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2D Box Traps for Bose-Einstein Condensates

Max Christenhusz

Published Jan 1, 2020• 0 citations
OpenAlex

Abstract

Optical dipole traps (ODTs) are among the most commonly used traps for confining Bose-Einstein condensates (BECs). A high degree of control of light allows for the realisation of a great variety of trapping geometries. The use of blue-detuned optical traps for BECs has many advantages over using red-detuned traps. As a blue-detuned trap acts as a repulsive potential, one of its main advantages is its ability to expel hot (thermal) atoms that introduce noise in measurements. We form our trap by creating an optical lattice of two interfering blue-detuned beams, and trap atoms vertically the nodal lattice planes. Another blue-detuned beam is used to trap atoms in the horizontal plane. This allows for creation of nearly uniform traps, closely resembling a finite-potential square well. The lattice spacing is easily adjusted, which provides stronger vertical confinement, making the trap a 2D box trap for BECs.

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•Jan 1, 2012•0 citations
Optical dipole traps (ODTs) are among the most commonly used traps for confining Bose-Einstein condensates (BECs). A high degree of control of light allows for the realisation of a great variety of trapping geometries. The use of blue-detuned optical traps for BECs has many advantages over using red-detuned traps. As a blue-detuned trap acts as a repulsive potential, one of its main advantages is its ability to expel hot (thermal) atoms that introduce noise in measurements. We form our trap by creating an optical lattice of two interfering blue-detuned beams, and trap atoms vertically the nodal lattice planes. Another blue-detuned beam is used to trap atoms in the horizontal plane. This allows for creation of nearly uniform traps, closely resembling a finite-potential square well. The lattice spacing is easily adjusted, which provides stronger vertical confinement, making the trap a 2D box trap for BECs.
The final stage in the creation of a Bose-Einstein condensate (BEC) is trapping and compressing in a strong magnetic trap. When the spin of an atom changes, it is lost from this magnetic trap. To be able to do experiments with varying atom spin, one can use an optical dipole trap, which traps atoms independent of their spin. This thesis presents a far red-detuned optical dipole trap, which is characterized by trapping BEC's. Furthermore, an approach to rapidly scan this trap is presented, creating a 'quasi-2D' potential. With this potential we attempt to create a horizontal sheet of condensed atoms, where the depth of the sheet of atoms is smaller than the range of the spin dynamics. This would enable a unique visualization of the spin dynamics using spin-dependent imaging.
Summary form only given. Bose-Einstein condensates (BEC) are generally produced by evaporative cooling of laser cooled atomic samples in magnetic traps. Though magnetic traps are indispensable for the production of a BEC, they pose restrictions on the possible studies with a BEC. Previous experiments of our group have demonstrated that it is possible to trap a BEC in the circular focus of a far-detuned infrared laser beam. This first realization of an optical dipole trap (ODT) for BEC was used to prepare and investigate spinor condensates of Na atoms in the F=1 hyperfine manifold. However, the initial peak density in this ODT after loading from the magnetic trap reached 10/sup 15/ atoms/cm/sup 3/, leading to rapid losses due to three-body collisions. To realize an ODT capable of storing large condensates for a long time, we have used a laser beam with an elliptical beam profile. The optical trap is loaded by superimposing the light field onto the magnetic trap and then slowly turning off the magnetic fields. The transfer efficiency is close to 100%, and thus more than 10/sup 7/ Bose-condensed atoms can be loaded into the ODT.
Optical dipole traps and atom chips are two very powerful tools for the quantum manipulation of neutral atoms. We demonstrate that both methods can be combined by creating an optical lattice potential on an atom chip. A red-detuned laser beam is retroreflected using the atom chip surface as a high-quality mirror, generating a vertical array of purely optical oblate traps. We transfer thermal atoms from the chip into the lattice and observe cooling into the two-dimensional regime. Using a chip-generated Bose-Einstein condensate, we demonstrate coherent Bloch oscillations in the lattice.
Summary form only given. The production of tight toroidal traps, or rings, for Bose-Einstein condensates (BECs) opens up the way to a variety of both fundamental and applied studies. By providing tight transverse confinement one can produce a quasi-one-dimensional motion for the BEC on a ring, which allows for studies of vortices and persistent currents. On the applied side, such mesoscopic rings could be used for rotation sensing via the Sagnac effect for counter-propagating atoms around the ring. In this paper, we show that Laguerre-Gaussian (LG) laser modes of high azimuthal mode index l and containing one radial ring can be used to create tight confined optical dipole traps for 2D BECs.
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.