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Superradiance and its implementation in cold atoms inside a hollow-core waveguide

Zhenghao Ding

Published May 23, 2019• 0 citations
OpenAlex

Abstract

In this thesis, I am intending to understand the cooperative effect of an ensemble of quantum emitters, which constitutes the preliminary elements of our current experimental investigations towards realization of an ultra-narrow linewidth superriant laser. In the first part of the thesis, I investigate the basics of the theory of superradiance (SR), which includes the full derivation of the Hamiltonian and the Lindblad equation for an ensemble of two-level atoms in both free-space and a single-mode waveguide. In addition, I construct the simulations for observing the transition from single-atom uncorrelated spontaneous emission to superradiance in various physical settings, as well as a simulation for the understanding of the cooperative effects of an ensemble of two-level atoms inside an optical cavity. Then, in the second part of the thesis, I introduce the experimental progress we have been making to observe SR with an ensemble of laser-cooled Cs atoms inside a hollow-core photonic crystal fiber (HCPCF). In our experiment, the Cs atoms, initially cooled using a magneto-optical trap (MOT), are guided and confined inside a short piece of HCPCF with a magic-wavelength dipole trap. Currently we have successfully implemented a novel detection methods for studying superradiance.

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In this thesis I describe experimental work and present data on the guiding of \nRubidium atoms along free-space propagating light beams as well as within hollow \ncore glass fibres, namely photonic crystal fibres. I describe experiments, laser systems \nand vacuum trap assemblies designed to facilitate this guiding. \nThese experiments are intended to aid progression within the field of cold \natom guidance wherein narrow diameter, long distance hollow-fibre guides are a \ncurrent goal. Realisation of these guides could lead to promising applications such as \natom interferometers and spatially accurate, multi-source, atom depositors. \nHerein, guided fluxes are observed in free-space guiding experiments for \ndistances up to 50mm and up to 10GHz red-detuning from resonance. Additionally \nhollow-core, Kagome structured, quasi- and true-photonic crystal fibres are \ncharacterised. Finally a number of detailed fibre-guiding magneto-optic traps are \ndeveloped. \nBoth cold atomic-beams and cold atomic clouds are reliably positioned above \nfibre entrance facets in conjunction with a guiding laser beam coupled into the fibre \ncore. \nIssues regarding optical flux detection outwith fibre confinement appear to \nhave hindered observation of guided atoms. A far more sensitive detection system has \nbeen developed for use in current, ongoing fibre-guide experiments.

