Dipole force free optical control and cooling of nanofiber trapped atoms

Abstract

The evanescent field surrounding nanoscale optical waveguides offers an efficient interface between light and mesoscopic ensembles of neutral atoms. However, the thermal motion of trapped atoms, combined with the strong radial gradients of the guided light, leads to a time-modulated coupling between atoms and the light mode, thus giving rise to additional noise and motional dephasing of collective states. Here, we present a dipole force free scheme for coupling of the radial motional states, utilizing the strong intensity gradient of the guided mode and demonstrate all-optical coupling of the cesium hyperfine ground states and motional sideband transitions. We utilize this to prolong the trap lifetime of an atomic ensemble by Raman sideband cooling of the radial motion which, to the best of our knowledge, has not been demonstrated in nano-optical structures previously. This Letter points towards full and independent control of internal and external atomic degrees of freedom using guided light modes only. Light guided by nano-optical waveguide and resonator structures propagates partly as an evanescent wave; its tight subwavelength confinement allows for strong interactions between guided light and single atoms [1][2][3][4][5] or atomic ensembles [6][7][8][9][10][11][12][13] trapped within the confined field.The inherent intensity gradients of evanescent modes are a necessity for the realization of dipole traps close to the surface of the structure. However, if the atoms are probed or manipulated also by evanescent modes, the gradients lead to detrimental effects, such as time-dependent coupling for moving atoms, additional quantum partition noise in probing atomic ensembles, and motional dephasing of collective internal quantum states. As strong gradients imply strong dipole forces for any Stark shift induced by guided light [14], a scheme for optical manipulation of the internal degrees of freedom without perturbation of the motional state is desirable.Additionally, any non-zero temperature above the motional quantum ground state potentially decreases the average interaction of atoms with the guided light mode, reducing the single atom optical depth.Previous results for addressing these challenges in the nanofiber platform [15] include microwave cooling of the azimuthal degree of freedom [16] by exploiting the state dependency of the trapping potentials for different Zeeman sub-states [17], as well as polarization gradient cooling [7].In this Letter, we present a Raman coupling scheme that allows us to drive coherent transfers on the hyperfine transition in cesium (Cs), as well as radial motional sideband transitions, while canceling all quadratic ac-Stark shifts and, thus, dipole forces. By driving Raman transitions with a single beam propagating through the waveguide, we implement a cooling protocol that relies on the gradient of the coupling strength, rather than its phase [18].A key ingredient in our experimental implementation is the Stark shift canceling the Raman coupling scheme presented in Fig. 1(a): w...

Membrane MOT: Trapping Dense Cold Atoms in a Sub-Millimeter Diameter Hole of a Microfabricated Membrane Device

“…In particular, efficient atom loading nearby the nanostructure is crucial for positioning and coupling many atoms, not a single atom, to the evanescent field of the guided mode. In so doing, we can connect the success efficient two-color evanescent-field atom trapping with nanofibers [49][50][51][52] to the unique features and possibilities of the photonic ATIP. Underpinning our approach is our introduction of a MOT produced in a sub-millimeter diameter hole on a microfabricated transparent membrane.…”

Measurement and simulation of atomic motion in nanoscale optical trapping potentials

“…Due to the strong spatial gradient of any light field that propagates through the nanofiber (see Figure 1(b)), it is important that the light used to monitor the atomic motion does not itself exert a force on the atoms. In [14], a dipole-force free scheme for driving Raman transitions in a nanofiber trap is introduced. We use a single phasemodulated beam, co-propagating with the probe light, to drive coherently Raman transitions.…”

Heating in Nanophotonic Traps for Cold Atoms

“…Frequency equation Figure 5 shows that there are three fundamental bands without a finite minimum frequency: L 01 , T 01 , and F 11 . Nanofiber-based cold atom traps have trap frequencies on the order of 100 kHz [5,9,24,74] as we discuss in section II, so only modes on the fundamental bands can resonantly couple to the atoms for typical parameters of the nanofiber. The fundamental bands are therefore of special importance in this article, and we provide approximate expressions for the dispersion relations and displacement fields in the low-frequency limit.…”