One of the most intriguing phenomena in physics is the localization of waves in disordered media 1 . This phenomenon was originally predicted by Anderson, fifty years ago, in the context of transport of electrons in crystals 2 . Anderson localization is actually a much more general phenomenon 3 , and it has been observed in a large variety of systems, including light waves 4,5 . However, it has never been observed directly for matter waves. Ultracold atoms open a new scenario for the study of disorder-induced localization, due to high degree of control of most of the system parameters, including interaction 6 . Here we employ for the first time a noninteracting Bose-Einstein condensate to study Anderson localization. The experiment is performed with a onedimensional quasi-periodic lattice, a system which features a crossover between extended and exponentially localized states as in the case of purely random disorder in higher dimensions. Localization is clearly demonstrated by investigating transport properties, spatial and momentum distributions. We characterize the crossover, finding that the critical disorder strength scales with the tunnelling energy of the atoms in the lattice. Since the interaction in the condensate can be controlled at will, this system might be employed to solve open questions on the interplay of disorder and interaction 7 and to explore exotic quantum phases 8,9 .

In 1970 V. Efimov predicted a puzzling quantummechanical effect that is still of great interest today. He found that three particles subjected to a resonant pairwise interaction can join into an infinite number of loosely bound states even though each particle pair cannot bind. Interestingly, the properties of these aggregates, such as the peculiar geometric scaling of their energy spectrum, are universal, i.e. independent of the microscopic details of their components. Despite an extensive search in many different physical systems, including atoms, molecules and nuclei, the characteristic spectrum of Efimov trimer states still eludes observation. Here we report on the discovery of two bound trimer states of potassium atoms very close to the Efimov scenario, which we reveal by studying three-particle collisions in an ultracold gas. Our observation provides the first evidence of an Efimov spectrum and allows a direct test of its scaling behaviour, shedding new light onto the physics of few-body systems.From nuclei, atoms and molecules up to galaxies, our complex world is made up of many kinds of aggregates whose properties depend on the details of the interactions between their components. This scenario is expected to drastically change as one moves to the world of few neutral quantum particles. The physics of these systems is typically dominated by two-body interactions, which in the limit of vanishing collision energies can be described by a single parameter, namely the s-wave scattering length, independently from the nature of the particles and of the microscopic shape of their interaction 1,2 . If the two-body scattering length becomes resonantly large, the binding of few such particles into larger aggregates is predicted to become universal, in the sense that its properties depend only on the scattering length and few other global parameters 3 .These expectations have been so far verified only for twobody bound states 2 , and even the seemingly simple case of three particles is still under investigation. In this frame, a landmark theoretical result was obtained in 1970 by Efimov 4,5 . He extended previous studies 6 to show that three identical bosons with large two-body scattering length a, even without two-body bound states, can form weakly bound trimer states with size greatly exceeding the characteristic range r 0 of the two-body potential. The binding properties of such states follow a universal behaviour, regardless of the microscopic peculiarities of their components and of their interaction. Efimov indeed identified an effective three-particle interaction potential of the form -(s 0 2 +1/4)/R 2 , where R is the overall size of the three-body system and s 0 1.00624 is a universal parameter 4 . This simple potential is known to support an infinite number of bound states whose energy spectrum exhibits a peculiar geometric scaling where two consecutive states are linked by the relation E n =E n-1 exp(-2/s 0 ). This perfect scaling is predicted to apply only for the special case of a system with infinite...

Transport of fermions is central in many fields of physics. Electron transport runs modern technology, defining states of matter such as superconductors and insulators, and electron spin, rather than charge, is being explored as a new carrier of information [1]. Neutrino transport energizes supernova explosions following the collapse of a dying star [2], and hydrodynamic transport of the quark-gluon plasma governed the expansion of the early Universe [3]. However, our understanding of non-equilibrium dynamics in such strongly interacting fermionic matter is still limited. Ultracold gases of fermionic atoms realize a pristine model for such systems and can be studied in real time with the precision of atomic physics [4,5]. It has been established that even above the superfluid transition such gases flow as an almost perfect fluid with very low viscosity [3,6] when interactions are tuned to a scattering resonance. However, here we show that spin currents, as opposed to mass currents, are maximally damped, and that interactions can be strong enough to reverse spin currents, with opposite spin components reflecting off each other. We determine the spin drag coefficient, the spin diffusivity, and the spin susceptibility, as a function of temperature on resonance and show that they obey universal laws at high temperatures. At low temperatures, the spin diffusivity approaches a minimum value set by /m, the quantum limit of diffusion, where is the reduced Planck's constant and m the atomic mass. For repulsive interactions, our measurements appear to exclude a metastable ferromagnetic state [7][8][9].Understanding the transport of spin, as opposed to the transport of charge, is of high interest for the novel field of spintronics [1]. While charge currents are unaffected by electron-electron scattering due to momentum conservation, spin currents will intrinsically damp due to collisions between opposite spin electrons, as their relative momentum is not conserved. This phenomenon is known as spin drag [10,11]. It is expected to contribute significantly to the damping of spin currents in doped semiconductors [12]. The random collision events also lead to spin diffusion, the tendency for spin currents to flow such as to even out spatial gradients in the spin density, which has been studied in high-temperature superconductors [13] and in liquid 3 He-4 He solutions [14,15]. Creating spin currents poses a major challenge in electronic systems where mobile spins are scattered by their environment and by each other. However, in ultracold atoms we have the freedom to first prepare an essentially non-interacting spin mixture, separate atoms spatially via magnetic field gradients, and only then induce strong interactions. Past observations of spin currents in ultracold Fermi gases [16,17] were made in the weaklyinteracting regime. Here we access the regime near a Feshbach resonance [5], where interactions are as strong as allowed by quantum mechanics (the unitarity limit). We measure spin transport properties, the spin drag coeffici...

