We report the production of matter-wave solitons in an ultracold lithium-7 gas. The effective interaction between atoms in a Bose-Einstein condensate is tuned with a Feshbach resonance from repulsive to attractive before release in a one-dimensional optical waveguide. Propagation of the soliton without dispersion over a macroscopic distance of 1.1 millimeter is observed. A simple theoretical model explains the stability region of the soliton. These matter-wave solitons open possibilities for future applications in coherent atom optics, atom interferometry, and atom transport.

We report Bose-Einstein condensation of weakly bound 6 Li2 molecules in a crossed optical trap near a Feshbach resonance. We measure a molecule-molecule scattering length of 170 +100−60 nm at 770 G, in good agreement with theory. We study the 2D expansion of the cloud and show deviation from hydrodynamic behavior in the BEC-BCS crossover region.PACS numbers: 03.75. Ss, 05.30.Fk, 32.80.Pj, By applying a magnetic field to a gas of ultra-cold atoms, it is possible to tune the strength and the sign of the effective interaction between particles. This phenomenon, known as Feshbach resonance, offers in the case of fermions the unique possibility to study the crossover between situations governed by Bose-Einstein and FermiDirac statistics. Indeed, when the scattering length a characterizing the 2-body interaction at low temperature is positive, the atoms are known to pair in a bound molecular state. When the temperature is low enough, these bosonic dimers can form a Bose-Einstein condensate (BEC) as observed very recently in 40 K [1] and 6 Li [2,3]. On the side of the resonance where a is negative, one expects the well known Bardeen-Cooper-Schrieffer (BCS) model for superconductivity to be valid. However, this simple picture of a BEC phase on one side of the resonance and a BCS phase on the other is valid only for small atom density n. When n|a| 3 > ∼ 1 the system enters a strongly interacting regime that represents a challenge for many-body theories [4,5,6] and that now begins to be accessible to experiments [7,8,9].In this letter, we report on Bose-Einstein condensation of 6 Li dimers in a crossed optical dipole trap and a study of the BEC-BCS crossover region. Unlike all previous observations of molecular BEC made in single beam dipole traps with very elongated geometries, our condensates are formed in nearly isotropic traps. Analyzing free expansions of pure condensates with up to 4×10 4 molecules, we measure the molecule-molecule scattering length a m = 170 +100 −60 nm at a magnetic field of 770 gauss. This measurement is in good agreement with the value deduced from the resonance position [9] and the relation a m = 0.6 a of ref. [10]. Combined with tight confinement, these large scattering lengths lead to a regime of strong interactions where the chemical potential µ is on the order of k B T C where T C ≃ 1.5 µK is the condensation temperature. As a consequence, we find an important modification of the thermal cloud time of flight expansion induced by the large condensate mean field. Moreover, the gas parameter n m a 3 m is no longer small but on the order of 0.3. In this regime, the validity of mean field theory becomes questionable [11,12,13]. We show, in particular, that the anisotropy and gas energy released during expansion varies monotonically across the Feshbach resonance.Our experimental setup has been described previously [14,15]. A gas of 6 Li atoms is prepared in the absolute ground state |1/2, 1/2 in a Nd-YAG crossed beam optical dipole trap. The horizontal beam (resp. vertical) propagates along x (y)...

We report the observation of coexisting Bose-Einstein condensate (BEC) and Fermi gas in a magnetic trap. With a very small fraction of thermal atoms, the 7Li condensate is quasipure and in thermal contact with a 6Li Fermi gas. The lowest common temperature is 0.28 microK approximately 0.2(1)T(C) = 0.2(1)T(F) where T(C) is the BEC critical temperature and T(F) the Fermi temperature. The 7Li condensate has a one-dimensional character.

Cavity quantum electrodynamics (cavity QED) describes the coherent interaction between matter and an electromagnetic field confined within a resonator structure, and is providing a useful platform for developing concepts in quantum information processing. By using high-quality resonators, a strong coupling regime can be reached experimentally in which atoms coherently exchange a photon with a single light-field mode many times before dissipation sets in. This has led to fundamental studies with both microwave and optical resonators. To meet the challenges posed by quantum state engineering and quantum information processing, recent experiments have focused on laser cooling and trapping of atoms inside an optical cavity. However, the tremendous degree of control over atomic gases achieved with Bose-Einstein condensation has so far not been used for cavity QED. Here we achieve the strong coupling of a Bose-Einstein condensate to the quantized field of an ultrahigh-finesse optical cavity and present a measurement of its eigenenergy spectrum. This is a conceptually new regime of cavity QED, in which all atoms occupy a single mode of a matter-wave field and couple identically to the light field, sharing a single excitation. This opens possibilities ranging from quantum communication to a wealth of new phenomena that can be expected in the many-body physics of quantum gases with cavity-mediated interactions.