•Jan 1, 2007•0 citations
In this thesis, I am intending to understand the cooperative effect of an ensemble of quantum emitters, which constitutes the preliminary elements of our current experimental investigations towards realization of an ultra-narrow linewidth superriant laser. In the first part of the thesis, I investigate the basics of the theory of superradiance (SR), which includes the full derivation of the Hamiltonian and the Lindblad equation for an ensemble of two-level atoms in both free-space and a single-mode waveguide. In addition, I construct the simulations for observing the transition from single-atom uncorrelated spontaneous emission to superradiance in various physical settings, as well as a simulation for the understanding of the cooperative effects of an ensemble of two-level atoms inside an optical cavity. Then, in the second part of the thesis, I introduce the experimental progress we have been making to observe SR with an ensemble of laser-cooled Cs atoms inside a hollow-core photonic crystal fiber (HCPCF). In our experiment, the Cs atoms, initially cooled using a magneto-optical trap (MOT), are guided and confined inside a short piece of HCPCF with a magic-wavelength dipole trap. Currently we have successfully implemented a novel detection methods for studying superradiance.
This thesis explores hollow-core fibres as a platform for quantum optics experiments with laser-cooled atomic ensembles. The non-diffracting, tightly-confined guided modes of these fibers grant us a ~µm-wide one-dimensional space to study atom-light interactions. In order to describe on-going experiments, simulations are carried out to understand atomic motion into the hollow fibers. Following which, a preliminary case study of a quantum optics experiment to convert wavelengths of single photons with Cs atomic ensembles inside the hollow fiber is presented. Lastly, basic optical properties of photonic crystal membranes are briefly explored. These can form novel cavities when appended to hollow fibers.
Our goal is to study quantum optical phenomena in a strong nonlinear optical system. For this purpose, we create a hollow-core photonic-crystal (HCPC) fiber-based platform that confines photons and atomic ensembles together. This enables strong light-matter interactions which are crucial for implementing optical nonlinearities at single-photon levels [1]. We use an optical dipole trap at cesium's magic wavelength to guide laser cooled cesium atoms into and inside the fiber core. This approach will allow us to perform experiments with the dipole trap continuously on as opposed to the previously reported fiber-loading systems [2,3] in which the dipole trap was rapidly amplitude-modulated to avoid the effect from the spatially dependent AC Stark shift. In this system, we perform experiments on electromagnetically induced transparency (EIT) and demonstrate slow light and all optical switch. We also aim to observe large cross-phase modulation (XPM) using a three-level ladder scheme [4,5]. Lastly, the approaches to integrate a cavity into HCPCFs to increase light-matter interactions, thus the optical nonlinearities, will be described.
Cold atoms confined inside a hollow core photonic crystal fiber with core diameter of a few photon wavelengths are a promising medium for studying nonlinear optical interactions at extremely low light levels. The high electric field intensity per photon, large optical depths, and interaction lengths not limited by diffraction are some of the unique features of this system. We describe recent progress in our experiment that uses a combination of magnetic trapping and a red-detuned optical dipole trap to transfer cold rubidium atoms into the hollow-core fiber. We present data on transfer efficiencies as well as preliminary experiments towards nonlinear optics with few photon pulses.
The transversely confined propagating modes of an optical fiber mediate virtually infinite range energy exchanges among atoms placed within their field, which adds to the inherent free space dipole–dipole coupling. Typically, the single atom free space decay rate largely surpasses the emission rate into the guided fiber modes. However, scaling up the atom number as well as the system size amounts to entering a collective emission regime, where fiber-induced superradiant spontaneous emission dominates over free space decay. We numerically study this super- and subradiant decay of highly excited atomic states for one or several transverse fiber modes as present in hollow core fibers. As particular excitation scenarios we compare the decay of a totally inverted state to the case of π /2 pulses applied transversely or along the fiber axis as in standard Ramsey or Rabi interferometry. While a mean field approach fails to correctly describe the initiation of superradiance, a second-order approximation accounting for pairwise atom–atom quantum correlations generally proves sufficient to reliably describe superradiance of ensembles from two to a few hundred particles. In contrast, a full account of subradiance requires the inclusion of all higher order quantum correlations. Considering multiple guided modes introduces a natural effective cut-off for the interaction range emerging from the dephasing of different fiber contributions.
Cold atoms trapped and guided in hollow-core photonic crystal fibers provide a scalable diffraction-free setting for atom–light interactions for quantum technologies. However, due to the mismatch of the depth and spatial extension of the trapping potential from free space to the fiber, the number of cold atoms in the fiber is mainly determined by the loading process from free space to waveguide confinement. Here, we provide a numerical study of the loading dynamics of cold atoms into a hollow-core photonic crystal fiber. We use the Monte Carlo method to simulate the trajectories of an ensemble of cold atoms from free space trapping potential to optical potential inside a hollow-core fiber and calculate the temperature, loading efficiency, and geometry of the ensemble. We also study the noise sources that cause heating and a loss of atoms during the process. Our result could be used to design and optimize the loading process of cold atoms into a hollow-core fiber for cold atom experiments.
In this thesis I describe experimental work and present data on the guiding of \nRubidium atoms along free-space propagating light beams as well as within hollow \ncore glass fibres, namely photonic crystal fibres. I describe experiments, laser systems \nand vacuum trap assemblies designed to facilitate this guiding. \nThese experiments are intended to aid progression within the field of cold \natom guidance wherein narrow diameter, long distance hollow-fibre guides are a \ncurrent goal. Realisation of these guides could lead to promising applications such as \natom interferometers and spatially accurate, multi-source, atom depositors. \nHerein, guided fluxes are observed in free-space guiding experiments for \ndistances up to 50mm and up to 10GHz red-detuning from resonance. Additionally \nhollow-core, Kagome structured, quasi- and true-photonic crystal fibres are \ncharacterised. Finally a number of detailed fibre-guiding magneto-optic traps are \ndeveloped. \nBoth cold atomic-beams and cold atomic clouds are reliably positioned above \nfibre entrance facets in conjunction with a guiding laser beam coupled into the fibre \ncore. \nIssues regarding optical flux detection outwith fibre confinement appear to \nhave hindered observation of guided atoms. A far more sensitive detection system has \nbeen developed for use in current, ongoing fibre-guide experiments.