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