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Nanofiber-based atomic Bragg structures

Periodic Reporting for period 1 - NanoBragg (Nanofiber-based atomic Bragg structures)

Reporting period: 2015-09-01 to 2017-08-31

In recent years, first devices that harness the effects of quantum physics for improved metrology and information processing have been succesfully commercialized, and multinational corporations have committed significant resources to quantum research. The technologies and methods employed are as diverse as potential applications. For optical quantum information processing, technologies based on optical fibers are particularly attractive, because they promise simple interconnection. At the TU Wien, we investigate quantum interfaces based on arrays of laser-cooled atoms coupled to the evanescent field of light that is efficiently guided in the waist of a tapered optical fiber. The waist diameter is only few hundred nanometers. These nanofiber-trapped atoms are extremely promising for quantum information processing and communication. The aim of this project was to begin the development of specialized nanofiber protocols that take advantage of the unique properties of this system by tuning the periodicity of the atomic array into Bragg resonance. The scattering of light by the atoms is then enhanced by coherent collective effects leading, for example, to atomic Bragg mirrors where an ensemble of only few thousand atoms reflects close to 100% of fiber-guided light.

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.
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.
The research that was conducted during the course of the project managed to surpass the state of the art on three fronts. A theoretical study of the spontaneous emission of a suitably-excited atom close to an optical nanofiber revealed a translationally invariant lateral scattering force. In contrast to all lateral forces that had previously been studied in the context of Casimir and Casimir-Polder physics, this force relies neither on a corrugation of the material surface, nor on higher-order interaction between electrically and magnetically induced dipoles. The magnitude and sign of the force can be controlled via the initial quantum state of the atom. We expect that this lateral force will have implications for laser cooling of atoms close to surfaces and nanophotonic structures.

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.
Illustration of the situation for an atom next to a nanofiber experiencing a lateral force.