Periodic Reporting for period 1 - NanoBragg (Nanofiber-based atomic Bragg structures)
Reporting period: 2015-09-01 to 2017-08-31
Before the start of the project we conjectured that specialized nanofiber protocols would enable radically new approaches for optical quantum information processing, and Bragg structures would be a first step in this direction. An extraordinary example that this conjecture was correct was given by theoretical work unrelated to this project by A. Asenjo-Garcia et al. (Phys. Rev. X 7, 031024, August 2017), who showed that exponential improvements for optical quantum memories can be achieved.
The objectives of the project were the experimental realization and investigation of an atomic Bragg structure in the vicinity of an optical nanofiber. While this objective could not be achieved during the funding period due to unforeseen experimental difficulties, an experimental apparatus with a record-high number of nanofiber-trapped atoms was realized, and results for collective scattering into a free-space Bragg resonance are imminent. In addition, symmetry-breaking lateral forces on the atoms arising during spontaneous emission were studied theoretically, and novel means to probe and manipulate the motional state of the atoms with the help of the particular polarization properties of nanofiber-guided light were realized.
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.
The second principal result obtained as part of the project was the exploitation of fictitious magnetic fields for probing and controlling the motion of atoms. Fictitious magnetic fields can be used to describe the interaction of with light fields that have strong polarization gradients. In particular, fictitious magnetic fields are naturally present for atoms trapped in strongly focused laser beams and evanescent fields. In the past, these fields were considered as a nuisance, and various techniques had been described to suppress their effects. Our work showed that they can be used to infer trap parameters and for thermometry of laser-cooled atoms via microwave spectroscopy. In addition, we used microwave sideband cooling to cool nanofiber-trapped atoms to the motional quantum ground state for the first time.
The third result, to be published soon, is the achievement of record-high optical depth of 1000 in an ensemble of nanofiber-trapped atoms, surpassing previously published results by one order of magnitude. This extreme optical depth could be realized by optimizing the loading of a 2-cm-long nanofiber-based dipole trap from a cigar-shaped ensemble of laser-cooled atoms. The optical depth is a key parameter for a multitude of protocols for optical quantum information processing, such as quantum memories and quantum switches.