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...

We study the spinor properties of S = 3 (52)Cr condensates, in which dipole-dipole interactions allow changes in magnetization. We observe a demagnetization of the Bose-Einstein condensate (BEC) when the magnetic field is quenched below a critical value corresponding to a phase transition between a ferromagnetic and a nonpolarized ground state, which occurs when spin-dependent contact interactions overwhelm the linear Zeeman effect. The critical field is increased when the density is raised by loading the BEC in a deep 2D optical lattice. The magnetization dynamics is set by dipole-dipole interactions.

We report on the realization of quantum magnetism using a degenerate dipolar gas in an optical lattice. Our system implements a lattice model resembling the celebrated t-J model. It is characterized by a nonequilibrium spinor dynamics resulting from intersite Heisenberg-like spin-spin interactions provided by nonlocal dipole-dipole interactions. Moreover, due to its large spin, our chromium lattice gases constitute an excellent environment for the study of quantum magnetism of high-spin systems, as illustrated by the complex spin dynamics observed for doubly occupied sites.

We study the interaction of a nearly resonant linearly polarized laser beam with a cloud of cold cesium atoms in a high finesse optical cavity. We show theoretically and experimentally that the cross-Kerr effect due to the saturation of the optical transition produces quadrature squeezing on both the mean field and the orthogonally polarized vacuum mode. An interpretation of this vacuum squeezing as polarization squeezing is given and a method for measuring quantum Stokes parameters for weak beams via a local oscillator is developed.PACS numbers: 42.50. Dv, 42.50.Lc, 03.67.Hk A great deal of attention has been recently given to the quantum features of the polarization states of the light, essentially because of their connections with quantum information technology. Several theoretical schemes to produce polarization squeezing using Kerr-like media have been proposed [1] and realized using optical fibers [2]. Other experimental realizations achieve polarization squeezing by mixing squeezed vacuum (generated by an OPO) with a strong coherent beam on a polarizing beam splitter [3] or mixing two independent quadrature squeezed beams (generated by an OPA) on a polarizing beam splitter [4]. Very recently it has been proposed to propagate a linearly polarized light beam through an atomic medium exhibiting self rotation to generate squeezed vacuum in the orthogonal polarization [5], which is equivalent to achieving polarization squeezing. In previous works [6] the interaction between a cloud of cold cesium atoms placed in a high finesse optical cavity and a circularly polarized laser beam nearly resonant with an atomic transition has been studied. Because of optical pumping, the atomic medium is conveniently modelled by an ensemble of two-level atoms. The saturation of the optical transition gives rise to an intensity-dependent refraction index. It is well known that the interaction of the light with a Kerr-like medium produces bistable behavior of the light transmitted by the cavity and that, at the turning point of the bistability curve, the quantum fluctuations of the light can be strongly modified and generate quadrature squeezing [7]. A noise reduction of 40% has thus been observed in our group [6]. In this paper we focus on the theoretical and experimental investigation of polarization squeezing via the interaction of a linearly polarized laser beam with cold cesium atoms. In this configuration, the two-level atom model is no longer applicable and the situation much more complicated. We describe the interaction between light and the atomic medium by means of an X-like four-level quantum model based on the linear input-output method. Our theoretical analysis shows clearly that competitive optical pumping may result in polarization switching, and polarization squeezing is predicted by the model [8]. In agreement with the model we observe quadrature squeezing in the probe laser mode and in the orthogonal vacuum mode. Experimentally, we obtain a polarization squeezing of 13% and we show for the first time that the ...

Understanding quantum thermalization through entanglement build up in isolated quantum systems addresses fundamental questions on how unitary dynamics connects to statistical physics. Spin systems made of long-range interacting atoms offer an ideal experimental platform to investigate this question. Here, we study the spin dynamics and approach towards local thermal equilibrium of a macroscopic ensemble of S = 3 chromium atoms pinned in a three dimensional optical lattice and prepared in a pure coherent spin state, under the effect of magnetic dipole–dipole interactions. Our isolated system thermalizes under its own dynamics, reaching a steady state consistent with a thermal ensemble with a temperature dictated from the system’s energy. The build up of quantum correlations during the dynamics is supported by comparison with an improved numerical quantum phase-space method. Our observations are consistent with a scenario of quantum thermalization linked to the growth of entanglement entropy.

We measure the excitation spectrum of a dipolar chromium Bose-Einstein condensate with Raman-Bragg spectroscopy. The energy spectrum depends on the orientation of the dipoles with respect to the excitation momentum, demonstrating an anisotropy that originates from the dipole-dipole interactions between the atoms. We compare our results with the Bogoliubov theory based on the local density approximation and, at large excitation wavelengths, with the numerical simulations of the time-dependent Gross-Pitaevskii equation. Our results show an anisotropy of the speed of sound.

We report on magneto-optical trapping of fermionic 53 Cr atoms. A Zeeman-slowed atomic beam provides loading rates up to 3 ϫ 10 6 s −1 . We present systematic characterization of the magneto-optical trap ͑MOT͒. We obtain up to 5 ϫ 10 5 atoms in the steady-state MOT. The atoms radiatively decay from the excited P state into metastable D states, and, due to the large dipolar magnetic moment of chromium atoms in these states, they can remain magnetically trapped in the quadrupole field gradient of the MOT. We study the accumulation of metastable 53 Cr atoms in this magnetic trap. We also report on the simultaneous magneto-optical trapping of bosonic 52 Cr and fermionic 53 Cr atoms. Finally, we characterize the light-assisted collision losses in this Bose-Fermi cold mixture.