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

Abstract

Alternatively, one loads the atoms first in a magnetic trap and performs evaporative cooling and afterward transfers a dense and compact atomic cloud into an ODT, which now requires a much lower ODT power, at the expense of experimental complexity. Within this last category, a very elegant approach is the hybrid trap (HT), introduced in Ref.[1], consisting of a simple quadrupole magnetic trap (QMT) and a single-beam ODT. Efficient evaporation and BoseEinstein condensation (BEC) have been demonstrated (see, e.g., [1][2][3]), using ODT powers of only a few Watts. By switching off the QMT completely, the atoms are transferred from the HT to a pure ODT.The hybrid trap has been mostly applied to 87 Rb, but is assumed to be generally applicable to other magnetically trappable atomic species [1]. However, the application of HT strongly depends on the mass of atom. Most importantly, the rates of Majorana loss and heating, which determine the temperature that can be reached by evaporative cooling in a QMT, scale inversely with mass [4,5]. This limits the transfer efficiency for light atoms, or puts constraints on the trap volume and trap depth, and therefore the power, of the ODT. Furthermore, for light atoms evaporative cooling in the HT is limited as the additional axial confinement provided by the QMT is small because of the small levitation gradient, below which the QMT has to operate in the HT. Finally, the small levitation gradient puts experimental limits on the control of the displacement of the QMT with respect to the ODT, which further limits the axial confinement.Here we report on the production of a metastable triplet helium ( 4 He * ) BEC using a single-beam HT with a moderate power of less than 3 W, demonstrating the application of HT for a light atom. Our work provides a novel and simple method for obtaining a 4 He * BEC, which can be used for atom optics experiments [6][7][8][9][10] or precision spectroscopy 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 × 10 6 atoms into the optical dipole trap, with an initial temperature of 14 µK, 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 × 10 5 atoms and a pure BEC of 5 × 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.

A simple 2 W continuous-wave laser system for trapping ultracold metastable helium atoms at the 319.8 nm magic wavelength

“…In this way, we extract temperatures of 0.62(2) and 0.51(2) µK W −1 respectively. This is approximately a factor 10 lower than the calculated trap depth and corresponds to a truncation parameter η = 10 which is typically found in a thermalized trapped gas [29].…”

“…The quadrupole field has a strong axis gradient of 0.54 Gauss/ cm, but gravity acts in a direction perpendicular to this axis. In this direction, the gradient is only half the magnitude which is well below the leviation gradient of mg/µ = 0.351 Gauss/cm [29]. Below this gradient gravity is stronger than the confining magnetic force so that atoms are not trapped in the absence of the UV beam.…”

“…We use two different methods to provide this confinement. The first is to add a magnetic field gradient along the beam direction to create a hybrid trap [29]. With this method, the loss rate and trap depth are straightforward to interpret.…”

An ultracold, optically trapped mixture of 87Rb and metastable 4He atoms

“…Details of our experimental setup have been given in reference [57] [59], in which we implement a hybrid trap (single-beam optical dipole trap combined with a weak quadrupole magnetic trap) to achieve Bose-Einstein condensation of 87 Rb and reference [60], where we demonstrate the production of 4 He * Bose-Einstein condensates in a single-beam optical dipole trap. In this paper, we focus on the scheme that is relevant for simultaneous loading of the two atomic species in an optical dipole trap.…”

“…In the following subsections, we briefly describe the different stages involved in the preparation of our ultracold mixture and discuss important issues regarding simultaneous loading as compared to our single-species experiments [59,60] and previous mixture experiment [57]. We also describe our simultaneous detection scheme and explain the preparation of different spin-state samples using MW and RF frequency sweeps.…”