Investigation of dipole blockade inultracold atomic ensembles

Owing to new available technologies, new research fields become available to scientists. Within the area of atomic physics, the ultracold gas technologies open new perspectives in the investigation of the regimes with strong interactions between atoms. A wide range of applications is associated with the manipulation in different configurations of ultracold atoms excited to highly excited states. Rydberg atoms have been intensively studies since nineteen seventy and due the merge of this field with ultracold atomic physics, cold Rydberg atoms brought several concepts to a new light. Rydberg atoms are known to exhibit strong dipolar moments hence flexible investigations of strong interactions between excited atoms become an interesting subject. For example it becomes possible to investigate interactions between individual particles. One of the implications of the strong interactions between Rydberg atoms is the phenomenon denominated as dipole blockade, that has been recently studied both theoretically, for instances, and experimentally for different atomic ensembles. This concept is presented in the context of atomic clocks, quantum computation, quantum cryptography and quantum information. Further research on the Rydberg-Rydberg interactions has produced collective coherent excitations and a non-linear dependence on the number of excited atoms as a function of intensity of the irradiation lasers and atomic density. In order to achieve the quantum computation targets, the implementation of quantum protocols consisting of a sequence of quantum gates requires an experimental realization of a quantum bit. As Rydberg atoms exhibit long range interactions they were proposed to be suitable candidates for realization of controllable quantum system. Yet, the implementation of a quantum gate is an ambitious task where a coherent manipulation of a great number of coupled states is needed. For this reason it was proposed that Rydberg excitations should be performed in samples with a precise spatial order. As a solution atoms in optical lattices were proposed. Already a strong interest in optical lattices as a tool of investigation of ultracold atoms or as a tool of addressing the atoms has been showed. The combination of the strong interactions between Rydberg atoms with the possibility of implementing a spatial order that will simplify coherent manipulation of a large number of coupled states provide us with exciting applications for the quantum information. The main goal of my thesis is to present new results on the excitation of Rydberg atoms from ultracold atomic samples obtained during my work as. The dipole blockade and its experimental implications without and with the presence of optical lattices are presented. This thesis shows results on Rydberg excitations of rubidium Bose condensates in one-dimensional periodic potentials. The coherent excitation dynamics of up to 30 Rydberg states in a condensate occupying around 100 sites of an optical lattice was observed. The zero-dimensional character of the system, in which at most one Rydberg excitation is present per lattice site, is ensured by expanding the condensate in a cylindrical trap in which the radial size of the atomic cloud is much less than the blockade radius of the Rydberg states with 55 < n < 80. This thesis encompasses six chapters and is organized as follows: - Chapter 2 is an introduction to general ideas of atomic physics. It introduces basic theoretical concepts such as the light shift or the three level atom. These concepts have applications while performing our experiments. Moreover, a short characterization of Rydberg atoms and their properties is included. The most important topic of this thesis the Rydberg dipole blockade is also described. - Chapter 3 describes the theoretical background of Bose Einstein condensation. The first part presents the trapping and cooling methods used in experimental realization of ultracold atoms. The second part of the chapter gives an overview of cold atoms in periodic potentials. It presents also how to realize a periodic potential by light interference. - Chapter 4 is devoted to experimental setup. The experimental apparatus of the Pisa BEC laboratory is described. An accurate description of the experimental procedure applied to reach the BEC quantum phase in dilute gases is included. This chapter also includes information about excitation of Rydberg atoms and the implementation of periodic potentials. The setup used in this thesis to prepare cold atomic samples was build at the end of last century and described in detail in the PhD thesis of Donatella Ciampini. The part of the setup used to create optical lattices was implemented and presented in the PhD thesis’s of Alessandro Zenesini and Carlo Sias . Therefore those setups will be not described with full details. The part of setup dedicated to Rydberg excitations was mainly build during my PhD work and is first described in details in the present thesis and also reported in Viteau et al. - Chapter 5 describes the characterization of experimental parameters such as the Rabi frequencies and the efficiency of the ion detection. The first results obtained on the photoionization of Bose Einstein condensates and cold atoms in magneto-optical traps are also shown. Moreover, the effects of excitation lasers and electric fields on the atomic ensembles are mentioned. All the material contained in this chapter, and in the following ones, is the result of my original research work, in collaboration with other members of the BEC team. -Chapter 6 presents our results on the excitations of Rydberg atoms from both BEC samples and cold atoms trapped in a MOT. The experiments exploring different BEC density regimes and their influence on the ion production are presented. A new method of estimating the dipole blockade radius is examined. The further part of chapter 5 reports the temporal dependence of the detected ions on the duration of the irradiating laser pulse. Results for different atomic density regimes and different quantum number n are shown. Clear signatures of sub-Poissonian counting statistics in the regime of strong interactions have been measured. -Chapter 7 is devoted to description of the cold Rydberg atoms experiments performed within the periodic potential of optical lattices. The influence of the Rydberg excitations in optical lattices on the phase coherence of a Bose Einstein condensate is examined. The coherent excitations of the ultracold atomic samples are described. The influence of atomic distributions is also taken into account. A new method of spatial distribution cleaning is presented.