We report on the production of 52 Cr Bose Einstein Condensates (BEC) with an all-optical method. We first load 5.10 6 metastable chromium atoms in a 1D far-off-resonance optical trap (FORT) from a Magneto Optical Trap (MOT), by combining the use of Radio Frequency (RF) frequency sweeps and depumping towards the 5 S2 state. The atoms are then pumped to the absolute ground state, and transferred into a crossed FORT in which they are evaporated. The fast loading of the 1D FORT (35 ms 1/e time), and the use of relatively fast evaporative ramps allow us to obtain in 20 s about 15000 atoms in an almost pure condensate.PACS numbers: 03.75. Hh , The study of the degenerate quantum phases of chromium is especially appealing for two main reasons. First, the atomic magnetic moment of 6 µ B (Bohr magneton) leads to large anisotropic long range dipole-dipole interactions, which are non negligible compared to the contact interaction [1], and can even become the dominant interaction close to a Feshbach resonance [2]. In this regime, the stability and excitation properties of dipolar BECs are completely modified by dipole-dipole interactions [3]. In addition, the large S = 3 spin in the ground state makes Cr a unique element for spinor physics [4]. Second, the existence of a fermionic isotope ( 53 Cr, 10 % natural abundance) opens the way to obtain a degenerate dipolar Fermi sea, and to study the interesting stability properties of a dipolar boson-fermion mixture [5].The historic [6] and still conventional way to produce quantum degenerate gases is evaporation inside a magnetic trap (MT). An other possibility, demonstrated first for Rb [7], is to evaporatively cool in an optical trap created by a far red detuned laser. These traps offer an interesting experimental alternative as the highly confining MTs required to evaporate efficiently demand either large currents, or the use of integrated structures [8]. For some atoms, the winning strategy to obtain condensation has been to use a FORT, either because of high inelastic collision rates (for Cs [9] and Cr [10],[11]), or because of the absence of a permanent magnetic moment (for Yb [12]). In the first case optically pumping the atoms to the lowest energy Zeeman substate suppresses all twobody inelastic collisions at low temperature, but these high field seeking states cannot be trapped magnetically: the use of optical traps is necessary. The evaporation is then performed in a crossed FORT with a standard procedure, for which the evaporation dynamics is well understood [13].However, efficiently loading a FORT is not straightforward in general and especially for Cr. In particular in our experiment, a direct loading of a Cr optical trap in the ground 7 S 3 state from a MOT leads to small number of atoms, presumably because of a high light assisted inelastic collision rate [14,15]. The loading procedure used to obtain the first Cr BEC [11] was to accumulate the atoms in metastable D states inside a MT, before transferring them first into an elongated Ioffe Pritchard MT, and then in ...

We study dipolar relaxation in both ultra-cold thermal and Bose-condensed chromium atom gases. We show three different ways to control dipolar relaxation, making use of either a static magnetic field, an oscillatory magnetic field, or an optical lattice to reduce the dimensionality of the gas from 3D to 2D. Although dipolar relaxation generally increases as a function of a static magnetic field intensity, we find a range of non-zero magnetic field intensities where dipolar relaxation is strongly reduced. We use this resonant reduction to accurately determine the S = 6 scattering length of chromium atoms: a6 = 103 ± 4a0. We compare this new measurement to another new determination of a6, which we perform by analysing the precise spectroscopy of a Feshbach resonance in d-wave collisions, yielding a6 = 102.5 ± 0.4a0. These two measurements provide by far the most precise determination of a6 to date. We then show that, although dipolar interactions are long-range interactions, dipolar relaxation only involves the incoming partial wave l = 0 for large enough magnetic field intensities, which has interesting consequences on the stability of dipolar Fermi gases. We then study ultra-cold chromium gases in a 1D optical lattice resulting in a collection of independent 2D gases. We show that dipolar relaxation is modified when the atoms collide in reduced dimensionality at low magnetic field intensities, and that the corresponding dipolar relaxation rate parameter is reduced by a factor up to 7 compared to the 3D case. Finally, we study dipolar relaxation in presence of radio-frequency (rf) oscillating magnetic fields, and we show that both the output channel energy and the transition amplitude can be controlled by means of rf frequency and Rabi frequency. Strong dipole-dipole interactions arise when an atomic or molecular species carries a strong permanent magnetic or electric dipole moment. Good candidates therefore include heteronuclear molecules with large electric dipole moments (which were recently produced at large phasespace densities [7]), or atoms with large electronic spin (so far erbium [8], dysprosium [9] and chromium). Up to now, chromium is the only species with large dipole moment for which a quantum degenerate gas has been produced [10,11]. Smaller dipolar effects were also observed in a BEC of potassium for which the scattering length can be precisely tuned to zero by means of a Feshbach resonance [12].Interesting new physics comes at play when one also considers the spin degree of freedom. Spin dynamics of optically trapped multi-component Bose-Einstein Condensates (also known as spinor condensates) [13] has been observed [14]. Coherent oscillations between the spin components is driven by centrally symmetric short range exchange interactions, and the total magnetization in the system is conserved. In [15] a first dipolar effect was observed on the spin texture of a Rb spinor condensate. Dipole-dipole interaction will introduce additional new features in spin dynamics as it couples the spin degree of fre...