Light propagation in ultracold atomic gases [Elektronische Ressource] / vorgelegt von Stefan Rist

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• Physik

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• Publié par: universitat_ulm
• Publié le: 01 janvier 2010
• Nombre de lectures: 21
• Langue: English
• Poids de l'ouvrage: 4 Mo

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Universitat Auto`noma de Barcelona Universita¨t Ulm
Grup d’Optica Abteilung fu¨r Quantenphysik
Directora de la Tesis: Betreuer der Doktorarbeit:
Prof. Dr. Giovanna Morigi Prof. Dr. W. P. Schleich
Light Propagation
In Ultracold Atomic Gases
Tesis doctoral del Dissertation
Departament de F´ısica zur Erlangung des Doktorgrades
de la Dr. rer. nat.
Universitat Auto`noma de Barcelona der Fakulta¨t der Naturwissenschaften
der Universita¨t Ulm
vorgelegt vonpresentat pel
Stefan Rist
￿ [e-mail]￿
nascut a Donaueschingen. aus Donaueschingen.
Barcelona/Ulm 2010That was a memorable day to me, for it made great changes in me.
But, it is the same with any life. Imagine one selected day struck out
of it, and think how different its course would have been. Pause you
who read this, and think for a moment of the long chain of iron or
gold, of thorns or flowers, that would never have bound you, but for
the formation of the first link on one memorable day.
Charles Dickens (Great Expectations)
The following information concerns the rules of the university of Ulm:
Amtierender Dekan: Prof. Dr. Axel Groß
Erstgutachter: Prof. Dr. Giovanna Morigi
Zweitgutachter: Prof. Dr. Wolfgang Peter Schleich
Tutor bei UAB: Prof. Dr. Gaspar Orriols
Tag der Promotion: 02.02.2011Contents
Introduction 1
1 Quantum Field Theory of Atoms Interacting with Photons 5
1.1 Classical Hamiltonian . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2 Quantum Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2.1 Atomic collisions . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2.2 Hamiltonian in the length gauge . . . . . . . . . . . . . . . . . . 11
1.2.3 Second quantization . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2 Ultracold Atoms in Optical Lattices 19
2.1 Optical lattices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2 Single particle dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.3 Bose-Hubbard Hamiltonian . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.3.1 Mean-Field Treatment . . . . . . . . . . . . . . . . . . . . . . . . 29
2.3.2 Bogoliubov Expansion . . . . . . . . . . . . . . . . . . . . . . . . 33
2.3.3 Particle-Hole Expansion . . . . . . . . . . . . . . . . . . . . . . . 36
2.3.4 Random phase approximation . . . . . . . . . . . . . . . . . . . . 38
3 Photonic Band Structure of a Bichromatic Optical Lattice 47
3.1 Theoretical model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.1.1 Hamiltonian . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.1.2 Weak excitation regime . . . . . . . . . . . . . . . . . . . . . . . 50
3.1.3 Spin waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.2 Monochromatic optical lattice . . . . . . . . . . . . . . . . . . . . . . . . 52
3.3 Biperiodic optical lattice . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.4 Bichromatic lattice inside a single-mode cavity . . . . . . . . . . . . . . 58
3.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4 Light Scattering from Ultracold Atoms in an Optical Lattice 65
4.1 The Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.2 Light scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
iii CONTENTS
4.2.1 Scattering cross section as a function of the atomic state . . . . . 70
4.2.2 Numerical results . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
4.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
5 Atomic homodyning 81
5.1 The Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
5.2 Scattered light intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5.2.1 Background contribution . . . . . . . . . . . . . . . . . . . . . . 88
5.2.2 Interference contribution . . . . . . . . . . . . . . . . . . . . . . . 91
5.3 A related experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
5.4 Measuring the atomic field . . . . . . . . . . . . . . . . . . . . . . . . . . 101
5.4.1 Temperature Measurement of a Bose-Einstein condensate . . . . 105
5.4.2 Measurement of the superfluid order parameter . . . . . . . . . 108
5.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
6 Summary and Outlook 115
Appendix 117
A Derivation of Eq. (5.15) 119
B Thermodynamics of trapped Bose-Einstein condensates 121
B.1 Non interacting case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
