We established superfluidity in a two-state mixture of ultracold fermionic atoms with imbalanced state populations. This study relates to the long-standing debate about the nature of the superfluid state in Fermi systems. Indicators for superfluidity were condensates of fermion pairs and vortices in rotating clouds. For strong interactions, near a Feshbach resonance, superfluidity was observed for a broad range of population imbalances. We mapped out the superfluid regime as a function of interaction strength and population imbalance and characterized the quantum phase transition to the normal state, known as the Pauli limit of superfluidity.

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We have observed Fermi polarons, dressed spin down impurities in a spin up Fermi sea of ultracold atoms. The polaron manifests itself as a narrow peak in the impurities' rf spectrum that emerges from a broad incoherent background. We determine the polaron energy and the quasiparticle residue for various interaction strengths around a Feshbach resonance. At a critical interaction, we observe the transition from polaronic to molecular binding. Here, the imbalanced Fermi liquid undergoes a phase transition into a Bose liquid coexisting with a Fermi sea.

The pairing of fermions lies at the heart of superconductivity and superfluidity. The stability of these pairs determines the robustness of the superfluid state, and the quest for superconductors with high critical temperature equates to a search for systems with strong pairing mechanisms. Ultracold atomic Fermi gases present a highly controllable model system for studying strongly interacting fermions. Tunable interactions (through Feshbach collisional resonances) and the control of population or mass imbalance among the spin components provide unique opportunities to investigate the stability of pairing-and possibly to search for exotic forms of superfluidity. A major controversy has surrounded the stability of superfluidity against an imbalance between the two spin components when the fermions interact resonantly (that is, at unitarity). Here we present the phase diagram of a spin-polarized Fermi gas of (6)Li atoms at unitarity, experimentally mapping out the superfluid phases versus temperature and density imbalance. Using tomographic techniques, we reveal spatial discontinuities in the spin polarization; this is the signature of a first-order superfluid-to-normal phase transition, and disappears at a tricritical point where the nature of the phase transition changes from first-order to second-order. At zero temperature, there is a quantum phase transition from a fully paired superfluid to a partially polarized normal gas. These observations and the implementation of an in situ ideal gas thermometer provide quantitative tests of theoretical calculations on the stability of resonant superfluidity.

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Bose-Einstein condensates of sodium atoms, prepared in an optical dipole trap, were distilled into a second empty dipole trap adjacent to the first one. The distillation was driven by thermal atoms spilling over the potential barrier separating the two wells and then forming a new condensate. This process serves as a model system for metastability in condensates, provides a test for quantum kinetic theories of condensate formation, and also represents a novel technique for creating or replenishing condensates in new locations.PACS numbers: 03.75.Lm, 64.60.My The characteristic feature of Bose-Einstein condensation is the accumulation of a macroscopic number of particles in the lowest quantum state. Condensate fragmentation, the macroscopic occupation of two or more quantum states, is usually prevented by interactions [1], but may happen in spinor condensates [2,3]. However, multiple condensates may exist in metastable situations. Let's assume that an equilibrium condensate has formed in one quantum state, but now we modify the system allowing for one even lower state. How does the original condensate realize that it is in the wrong state and eventually migrate to the true ground state of the system? What determines the time scale for this equilibration process? This is the situation which we experimentally explore in this paper using a double-well potential.The process we study is relevant for at least four different questions.(1) The description of the formation of the condensate is a current theoretical frontier and requires finite-temperature quantum kinetic theories. There are still discrepancies between theoretical predictions and experimental results [4,5]. Our double-well system has the advantage of being an almost closed system (little evaporation) with well defined initial conditions and widely adjustable time scales (through the height of the barrier). (2) Spinor condensates show rich ground states and collective excitations due to the multi-component order parameter [2]. Several groups have observed longlived metastable configurations [6,7,8,9] and speculated about transport of atoms from one domain to another via the thermal cloud [6,8]. The double-well potential allows us to characterize such distillation processes in their simplest realization. (3) The incoherent transport observed here in a double well-potential imposes stringent limitations on future experiments aiming at the observation of coherent transport in Josephson junctions [10,11,12]. (4) Our observation of condensate growth in one potential well due to the addition of thermal atoms realizes the key ideas of proposals on how to achieve a continuous atom laser [13] which is different from the experiment where condensates were replenished with transported condensates [14]. The whole system has equilibrated. V denotes the height of the potential barrier between the two wells, which is measured with respect to the bottom of the left well, and ∆U the trap depth difference between the two wells.The scheme of the experiment is shown in Fig....

Phase transitions are dramatic phenomena: water freezes into ice, atomic spins spontaneously align in a magnet, and liquid helium becomes superfluid. Sometimes, such a drastic change in behaviour is accompanied by a visible change in appearance. The hallmark of Bose-Einstein condensation and superfluidity in trapped, weakly interacting Bose gases is the sudden formation of a dense central core inside a thermal cloud. However, in strongly interacting gases--such as the recently observed fermionic superfluids--there is no longer a clear separation between the superfluid and the normal parts of the cloud. The detection of fermion pair condensates has required magnetic field sweeps into the weakly interacting regime, and the quantitative description of these sweeps presents a major theoretical challenge. Here we report the direct observation of the superfluid phase transition in a strongly interacting gas of 6Li fermions, through sudden changes in the shape of the clouds--in complete analogy to the case of weakly interacting Bose gases. By preparing unequal mixtures of the two spin components involved in the pairing, we greatly enhance the contrast between the superfluid core and the normal component. Furthermore, the distribution of non-interacting excess atoms serves as a direct and reliable thermometer. Even in the normal state, strong interactions significantly deform the density profile of the majority spin component. We show that it is these interactions that drive the normal-to-superfluid transition at the critical population imbalance of 70 +/- 5 per cent (ref. 12).

Precise understanding of strongly interacting fermions, from electrons in modern materials to nuclear matter, presents a major goal in modern physics. However, the theoretical description of interacting Fermi systems is usually plagued by the intricate quantum statistics at play. Here we present a cross-validation between a new theoretical approach, Bold Diagrammatic Monte Carlo (BDMC), and precision experiments on ultra-cold atoms. Specifically, we compute and measure with unprecedented accuracy the normal-state equation of state of the unitary gas, a prototypical example of a strongly correlated fermionic system. Excellent agreement demonstrates that a series of Feynman diagrams can be controllably resummed in a non-perturbative regime using BDMC. This opens the door to the solution of some of the most challenging problems across many areas of physics.