We have observed the persistent flow of Bose-condensed atoms in a toroidal trap. The flow persists without decay for up to 10 s, limited only by experimental factors such as drift and trap lifetime. The quantized rotation was initiated by transferring one unit variant Planck's over 2pi of the orbital angular momentum from Laguerre-Gaussian photons to each atom. Stable flow was only possible when the trap was multiply connected, and was observed with a Bose-Einstein condensate fraction as small as 20%. We also created flow with two units of angular momentum and observed its splitting into two singly charged vortices when the trap geometry was changed from multiply to simply connected.

We demonstrate the coherent transfer of the orbital angular momentum of a photon to an atom in quantized units of , using a 2-photon stimulated Raman process with Laguerre-Gaussian beams to generate an atomic vortex state in a Bose-Einstein condensate of sodium atoms. We show that the process is coherent by creating superpositions of different vortex states, where the relative phase between the states is determined by the relative phases of the optical fields. Furthermore, we create vortices of charge 2 by transferring to each atom the orbital angular momentum of two photons.PACS numbers: 03.75. Lm, 42.50.Vk Light can carry two kinds of angular momentum: Internal or spin angular momentum (SAM) associated with its polarization and external or orbital angular momentum (OAM) associated with its spatial mode [1]. A light beam with a phase singularity, e.g., a Laguerre-Gaussian (LG) beam, has a well-defined OAM along its propagation axis [2]. Beams with phase singularities have only recently been generated [3,4,5], and are now routinely created so as to carry specific values of OAM [6,7].Interaction of light with matter inevitably involves the exchange of momentum. For linear momentum (LM), the mechanical effects of light range from comet tails to laser cooling of atoms. The transfer of optical SAM to atoms has been studied for over a century [8], and the mechanical effect of SAM on macroscopic matter was first demonstrated 70 years ago in an experiment where circularly polarized light rotated a birefringent plate [9]. More recently, the mechanical effects of optical OAM on microscopic particles and atoms have been investigated [6]. SAM and OAM of light has been used to rotate micron-sized particles held in optical tweezers [10,11,12]. The forces on atoms due to optical OAM [13] An atomic gas Bose-Einstein condensate (BEC) allows the study of macroscopic quantum states. For example, BEC superfluid properties can be explored using vortex states (macroscopic rotational atomic states with angular momentum per atom quantized in units of ). The many-body wavefunction of the BEC is very well approximated by the product of identical single-particle wavefunctions, so for a BEC in a vortex state, each particle carries quantized OAM. The first generation of a vortex in a BEC used a "phase engineering" scheme involving a rapidly rotating G laser beam coupling the external motion to internal state Rabi oscillations [17,18]. Later schemes included mechanically stirring the BEC with a focused laser beam [19] and "phase imprinting" by adiabatic passage [16,20]. However, transfer of OAM from the rotating light beams in these earlier schemes is not well-defined.Here, we report the direct observation of the quantized transfer of well-defined OAM of photons to atoms. Using a 2-photon stimulated Raman process, similar to Bragg diffraction [21], but with a LG beam carrying OAM of per photon, we generate an atomic vortex state in a BEC. Over the past decade, numerous papers [22,23] proposed generating vortices in a BEC using stimulate...

We have observed high-order quantum resonances in a realization of the quantum delta-kicked rotor, using Bose-condensed Na atoms subjected to a pulsed standing wave of laser light. These resonances occur for pulse intervals that are rational fractions of the Talbot time, and are characterized by ballistic momentum transfer to the atoms. The condensate's narrow momentum distribution not only permits the observation of the quantum resonances at 3/4 and 1/3 of the Talbot time, but also allows us to study scaling laws for the resonance width in quasimomentum and pulse interval.

Neutral atoms stored in optical traps are strong candidates for a physical realization of a quantum logic device 1,2 . Far off-resonance optical traps provide conservative potentials and excellent isolation from the environment, and they may be arranged to produce arbitrary arrays of traps, where each trap is occupied by a single atom that can be individually addressed [3][4][5][6] . At present, significant effort is being expended on developing two-qubit gates based on coupling individual Rydberg atoms in adjacent optical microtraps [7][8][9] . A major challenge associated with this approach is the reliable generation of single-atom occupancy in each trap, as the loading efficiency in the past experiments has been limited to 50% (refs 4,7,8,10-12). Here we report a loading efficiency of 82.7% in an optical microtrap. We achieve this by manipulating the collisions between pairs of trapped atoms through tailored optical fields and directly observing the resulting single atoms in the trap.Deterministic control of single neutral atoms is a long-standing goal in atomic physics. Not only would it represent a milestone in scientists' ability to control the microscopic world, but also because it would enable a neutral-atom-based quantum logic device 7-9 . Two approaches have successfully led to direct observation of subPoissonian number distributions of atoms in optical microtraps, without consecutive atom sorting 13 . In the first, the Mott insulator transition of a Bose-Einstein condensate provides an efficient route for high occupancy of individual atoms in optical lattices where atoms can tunnel between adjacent lattice sites [14][15][16][17] . The second approach, which may be applied in arbitrary geometries 18,19 , is to employ light-assisted collisions 4,[10][11][12] . This method makes use of the change in the atom-atom interaction that arises when light drives one of the atoms undergoing a collision to the electronic excited state. In the case of light with a frequency below resonance (red detuned), the atom pair is excited to an attractive potential leading to the atoms forming a molecule and/or gaining a large amount of kinetic energy. In each case, both atoms are lost, leading to a maximal 50% chance of ending with one atom in the trap, depending on whether the initial atom number is even or odd 10,12 . However, a process where only one atom is lost as a result of a two-body collision would lead to deterministic preparation of a single atom in a given site. In the past, it has been shown that various forms of collisional trap loss can be suppressed by the application of optical control fields, and in particular, the use of blue-detuned light to effect so-called optical shielding 20,21 . In this Letter, we study light-assisted collisions at the single-event level. We prepare individual pairs of atoms in an optical microtrap and expose them to near-resonant light. We directly observe that light-assisted collisions between these atoms can lead to only one atom being lost. By choice of the frequency and intensi...