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Simple method for producing Bose-Einstein condensates of metastable helium using a single beam optical dipole trap

Flores, A.S., Mishra, H.P., W. Vassen, Knoop, S.

Published Jan 1, 2015• 4 citations
OpenAlex

Abstract

We demonstrate a simple scheme to reach Bose–Einstein condensation (BEC) of metastable triplet helium atoms using a single-beam optical dipole trap with moderate power of less than 3 W. Our scheme is based on RF-induced evaporative cooling in a quadrupole magnetic trap and transfer to a single-beam optical dipole trap that is located below the magnetic trap center. We transfer $$1\times 10^6$$ atoms into the optical dipole trap, with an initial temperature of $$14\,\upmu\hbox{K}$$ , and observe efficient forced evaporative cooling both in a hybrid trap, in which the quadrupole magnetic trap operates just below the levitation gradient, and in the pure optical dipole trap, reaching the onset of BEC with $$2\times 10^5$$ atoms and a pure BEC of $$5\times 10^4$$ atoms. Our work shows that a single-beam hybrid trap can be applied for a light atom, for which evaporative cooling in the quadrupole magnetic trap is strongly limited by Majorana spin-flips, and the very small levitation gradient limits the axial confinement in the hybrid trap.

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We report the realization of Bose-Einstein condensation (BEC) of metastable helium atoms using an in-vacuum coil magnetic trap and a crossed-beam optical dipole trap. A quadrupole-Ioffe configuration magnetic trap made from in-vacuum hollow copper tubes provides fast switching times while generating traps with a 10-G bias, without compromising optical access. The bias enables in-trap one-dimensional Doppler cooling to be used, which is the only cooling stage between the magneto-optic trap and the optical dipole trap. This allows direct transfer to the dipole trap without the need for any additional evaporative cooling in the magnetic trap. The entire experimental sequence takes 3.3 s, with essentially pure BECs observed with $\ensuremath{\sim}{10}^{6}$ atoms after evaporative cooling in the dipole trap.
We report on a simple trapping scheme for the generation of Bose-Einstein condensates of 87Rb^{87}\mathrm{Rb}87Rb atoms. This scheme employs a near-infrared single-beam optical dipole trap combined with a weak magnetic quadrupole field as used for magneto-optical trapping to enhance the confinement in axial direction. Efficient forced evaporative cooling to the phase transition is achieved in this weak hybrid trap via reduction of the laser intensity of the optical dipole trap at constant magnetic field gradient.
We have realized Bose-Einstein condensation (BEC) of 87Rb in the F = 2, m F = 2 hyperfine substate in a hybrid trap, consisting of a quadrupole magnetic field and a single optical dipole beam. The symmetry axis of the quadrupole magnetic trap coincides with the optical beam axis, which gives stronger axial confinement than previous hybrid traps. After loading 2 × 106 atoms at 14 μK from a quadrupole magnetic trap into the hybrid trap, we perform efficient forced evaporation and reach the onset of BEC at a temperature of 0.5 μK and with 4 × 105 atoms. We also obtain thermal clouds of 1 × 106 atoms below 1 μK in a pure single beam optical dipole trap, by ramping down the magnetic field gradient after evaporative cooling in the hybrid trap.
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We describe an apparatus for quickly and simply producing 87Rb^{87}\text{R}\text{b}87Rb Bose-Einstein condensates. It is based on a magnetic quadrupole trap and a red-detuned optical dipole trap. We collect atoms in a magneto-optical trap (MOT) and then capture the atoms in a magnetic quadrupole trap and force rf evaporation. We then transfer the resulting cold dense cloud into a spatially mode-matched optical dipole trap by lowering the quadrupole field gradient to below gravity. This technique combines the efficient capture of atoms from a MOT into a magnetic trap with the rapid evaporation of optical dipole traps; the approach is insensitive to the peak quadrupole gradient and the precise trapping beam waist. Our system reliably produces a condensate with $N\ensuremath{\approx}2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms every 16 s.