B.2 Interacting case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
C Derivation of Eq. (5.56) 127
Bibliography 131
List of Publications 143
Acknowledgements 145Introduction
The wave nature of ultracold matter is perhaps most spectacularly visible in atomic
physics experiments, where atomic gases can be prepared at ultralow temperatures,
such that the De Broglie wave length becomes of the order of the interparticle distance.
This regime where the quantum statistics of the atoms becomes relevant has been first
reached experimentally with the achievement of Bose-Einstein condensation in weakly
interacting atomic gases in 1995 [1, 2, 3]. These experiments can be considered as
benchmark, starting the field of ultracold atoms.
In the subsequent years the main focus of the research of ultracold atomic gases
was on exploring the properties of atomic Bose-Einstein condensates. Prominent exam-
ples are the interference of two condensates [4], the measurement of long-range phase
coherence [5] and the observation of quantized vortices [6, 7, 8] in a Bose-Einstein con-
densate. We remark here that there are close optical analogues for these experiments,
such as the observation of interference of light from two independent sources [9, 10] or
the occurrence of optical vortices in the modal structure of some lasers [11].
Drawing on these studies, increasing attention has been devoted to atomic gases
with strong correlations, in the regime in which the many-body dynamics essentially
determines the system properties. This has been achieved for instance with optical
lattices. It was shown theoretically by Jaksch et al. in 1998 [12] that the lowest lying
excitations of ultracold bosonic atoms in a dispersive optical lattice can be described by
theBose-HubbardHamiltonian. TheimplementationoftheBose-HubbardHamiltonian
inanopticallatticeandtheobservationofthequantumphasetransitionfromsuperfluid
to Mott insulator phase has been first reported by Greiner et al. in 2002 [13]. It opened
the door to study ultracold atomic gases in a regime that is typically studied in solid
state and condensed matter physics and constitutes a benchmark in the research with
ultracold atomic gases. Further reading about the progress so far in the research of
ultracold atomic gases can be found in [14, 15] and references therein.
A crucial point for comparing experiment with theoretical predictions is the char-
acterization of the many-body state of the atoms. For ultracold atoms detection of the
quantum state is often performed by releasing the atoms from the confining region and
measuring their density distribution after the time of flight to the detecting region by
absorptionimaging. Forsufficientlylongtimeofflightthemeasureddensitydistribution
isproportionaltotheinitialmomentumdistributioninthetrap[14]. Beyondmeasuring2 Introduction
the initial momentum distribution several experimental techniques allow one to extract
distinct physical quantities characterising the quantum state of the atoms. Applying
a Bragg-pulse initially leads to a well defined momentum and energy transfer to the
atoms and permits the study of collective excitations of the system [16, 17, 18, 19],
thereby measuring the structure form factor, which is essentially the Fourier transform
of the density-density correlation function of the gas [20, 21, 22]. The single-particle
correlation function of the atomic gas can be determined by spatially resolved outcou-
pling of atoms from the trap and measuring the resulting interference pattern [5]. This
technique has been used to study the emergence of off diagonal long range order in
Bose-Einstein condensation [23]. It was also shown theoretically that one may be able
tomeasurethesuperfluidfractionofanultracoldatomicgasbyapplyingalightinduced
vector potential to simulate rotation [24].
Further measurement techniques based on the so called noise correlation measure-
mentstakeintoaccountquantumfluctuationsbycomparingabsorptionimagesofdiffer-
ent experimental runs [25]. Applying noise correlation measurements to the absorption
image of two interfering identical atomic samples it has been shown that one can com-
pletely determine the quantum state of the atomic system for one and two dimensional
Bose-liquids[26,27,28]. Ingeneraltheyhaveproventobeavaluabletoolforextracting
additional information about the many-body state of the atoms [29, 30, 31].
Despite the vast amount of information that can be obtained about the atomic
systems using absorption imaging after time of flight, the main drawback is its de-
structive character. An alternative approach to determine the quantum state of the
ultraco