reduced experimental effort. However, the challenge is to combine the different constraints which the individual atomic species enforce on the set of trapping and cooling parameters. Especially, arXiv:0710.2779v1 [cond-mat.other]