Optimized coupling of cold atoms into a fiber using a blue-detuned hollow-beam funnel

Abstract

We theoretically investigate the process of coupling cold atoms into the core of a hollow-core photonic-crystal optical fiber using a blue-detuned Laguerre-Gaussian beam. In contrast to the use of a red-detuned Gaussian beam to couple the atoms, the blue-detuned hollow-beam can confine cold atoms to the darkest regions of the beam thereby minimizing shifts in the internal states and making the guide highly robust to heating effects. This single optical beam is used as both a funnel and guide to maximize the number of atoms into the fiber. In the proposed experiment, Rb atoms are loaded into a magneto-optical trap (MOT) above a vertically-oriented optical fiber. We observe a gravito-optical trapping effect for atoms with high orbital momentum around the trap axis, which prevents atoms from coupling to the fiber: these atoms lack the kinetic energy to escape the potential and are thus trapped in the laser funnel indefinitely. We find that by reducing the dipolar force to the point at which the trapping effect just vanishes, it is possible to optimize the coupling of atoms into the fiber. Our simulations predict that by using a low-power (2.5 mW) and far-detuned (300 GHz) Laguerre-Gaussian beam with a 20-µm radius core hollow-fiber it is possible to couple 11% of the atoms from a MOT 9 mm away from the fiber. When MOT is positioned further away, coupling efficiencies over 50% can be achieved with larger core fibers.

Loading and spatially-resolved characterization of a cold atomic ensemble inside a hollow-core fiber

“…These results show that the atomic number density distribution is also in our HCPBGF well-represented by Eq. ( 2), i.e., the density is highest on the fiber axis as observed in [27], in contrast to other predictions [14,49], which favored ring-shaped distributions.…”

High-efficiency cold-atom transport into a waveguide trap

“…The atomic coherence time will be limited by the faster of two effects: decoherence induced by interaction between trapped atoms and the optical guide field, or the loss of atoms. Photon scattering in the current experiment limits the atomic coherence time to around 200 µs, although this rate could be much reduced by replacing the attractive guide with a hollow repulsive guide mode [48]. This trap geometry would confine atoms predominantly within the low intensity region, vastly reducing the scattering rate.…”

Engineering Photon-Photon Interactions within Rubidium-Filled Waveguides

“…These techniques, however, do not extend the coherent interaction time with an optically excited state: in this case one requires no collisions with other atoms or the fiber wall. One solution to extend excited state coherence is to reduce the average motion of the atoms, pointing towards laser-cooled atoms as a route to create strong photon-photon interaction mediated by light-atom interactions in waveguide geometries [8,19,35,51,52].…”