Quantized Rotation of Atoms from Photons with Orbital Angular Momentum

Abstract

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...

Vortices in Polariton OPO Superfluids

“…Here, vortices are stable as far as the system is kept rotating and become unstable when the imposed rotation is halted [77]. However, persistent flow can be observed when a BEC is confined into a toroidal trap and the quantised rotation is initiated by a pulsed Laguerre-Gauss beam [88,89,90]. The toroidal trap is essential to allow the vortex stability, because of the energy cost of the vortex core to move through the high density region from the center of the torus where the density is zero.…”

Optical manipulation of matter-wave vortices: An analog of circular dichroism

“…As mentioned in the previous paragraph, we consider sodium BEC containing 10 4 number of atoms initially prepared in electronic state |3S 1/2 , F = 1, m F = −1 [11]. The atoms are trapped in an anisotropic trap.…”

“…We consider sodium BEC is initially prepared in electronic state |3S 1/2 , F = 1, m F = −1 [11]. Our proposed experimental scheme is shown in figure 1. LG l 0 and G 2 pulses propagate along +Z axis and LG −l 0 and G 1 pulses propagate along −Z.…”

“…One can detect the presence and sign of the undisturbed vortex by doing measurements on the separated part. In recent experiments of [11,14,15], two-photon transitions are used to transfer the OAM to the atomic BEC employing an LG and a Gaussian beam.…”

“…Numerous theoretical and experimental studies related to creation of matter-wave vortex states, persistent current in BEC and coherent control of the OAM of atoms using interaction of ultra-cold atoms with optical vortex [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] have been reported over past two decades. Quantized vortex states play an essential role in macroscopic quantum phenomena like superfluidity and superconductivity.…”