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arXiv:0712.3521 · Physical Review A

All-optical production of chromium Bose-Einstein condensates

Quentin Beaufils, Radu Chicireanu, T. Zanon-Willette, B. Laburthe-Tolra, É. Maréchal, L. Vernac, J. Keller, O. Gorceix

Published Jun 2, 2008• 214 citations
arXivHTMLPDF

Abstract

We report on the production of $^{52}\mathrm{Cr}$ Bose-Einstein condensates with an all-optical method. We first load $5\ifmmode\times\else\texttimes\fi{}{10}^{6}$ metastable chromium atoms in a one-dimensional (1D) far-off-resonance optical trap (FORT) from a magneto-optical trap, by combining the use of radio-frequency sweeps and depumping toward the $^{5}S_{2}$ 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 ($35\phantom{\rule{0.3em}{0ex}}\mathrm{ms}$ $1∕e$ time) and the use of relatively fast evaporative ramps allow us to obtain in $20\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ about $15\phantom{\rule{0.2em}{0ex}}000$ atoms in an almost pure condensate.

DOI
10.1103/physreva.77.061601

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We report on the production of 52Cr^{52}\mathrm{Cr}52Cr Bose-Einstein condensates with an all-optical method. We first load 5\ifmmode×\else\texttimes\fi1065\ifmmode\times\else\texttimes\fi{}{10}^{6}5\ifmmode×\else\texttimes\fi106 metastable chromium atoms in a one-dimensional (1D) far-off-resonance optical trap (FORT) from a magneto-optical trap, by combining the use of radio-frequency sweeps and depumping toward the 5S2^{5}S_{2}5S2​ 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 (35ms35\phantom{\rule{0.3em}{0ex}}\mathrm{ms}35ms 1∕e1∕e1∕e time) and the use of relatively fast evaporative ramps allow us to obtain in 20s20\phantom{\rule{0.3em}{0ex}}\mathrm{s}20s about 1500015\phantom{\rule{0.2em}{0ex}}00015000 atoms in an almost pure condensate.
We report the all-optical production of Bose-Einstein condensates (BEC) of 39K^{39}\mathrm{K}39K atoms. We directly load $3\ifmmode\times\else\texttimes\fi{}{10}^{7}$ atoms in a large volume optical dipole trap from gray molasses on the D1 transition. We then apply a small magnetic quadrupole field to polarize the sample before transferring the atoms in a tightly confining optical trap. Evaporative cooling is finally performed close to a Feshbach resonance to enhance the scattering length. Our setup allows one to cross the BEC threshold with $3\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms every 7 s. As an illustration of the interest of the tunability of the interactions we study the expansion of Bose-Einstein condensates in the one-dimensional to three-dimensional crossover.
Full text: The study of quantum gases made of chromium atoms is compelling for several reasons. Being accessible to laser manipulation, chromium has a most abundant bosonic isotope {sup 52}Cr and a 9-percent abundant fermionic isotope {sup 53}Cr. Most importantly, Cr atoms carry an exceptionally large magnetic moment of 6 {mu}{sub B}. Consequently, Cr provides a valuable tool to study the physics of dipolar quantum gases as demonstrated elsewhere. We present our recent achievement of a chromium Bose-Einstein Condensation (Cr-BEC) using an all-optical procedure along with two innovative techniques: continuous accumulation of metastable {sup 52}Cr atoms in a mixed optical and magnetic trap; fast and intense rf sweeps to average to zero the magnetic potential and optimize the transfer efficiency from the Cr-MOT to the optical trap. We also report on the rf spectroscopy and association of weakly bound Cr{sub 2} molecules in the decatriplet {sup 13}{sigma}{sub g}{sup +} state. These latter experiments are performed in the vicinity of a d-wave Feshbach resonance at low magnetic field. Though the association rate is at present fairly low, we can study the spectroscopic properties of these cold trapped high-spin chromium molecules. (author)
Research on degenerate quantum gases of dilute atomic gases has become a well established field in modern physics and revealed many astonishing phenomena. The first experiments on Bose and Fermi gases at low temperatures aimed to prove the predictions of quantum mechanics on a macroscopic scale. Since then we make use of Bose and Fermi gases and mixtures of any combination to model other many body systems. A property of Bose Einstein condensates is superfluidity which we want to investigate further in toroidal potentials with a new apparatus which can produce an all-optical BEC. This means trapping thermal atoms in an optical dipole trap and cooling the sample by evaporation into the quantum degenerate regime. This new technique was first successfully demonstrated by M.D. Barrett et al. in 2001 and promises to be faster and simpler than the well established and common way of evaporation in magnetic traps. A new apparatus was built by the author and we report on the experimental details of the trapping of 87Rb atoms from a hot vapour and cooling them in a magneto-optical trap, and further evaporation to quantum degeneracy in a crossed optical dipole trap operating at a wavelength of l = 1064nm, with accurate control of the power in both beams via a feedback loop. The evaporation of neutral atoms of 87Rb in far red detuned optical dipole traps using linearly polarised laser light is spin independent, and mixed spinor condensates of the F=1 manifold can be formed. As an empirical technique we found by applying a magnetic gradient field during the final evaporation that we can selectively populate mF spin states or prepare mixtures. This intriguing mechanism was found earlier as well by M. S. Chang, but is yet not fully understood and subject of our future research. We can now routinely prepare an almost pure condensate containing up to 7000 atoms in the condensed phase purely in the mF = 0 spin state. This thesis contains the theoretical framework of Bose-Einstein condensation with in mean-field theory, the concepts of cooling of atoms in magneto-optical traps, the trapping and evaporation in optical dipole traps and the experimental realisation. In the future this experiment will be extended by a fast scanning dipole trap to apply arbitrary time averaged potentials of more complex geometries, such as toroidal traps to study the effects of superfluidity in a periodic potential. The production of toroidal traps and cold atoms in such a trap are outlined in this thesis.
