## Tesi etd-12122010-122600 |

Tipo di tesi

Tesi di dottorato di ricerca

Autore

RADOGOSTOWICZ, JAGODA

Indirizzo email

radogost@df.unipi.it

URN

etd-12122010-122600

Titolo

Investigation of dipole blockade inultracold atomic ensembles

Settore scientifico disciplinare

FIS/03

Corso di studi

FISICA APPLICATA

Commissione

**tutor**Prof. Ennio Arimondo

Parole chiave

- Optical lattices
- Qunatum Physics
- BEC

Data inizio appello

2010-12-20

Disponibilità

completa

Riassunto analitico

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.

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.

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