Twisting Light to Trap Atoms

Davıd L. Andrews, A. C. Carter, M. Babiker +1 more

Theoretical work has already established the existence of a light-induced torque acting on the centre of mass of an atom, ion or molecule immersed in twisted light, where the transition frequency is suitably detuned from that of the twisted light beam. The twisted beam carries l units of orbital angular momentum per photon, and the steady-state saturation form of the torque is also determined by the width of the upper state in the atomic transition. It has been shown that, to leading order, the transfer of orbital angular momentum can only occur between the twisted light and the centre of mass motion. We argue here that, for small linewidth, the full time-dependence of the torque is needed to account correctly for the dynamics of atoms in a twisted light beam. We outline the theoretical framework needed to derive this full time-dependence, applying the theory to the motion in a twisted light beam of Eu3+ ions, which possess a particularly narrow linewidth state. For relatively large linewidth, the steady-state forces and torque are appropriate, but the processes of cooling and trapping require the application of several suitably oriented twisted beams. The description of atomic motion in multiple twisted beams demands the application of special coordinate transformations. We show how to construct the appropriate transformation matrices to represent a twisted light beam propagating in an arbitrary direction, and we proceed to investigate the cooling and trapping of Mg+ ions in sets of pairs of counter-propagating twisted beams in two-dimensional and three-dimensional molasses configurations.

John Maddox

Anonymous

A. R. Carter, M. Babiker

We explain how a proper specification of the spatial field distribution and angular momentum content of a twisted light beam propagating in an arbitrary direction can be made using appropriate transformations. Such transformations are needed whenever a light beam is to be combined with other similar beams propagating in specified directions. The transformation procedure greatly facilitates the evaluations of the radiation forces acting on atoms subject to an arbitrary set of twisted light beams. It allows atomic trajectories in one-, two-, and three-dimensional optical molasses configurations to be correctly evaluated. We find that multiple twisted beams offer a flexible means of controlling the motion of atoms, but there are notable features attributable only to the orbital optical angular momentum property of the twisted light. Under suitable conditions, atoms can be decelerated and ultimately made to congregate in specific regions of space. The implications of this for all-optical atomic cooling and trapping using twisted light are pointed out and discussed.

A. R. Carter, M. Babiker, M. Al-Amri +1 more

We propose a scheme for a viable and highly flexible all-optical atomic cooling and trapping using twisted light. In particular, we explain how one-dimensional twisted optical molasses should lead to a microscale atomic ring or a picoampere ionic current. Two-dimensional and three-dimensional molasses lead, respectively, to the creation of atom or ion loops and discrete atom clusters positioned at the eight corners of a microcube. These features at the microscale should find applications in physics and in quantum information processing using optically trapped atoms and ions.

Andreas Osterwalder