We produce a quantum degenerate mixture composed by two Bose-Einstein condensates of different atomic species, 41 K and 87 Rb. We study the dynamics of the superfluid system in an elongated magnetic trap, where off-axis collisions between the two interacting condensates induce scissors-like oscillation.The long-standing interest in mixtures of superfluids, originally focused on helium systems [1], has recently been renewed by the achievement of Bose Einstein condensation (BEC) in dilute atomic gases [2]. Already using a single atomic species, multiple condensates were realized by exploiting the magnetic structure of the ground electronic state of alkali atoms. Mixtures of two hyperfine spin states of 87 Rb in magnetic traps allowed to study the effect of the mutual interaction in the dynamic of miscible BECs [3]. Superposition of spinor condensates of 23 Na in an optical trap led to a first observation of both weakly miscible and immiscible superfluids [4] and of the occurrence of metastable states [5]. These experimental achievements stimulated an extensive theoretical research on the properties of a mixture of two BECs, and the role of the interparticle interaction in determining its static and dynamical properties has been recognized [6][7][8][9][10].As early suggested [6], an even wider scenario for the study of superfluid systems would be opened by BEC in mixtures of different atoms. Considering the species condensed so far, the use of different isotopes of the same species would be restricted to the case of rubidium [11], while a wider choice would be offered the use of different atomic species. Recently, two-species mixtures were successful for the realization of Fermi-Bose degenerate gases [12].In this Letter, we report the realization of a mixture of Bose-Einstein condensates of different atomic species, using potassium and rubidium. Simultaneous condensation of 41 K and 87 Rb is achieved by means of two-species sympathetic cooling [13] in a magnetic trap. The stability against collapse of the degenerate mixture, already * also at

We report on the experimental observation of vortex tangles in an atomic Bose-Einstein condensate (BEC) of ;{87}Rb atoms when an external oscillatory perturbation is introduced in the trap. The vortex tangle configuration is a signature of the presence of a turbulent regime in the cloud. We also show that this turbulent cloud suppresses the aspect ratio inversion typically observed in quantum degenerate bosonic gases during free expansion. Instead, the cloud expands keeping the ratio between their axis constant. Turbulence in atomic superfluids may constitute an alternative system to investigate decay mechanisms as well as to test fundamental theoretical aspects in this field.

A degenerate gas of identical fermions is brought to collapse by the interaction with a Bose-Einstein condensate. We used an atomic mixture of fermionic potassium-40 and bosonic rubidium-87, in which the strong interspecies attraction leads to an instability above a critical number of particles. The observed phenomenon suggests a direction for manipulating fermion-fermion interactions on the route to superfluidity.

We employ radio-frequency spectroscopy to investigate a polarized spin-mixture of ultracold 6 Li atoms close to a broad Feshbach scattering resonance. Focusing on the regime of strong repulsive interactions, we observe well-defined coherent quasiparticles even for unitarity-limited interactions. We characterize the many-body system by extracting the key properties of repulsive Fermi polarons: the energy E+, the effective mass m * , the residue Z and the decay rate Γ. Above a critical interaction, E+ is found to exceed the Fermi energy of the bath while m * diverges and even turns negative, thereby indicating that the repulsive Fermi liquid state becomes energetically and thermodynamically unstable.Landau's idea of mapping the behavior of impurity particles interacting with a complex environment into quasiparticle properties [1] plays a fundamental role in physics and materials science, from helium liquids [2] and colossal magnetoresistive materials [3,4] to polymers and proteins [5,6]. In the field of ultracold gases, the impurity problem and the associated concept of polaron quasiparticle have attracted over the last decade a growing interest [7][8][9][10]. Initiated with the investigation of polarized Fermi gases in the BEC-BCS crossover [11][12][13][14][15][16], the study of polaron physics has been extended to mass-imbalanced [17,18], low-dimensional fermionic systems [19], and also to bosonic environments [20][21][22]. The polaron properties are fundamentally relevant for understanding the more complex scenario of partially-polarized and balanced Fermi mixtures: the impurity limit exhibits some of the critical points of the full phase diagram, whose topology we can thus learn about by investigating polarized systems [8,16].While researchers initially focused on attractive interactions [14,15], more recently they have explored novel quasiparticles associated with repulsive interactions: these repulsive polarons [23][24][25][26][27] are centrally important for realizing repulsive many-body states [23,24,28,29] and therein exploring itinerant ferromagnetism [30][31][32]. In particular, if the polaron energy exceeds the Fermi energy of the surrounding medium, a fullyferromagnetic phase is favored against the paramagnetic Fermi liquid [23][24][25]27]. However, short-ranged strong repulsion always require an underlying weakly-bound molecular state, into which the system may rapidly decay [31,33], making the repulsive polaron an excited manybody state, whose theoretical and experimental investigation are challenging. In three dimensions, repulsive Fermi polarons have been first unveiled in a 6 Li -40 K mixture at a comparatively narrow Feshbach resonance [17], but they lack observation in the universal, broad * scazza@lens.unifi.it resonance case, for which the decay rate is expected to be the largest [10].In this Letter we report on reverse radio-frequency (RF) spectroscopy [17,34,35] experiments to unveil the existence and characterize the properties of repulsive polarons in a polarized Fermi mixture of lithium ...

We report on the Bose-Einstein condensation of potassium atoms, whereby quantum degeneracy is achieved by sympathetic cooling with evaporatively cooled rubidium. Because of the rapid thermalization of the two different atoms, the efficiency of the cooling process is high. The ability to achieve condensation by sympathetic cooling with a different species may provide a route to the production of degenerate systems with a larger choice of components.