We investigate the strongly interacting regime in an optically trapped 6 Li Fermi mixture near a Feshbach resonance. The resonance is found at 800(40) G in good agreement with theory. Anisotropic expansion of the gas is interpreted by collisional hydrodynamics. We observe an unexpected and large shift (80 G) between the resonance peak and both the maximum of atom loss and the change of sign of the interaction energy.

We create weakly bound Li2 molecules from a degenerate two component Fermi gas by sweeping a magnetic field across a Feshbach resonance. The atom-molecule transfer efficiency can reach 85% and is studied as a function of magnetic field and initial temperature. The bosonic molecules remain trapped for 0.5 s and their temperature is within a factor of 2 from the Bose-Einstein condensation temperature. A thermodynamical model reproduces qualitatively the experimental findings.

We report the observation of three p-wave Feshbach resonances of 6 Li atoms in the lowest hyperfine state f = 1/2. The positions of the resonances are in good agreement with theory. We study the lifetime of the cloud in the vicinity of the Feshbach resonances and show that depending on the spin states, 2-or 3-body mechanisms are at play. In the case of dipolar losses, we observe a non-trivial temperature dependence that is well explained by a simple model. PACS numbers: 03.75.Ss, 05.30.Fk, 32.80.Pj, In the presence of a magnetic field, it is possible to obtain a quasi-degeneracy between the relative energy of two colliding atoms and that of a weakly bound molecular state. This effect, known as a Feshbach resonance, is usually associated with the divergence of the scattering length and is the key ingredient that led to the recent observation of superfluids from fermion atom pairs of 6 Li [1, 2, 3, 4] and 40 K [5]. Up to now these pairs were formed in s-wave channels but it is known from condensed matter physics that fermionic superfluidity can arise through higher angular momentum pairing: p-wave Cooper pairs have been observed in 3 He [6] and d-wave in high-T c superconductivity [7]. Although Feshbach resonances involving p or higher partial waves have been found in cold atom systems [8,9,10], p-wave atom pairs have never been directly observed.In this paper we report the observation of three narrow p-wave Feshbach resonances of 6 Li in the lowest hyperfine state f = 1/2. We measure the position of the resonance as well as the lifetime of the atomic sample for allWe show that the position of the resonances are in good agreement with theory. In the case of atoms polarized in the ground state (1/2, 1/2), the atom losses are due to 3-body processes. We show that the temperature dependence of the losses at resonance cannot be described by the threshold law predicted by [11] on the basis of the symmetrization principle for identical particles. In the case of atoms polarized in (-1/2,-1/2) or that of a mixture (1/2,-1/2), the losses are mainly due to 2-body dipolar losses. These losses show a non trivial temperature dependence, that can nevertheless be understood by a simple theoretical model with only one adjustable parameter. In the (1/2,-1/2) channel, we take advantage of a sharp decrease of the 2-body loss rate below the Feshbach resonance to present a first evidence for the generation of p-wave molecules.The p-wave resonances described in these paper have their origin in the same singlet (S = 0) bound state that Magnetic Field (G) leads to the s-wave Feshbach resonances located at 543 G and ∼ 830 G. The latter has been used to generate stable molecular Bose-Einstein condensates [1,2,3,4]. In order to discuss the origin of these resonances, it is useful to introduce the molecular basis quantum numbers S, I, and l, which correspond to the total electron spin S = s 1 + s 2 , total nuclear spin I = i 1 + i 2 , and orbital angular momentum l. Furthermore, the quantum numbers must fulfill the selection rulewhich is a r...

The phase transition of Bose-Einstein condensation is studied in the critical regime, when fluctuations extend far beyond the length scale of thermal de Broglie waves. Using matter-wave interference we measure the correlation length of these critical fluctuations as a function of temperature. The diverging behavior of the correlation length above the critical temperature is observed, from which we determine the critical exponent of the correlation length for a trapped, weakly interacting Bose gas to be $\nu=0.67\pm 0.13$. This measurement has direct implications for the understanding of second order phase transitions.Comment: 4 pages, 3 figures, accepted version; published document available at http://www.quantumoptics.ethz.ch/publications.htm