We have investigated the ultracold interspecies scattering properties of metastable triplet He and Rb. We performed state-of-the-art ab initio calculations of the relevant interaction potential, and measured the interspecies elastic cross section for an ultracold mixture of metastable triplet 4 He and 87 Rb in a quadrupole magnetic trap at a temperature of 0.5 mK. Our combined theoretical and experimental study gives an interspecies scattering length a4+87 = +17 +1 −4 a0, which prior to this work was unknown. More general, our work shows the possibility of obtaining accurate scattering lengths using ab initio calculations for a system containing a heavy, many-electron atom, such as Rb.

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.

Abstract. We have realized Bose-Einstein condensation (BEC) of87 Rb in the F = 2, mF = 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 × 10 6 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 × 10 5 atoms. We also obtain thermal clouds of 1 × 10 6 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.

In an ultracold, optically trapped mixture of 87 Rb and metastable triplet 4 He atoms we have studied trap loss for different spin-state combinations, for which interspecies Penning ionization is the main two-body loss process. We observe long trapping lifetimes for the purely quartet spinstate combination, indicating strong suppression of Penning ionization loss by at least two orders of magnitude. For the other spin-mixtures we observe short lifetimes that depend linearly on the doublet character of the entrance channel. We compare the extracted loss rate coefficient with recent predictions of multichannel quantum-defect theory for reactive collisions involving a strong exothermic loss channel and find near-universal loss for doublet scattering. Our work demonstrates control of Penning ionization reactive collisions by internal atomic state preparation.Ultracold inelastic and reactive collisions are important processes in atomic and molecular samples [1,2], determining their trapping lifetimes and the success of evaporative and sympathetic cooling. Conversely, measurements of these lifetimes reveal the rate coefficients of the dominant inelastic or reactive collision processes, opening the fields of ultracold few-body physics [3,4] and ultracold chemistry [5,6]. The ultracold regime offers exquisite control over the initial internal and external quantum states, and the possibility to experimentally control collision properties or even steer chemical reactions with external fields [7].Understanding of inelastic and reactive collisions is in general very difficult due to the many degrees of freedom involved. This has motivated recent work based on multichannel quantum-defect theory (MQDT) [8][9][10], in which analytic expressions of collision rates were derived in the case of a strong exothermic reactive channel. In particular, if the probability of an inelastic or reactive process in the short-range part of the collision is 100%, i. e. if P re = 1, theory predicts universal rate constants that only depend on the reduced mass of the collision partners and the leading long-range coefficient [8,9], independent of the complicated short-range dynamics. If the reaction probability is less than 100% (P re < 1), still only two parameters are required to include the (nonuniversal) short-range physics, i. e. the scattering length a and P re [10]. These analytical models have been applied to atom-exchange reactions between ground state KRb molecules below 1 µK [5,8], and Penning ionization reactions between argon and helium atoms in the metastable triplet 2 3 S 1 state (He * ) in merged-beam experiments from 10 mK up to 30 K [10,11].In this Rapid Communication we study ultracold Penning ionizing collisions between He * atoms (internal energy 19.8 eV) and alkali atoms A in their electronic ground state:

Abstract. We report on the realization of an ultracold (<25 μK) mixture of rubidium ( 87 Rb) and metastable triplet helium ( 4 He) in an optical dipole trap. Our scheme involves laser cooling in a dual-species magneto-optical trap, simultaneous MW-and RF-induced forced evaporative cooling in a quadrupole magnetic trap, and transfer to a single-beam optical dipole trap. We observe long trapping lifetimes for the doubly spin-stretched spin-state mixture and measure much shorter lifetimes for other spin-state combinations. We discuss prospects for realizing quantum degenerate mixtures of alkali-metal and metastable helium atoms.