An all optical BEC apparatus for the investigation of magnetic order in spinor atomic gases in ultralow magnetic fields

Chad Fertig, Joshua Hughes, Wesam El Qadi

Mark Brannan

This thesis outlines the design and construction of an experiment to investigate dipolar interactions within a rubidium-87 (\\(^8\\)\\(^7\\)Rb) spinor Bose-Einstein condensate (BEC). For dipolar interactions to drive population transfer between the internal states of a \\(^8\\)\\(^7\\)Rb BEC the ambient magnetic field must be reduced below 100 pT. To this end I have designed an active magnetic stabilisation system which is able to reduce the ambient magnetic field to a given level with 30 nT peak-to-peak fluctuations. A design is presented for a complementary passive magnetic shield in order to further reduce the ambient magnetic field, which electromagnetic simulations show should provide a factor of >10\\(^8\\) reduction in the ambient magnetic field. \n \n

Rebekah Schiller, Stephan Gerhard Albert, Daniel Pertot +4 more

Sean Lourette, G. Edward Marti, Ryan Olf +1 more

Benjamin Pasquiou, É. Maréchal, G. Bismut +4 more

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.

I. L. H. Humbert

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.