The first year of the project was spent on the completion of the experimental apparatus for the investigation of the Bragg stucture. The project was significantly delayed by an unexpected extension of the lead time of an essential vacuum component by 6 months. We took advantage of this delay to conclude a theoretical investigation of the force acting on an atom that undergoes spontaneous decay in the vicinity of a nanofiber. Surprisingly, the force breaks the translational symmetry along the axis of the nanofiber and is one of few examples of a lateral Casimir-Polder force. The results were published (Scheel at al., Phys. Rev. A 92, 43819, 2015). In addition, a different experimental apparatus was used to investigate the effect of so-called ficititious magnetic fields on nanofiber-trapped atoms. As can be shown from Maxwell's equations, such fields are in general present when the optical intensity varies significantly on the length scale of the optical wavelength. This is, for example, the case in the evanescent field surrounding the nanofiber. We showed that this effect can be exploited to probe and manipulate the motional state of nanofiber-trapped atoms using microwaves, and demonstrated cooling of the atoms to the motional ground state (Albrecht et al., Phys. Rev. A 94, 61401, 2016).
When the missing vacuum part arrived, the new experimental apparatus was completed and characterized. In view of the time lost, we decided to not add the experimental complication of an additional trapping laser for tuning the spacing of the atoms into Bragg resonance. Instead, we decided to make use of the periodicity imposed by the existing, fiber-guided trap lasers. Also here one can speak of a resonance, which manifests itself in the collective scattering of fiber-guided light by the atomic array into a specific angle with respect to the fiber axis. Hence, the light is emitted into a cone around the nanofiber, the exact emission pattern depending on the specific characteristics of the atomic ensemble. Theoretical calculations regarding this effect have been presented at various conferences. The experimental search for this collective scattering was hindered by a steady degradation of the experimental conditions, which eventually could be attributed to the hitherto unreported formation of cesium salt on the nanofiber. The exact conditions that enabled this formation are not understood. We recovered good experimental conditions by replacing the nanofiber, and results on the collective scattering are imminent. As a by-product we obtained record-high numbers of nanofiber-trapped atoms, which also results in extreme optical depths. Also these results will soon be published.
