Cold atom physics using ultra-thin optical fibres

In this thesis I present experiments concerning the investigation and manipulation of cold neutral atoms using ultra-thin optical fibres with a diameter smaller than the wavelength of the guided light. In such a fibre-field configuration the guided light exhibits a large evanescent field that penetrates into the free-space surrounding the fibre thus enabling to couple laser cooled atoms to the fibre mode. By trapping and cooling caesium atoms in a magneto-optical trap formed around the fibre I investigated the interaction of the atoms with the evanescent field at sub-micrometre distances from the fibre surface. Chapters 1 and 2 provide the theoretical foundations of this work. Chapter 1 describes the propagation of light in optical fibres. The general solution of the Maxwell's equations in the fibre that complements the description is provided in Appendix A. In Chapter 2, the theory of the interaction of atoms with time-varying electric fields is described. In Chapter 3 the resonant interaction of laser cooled caesium atoms with the evanescent field of a probe laser launched through a 500-nm diameter fibre is studied. A detailed analysis of the atomic absorption at sub-micrometre distances from the fibre surface is given. I have performed Monte-Carlo simulations of atomic trajectories inside the cold atom cloud surrounding the fibre. From the simulations, the atomic density at the vicinity of the fibre is deduced and the absorbance profiles of the atoms measured during the experiments can be modelled. By carefully investigating the linewidths of these profiles, clear evidence of dipole forces, van der Waals interaction, and a significant enhancement of the spontaneous emission rate of the atoms is found. The atomic spontaneous emission into the guided mode of a 500-nm diameter optical fibre is the focus of Chapter 4. Here, I show that the fibre can be used as an efficient tool to collect and guide the spontaneous emission of the atoms. The dipole force induced by the evanescent field on the atoms is the central idea of the experiments performed in Chapter 5. I have built a new version of the experimental setup that opens the route towards atom trapping in the evanescent field in an array of surface microtraps around the fibre. Such traps are created by the combination of two laser fields with opposite sign of the detuning with respect to the excitation frequency of the atoms. The first experimental results reporting the influence of the two-colour evanescent field on the spectral properties of the atoms are presented.Ultradünne Glasfasern zur Manipulation kalter Cäsiumatome Die vorliegende Arbeit berichtet über Experimente zur Untersuchung und Manipulation kalter neutraler Atome mittels ultradünner Glasfasern, deren Durchmesser kleiner als die Wellenlänge des geführten Lichts ist. In dieser Konfiguration besitzt die geführte Fasermode ein starkes evaneszentes Feld, das in den freien Raum um die Faser hineinreicht und damit die Kopplung kalter Cäsiumatome ermöglicht. Die Kapitel 1 und 2 liefern die theoretischen Grundlagen dieser Dissertation. In Kapitel 1 werden die Eigenschaften der Lichtleitung in Glasfasern beschrieben. Die allgemeine Lösung der Maxwellgleichungen in der Faser liefert Anhang A. In Kapitel 2 wird die Theorie der Wechselwirkung neutraler Atome mit einem zeitabhängigen elektrischen Feld behandelt. In Kapitel 3 wird die Wechselwirkung lasergekülter Atome mit dem evaneszenten Feld resonanter Laserstrahlung untersucht, die in einer Glasfaser mit einem Durchmesser von 500~nm geleitet wird. Es wird eine detaillierte Analyse der atomaren Absorption in einer Submikrometerentfernung von der Faseroberfläche präsentiert. Ich habe Monte-Carlo Simulationen von Trajektorien der lasergekühlten Atome durchgeführt. Daraus kann die atomare Dichte in der Nähe der Faser abgeleitet werden. Dies erlaubt die Modellierung der gemessenen atomaren Absorbanzkurven. Eine sorfältige Untersuchung der Linienbreite solcher Absorbanzkurven liefert eine deutliche Signatur lichtinduzierter Dipolkräfte, der van der Waals Kraft und einer Erhöhung der spontanen Emissionsrate der Atome. Die spontane Emission in die Mode einer ultradünnen Glasfaser steht im Zentrum von Kapitel 4. Hier untersuche ich die Effizienz solcher Fasern als Werkzeug zum Sammeln und Übertragen atomarer spontanen Emission. Die durch das evaneszente Feld induzierte Dipolkraft ist der Kerngedanke des Experiments von Kapitel 5. Ich habe eine erweiterte Version des expereimentellen Aufbaus entwickelt, die den Weg zum Fangen von Cäsiumatomen im evaneszenten Feld der Faser geebnet hat. Solche Fallen bestehen aus einer Kombination von zwei Lasern mit entgegengesetzter Verstimmung gegenüber der atomaren Anregungsfrequenz. Zudem werden die ersten Messungen des spektralen Einflusses des zweifarbigen evaneszenten Feldes auf die kalte Atomwolke präsentiert

Cold Atom Physics Using Ultra-Thin Optical Fibers: Light-Induced Dipole Forces and Surface Interactions

The strong evanescent field around ultra-thin unclad optical fibers bears a high potential for detecting, trapping, and manipulating cold atoms. Introducing such a fiber into a cold atom cloud, we investigate the interaction of a small number of cold Caesium atoms with the guided fiber mode and with the fiber surface. Using high resolution spectroscopy, we observe and analyze light-induced dipole forces, van der Waals interaction, and a significant enhancement of the spontaneous emission rate of the atoms. The latter can be assigned to the modification of the vacuum modes by the fiber.Comment: 4 pages, 4 figure

Análisis de la evolución de pacientes con patología cardiovascular e infección por COVID - 19

La infección por Coronavirus 2019 (COVID-19) se caracteriza por una gran variabilidad clínica entre los distintos pacientes. Los factores de riesgo cardiovascular y las cardiopatías son un denominador común que encontramos en un gran número de pacientes con formas de infección grave por COVID-19. Las complicaciones cardiovasculares son frecuentes y suponen una causa importante de mortalidad en los sujetos que las desarrollan. Nuestro objetivo es analizar el impacto de la patología cardiovascular en la evolución de los pacientes con infección por COVID-19.Grado en Medicin

Nanofiber-Based Double-Helix Dipole Trap for Cold Neutral Atoms

A double-helix optical trapping potential for cold atoms can be straightforwardly created inside the evanescent field of an optical nanofiber. It suffices to send three circularly polarized light fields through the nanofiber; two counterpropagating and far red-detuned with respect to the atomic transition and the third far blue-detuned. Assuming realistic experimental parameters, the transverse confinement of the resulting potential allows one to reach the one-dimensional regime with cesium atoms for temperatures of several \muK. Moreover, by locally varying the nanofiber diameter, the radius and pitch of the double-helix can be modulated, thereby opening a realm of applications in cold-atom physics.Comment: 9 pages, 4 figure

Optical interface created by laser-cooled atoms trapped in the evanescent field surrounding an optical nanofiber

Trapping and optically interfacing laser-cooled neutral atoms is an essential requirement for their use in advanced quantum technologies. Here we simultaneously realize both of these tasks with cesium atoms interacting with a multi-color evanescent field surrounding an optical nanofiber. The atoms are localized in a one-dimensional optical lattice about 200 nm above the nanofiber surface and can be efficiently interrogated with a resonant light field sent through the nanofiber. Our technique opens the route towards the direct integration of laser-cooled atomic ensembles within fiber networks, an important prerequisite for large scale quantum communication schemes. Moreover, it is ideally suited to the realization of hybrid quantum systems that combine atoms with, e.g., solid state quantum devices