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 obtain subnatural linewidth (i.e. < Γ) for probe absorption in room-temperature Rb vapor using electromagnetically induced transparency (EIT) in a Λ system. For stationary atoms, the EIT dip for a resonant control laser is as wide as the control Rabi frequency Ωc. But in thermal vapor, the moving atoms fill the transparency band so that the final EIT dip remains subnatural even when Ωc > Γ. We observe linewidths as small as Γ/7 in the D2 line of Rb.PACS numbers: 42.50. Gy,32.80.Qk,42.50.Md,32.80.Wr Laser spectroscopy in a room-temperature gas is often limited by Doppler broadening due to the thermal velocity of gas particles. While techniques such as saturatedabsorption spectroscopy can be used to eliminate the first-order Doppler effect and get linewidths close to the natural linewidth, the natural linewidth itself appears as a fundamental limit to the resolution that can be achieved in precision spectroscopy. In addition, when lasers are locked to atomic transitions (for use as frequency standards), the natural linewidth determines the tightness of the lock. It is therefore desirable to develop techniques for getting below the natural linewidth.In this work, we propose and demonstrate a technique to obtain subnatural linewidth in a Doppler broadened medium. The technique has been adapted from recent developments in the use of control lasers in three level systems as a means of modifying the absorption properties of a probe beam [1], in what is generally called coherent-control spectroscopy. More specifically, we use the phenomenon of electromagnetically induced transparency (EIT) in a Λ-type system, in which an initially absorbing medium is rendered transparent to a weak probe when a strong control laser is applied to a second transition [2]. It is well known that the EIT dip on resonance for stationary atoms can be subnatural if the Rabi frequency of the control laser is sufficiently small [3]. However, in thermal vapor, the effect of the large Doppler width was thought to have a detrimental effect on observing any subnatural features. Indeed, theoretical work in such Doppler broadened media predicted that one can at best achieve sub-Doppler resolution by detuning the control laser [4]. However, in earlier work [5], we have shown that we can obtain sub-natural linewidth either by detuning the control by an amount that is larger than the Doppler width, or by using a slightly-detuned control along with a counterpropagating pump beam that allows the probe to address only zero-velocity atoms. Here, we show that one can observe subnatural linewidth for the EIT dip even when the control is on resonance. * Electronic address: vasant@physics.iisc.ernet.in; URL: www.physics.iisc.ernet.in/~vasant There has been a previous report of subnatural linewidth for the EIT dip using a Λ system in the D 1 line of Rb [3]. However, the dip was subnatural only when the control Rabi frequency was less than the natural linewidth, Γ, and reached a smallest value of Γ/4 partly limited by the 3-MHz linewidth of the laser. By contr...
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