We describe an all-optical method of making a 87Rb^{87}\mathrm{Rb}87Rb Bose-Einstein condensate (BEC). Atoms are first cooled in an optical lattice and loaded into a crossed dipole trap with $1.06\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ light. Then the density is increased by dynamically reducing the trap size, while the atoms are evaporatively cooled by reducing the light intensity. Every 3.3s3.3\phantom{\rule{0.3em}{0ex}}\mathrm{s}3.3s, we obtain a nearly pure BEC with $3.5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms in their lowest internal energy state.
In 1925, Einstein predicted the condensation of bosons into the ground state of the system for low (but finite) temperatures. Several phenomena, including superfluidity and superconductivity have been associated with Bose-Einstein condensation, but these systems interact strongly with their environment and pure Bose-Einstein condensation could not be established. It took 70 years, in which time the laser was discovered, and laser cooling techniques to manipulate atoms in a dilute atomic gas, before Bose-Einstein condensation in dilute atomic gases could be demonstrated in 1995. In the first condensation experiments, BECs were created in a magnetic trap. Since in a magnetic trap not all mF states of the atom can be trapped simultaneously, thereby limiting the number of experiments that can be done, other ways of trapping and generating BECs were sought and found. In 2001, the first all-optical BEC was made, where the dipole force was used to trap atoms in the crossing of two far red detuned laser beams. In an optical dipole trap not only atoms in different internal states can be trapped, but also different atomic species simultaneously. In this thesis, the formation of an all-optical Bose-Einstein condensate with rubidium atoms is presented. Conventional all-optical BECs are usually created in high power CO2 laser dipole traps, or have complicated laser cooling schemes and complex dipole trap setups. In our simple and straightforward setup, we load rubidium atoms from a magneto-optical trap into a crossed optical dipole trap created by a single frequency Yb:YAG laser with a wavelength at 1030 nm. The small wavelength allows for a small diffraction limit, and permits us to use standard optical materials, thus making the experimental setup cost effective. Other attempts to achieve Bose-Einstein condensation in a multi-mode (frequency) fiber laser at 1064 nm failed, because the atom loss was quite high. It is assumed that the multi-mode character of the fiber laser induces Raman transitions in rubidium atoms, thereby heating them. We can trap about ∼ 5E7 atoms in a single beam dipole trap out of ∼ 5E9 atoms trapped in theMOT, and ∼ 350,000 atoms can be trapped in a crossed beam dipole trap due to the smaller trap volume. 70% of the atoms in the dipole trap is optically pumped into one mF state. Quantum degeneracy is reached by evaporatively cooling the atoms in the crossed dipole trap by ramping down the laser power with three linear ramps. We can independently change the power of each beam by an AOM. This allows us to use one beam as an atom waveguide for future experiments. After evaporation, we typically have about 10,000 atoms at a temperature below the critical temperature. We have proved Bose-Einstein condensation by using the anisotropic expansion of a quantum degenerate gas trapped in an anisotropic potential. The aspect ratio of our atom cloud changed during a time of flight from 0.7 to 1.2 in 10 ms, thus proving that we have reached quantum degeneracy. We have about 5,000 condensed atoms in our optical dipole trap at a temperature less than 100 nK. The remaining atoms are thermal. Bose-Einstein condensation is obtained within 8 s, and we can repeat the experiment every 30 s. It should be mentioned that the Bose-Einstein experiment was moved from the ”Leibniz Universität Hannover” to the ”Technische Universität Darmstadt”, and had to be completely rebuilt. All-optical Bose-Einstein condensation was reached within one year after the move. Our Bose-Einstein condensation setup presents an ideal starting point for using our condensates in combination with miniaturized atom optical setups based on our novel microfabricated optical elements. With our microlenses, we can create a number of possible dipole trap configurations, such as the dipole trap array or the cylindrical microlens array. Using microlenses in miniaturized atom optical setups opens a completely new field of coherent atom optics. Also because the tight confinement of the microtraps allows us to load a 3D BEC, a 1D BEC, or a Tonks-Girardeau gas in the micropotentials depending on the density.
We demonstrate runaway evaporative cooling directly with a tightly confining optical-dipole trap and achieve fast production of condensates of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ 87Rb^{87}\text{R}\text{b}87Rb atoms. Our scheme uses a misaligned crossed-beam far off-resonance optical-dipole trap (MACRO-FORT). It is characterized by independent control of the trap confinement and depth allowing forced all-optical evaporation in the runaway regime. Although our configuration is particularly well suited to the case of 87Rb^{87}\text{R}\text{b}87Rb atoms in a 1565 nm optical trap, where an efficient initial loading is possible, our scheme is general and will allow all-optical evaporative cooling at constant stiffness for every optically trappable atomic or even molecular species.