Ultracold thermal atoms and Bose-Einstein condensates interacting with a single carbon nanofiber [Elektronische Ressource] / vorgelegt von Philipp Schneeweiß

Sujets

• Physik

Informations

• Publié par: eberhard_karls_universitat_tubingen
• Publié le: 01 janvier 2011
• Nombre de lectures: 7
• Langue: English
• Poids de l'ouvrage: 15 Mo

Extrait

Ultracold Thermal Atoms and
Bose-Einstein Condensates Interacting
with a Single Carbon Nano ber
Dissertation
zur Erlangung des Grades eines Doktors
der Naturwissenschaften
der Mathematisch-Naturwissenschaftlichen Fakult at
der Eberhard Karls Universit at Tubingen
vorgelegt von
Philipp Schneewei
aus Potsdam
2011.
Tag der mundlic hen Prufung: 27. Januar 2011
Dekan: Prof. Dr. Wolfgang Rosenstiel
1. Berichterstatter: Prof. Dr. J ozsef Fort agh
2. Berichr: Prof. Dr. Claus ZimmermannAbstract
The present thesis investigates the decay of ultracold atoms from a magnetic trap due
to the interaction with a single carbon nano ber. The latter is spatially overlapping
with the atomic cloud. For both an ultracold thermal cloud and a Bose-Einstein
condensate, the atomic loss has been measured for di erent interaction times and
degrees of cloud-nano ber overlap. Relevant theoretical concepts to analyze the
measurements are derived and applied to the experimental results. For the thermal
as well as the degenerate gas case, the atom loss is signi cantly faster than expected
from the geometry of the nano ber. The experimental data is consistent with an
enhanced atom loss due to an attractive Casimir-Polder force between the nano ber
and the ultracold atoms. Using a power-law approximation, the Casimir-Polder
potential of the nano ber is quantitatively obtained by tting the experimental
data.
Kurzfassung
In der vorliegenden Arbeit wird der Verlust von ultrakalten Atomen aus einer mag-
netischen Falle aufgrund von Wechselwirkungen mit einer Kohlensto -Nanofaser un-
tersucht. Die Nanofaser wird dabei mit der ultrakalten Wolke aumlicr h ub erlappt.
Sowohl fur eine ultrakalte thermische Wolke als auch fur ein Bose-Einstein Kon-
densat ist der Verlust von Atomen fur verschiedene Wechselwirkungszeiten und
Uberlapp-Parameter gemessen worden. Die relevanten theoretischen Konzepte fur
die Analyse der Messungen werden hergeleitet und auf die experimentellen Ergeb-
nisse angewendet. Fur beide F alle, d.h. fur das ultrakalte thermische Gas und fur das
Bose-Einstein Kondensat, auftl der Verlust der Atome signi kant schneller ab als
aufgrund der Geometrie der Nanofaser erwartet. Die experimentellen Daten sind
konsistent mit einem durch attraktive Casimir-Polder Kr afte zwischen Nanofaser
und ultrakalten Atomen verst arkten Verlust. Das Casimir-Polder Potential der
Nanofaser wurde dabei quantitativ, unter Annahme eines Potenzgesetzes fur dessen
Verlauf, bestimmt.
iiiivContents
Contents v
1 Introduction 1
1.1 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Dispersion Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.1 Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.2 Casimir-Polder Potential of (Nano)-Cylinders . . . . . . . . . 5
2 Technical Implementation 9
2.1 Magnetic Trapping of Ultracold Atoms . . . . . . . . . . . . . . . . . 9
2.1.1 Principle of Magnetic Trapping . . . . . . . . . . . . . . . . . 9
2.1.2 On-Chip Waveguide . . . . . . . . . . . . . . . . . . 11
2.2 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2.1 Vacuum System . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2.2 Macroscopic Electromagnets . . . . . . . . . . . . . . . . . . . 16
2.2.3 Carrier Chip . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.4 Nanostructured Chip . . . . . . . . . . . . . . . . . . . . . . . 21
2.2.5 Nanochip and Carrier Chip Assembly . . . . . . . . . . . . . . 23
2.2.6 Laser System . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.3 Preparation of the Measurement . . . . . . . . . . . . . . . . . . . . . 30
2.3.1 Experimental Cycle . . . . . . . . . . . . . . . . . . . . . . . . 30
2.3.2 Center-of-Mass Oscillations . . . . . . . . . . . . . . . . . . . 32
2.3.3 Surface Gauging . . . . . . . . . . . . . . . . . . . . . . . . . 34
3 Measurements 39
3.1t Locations and Shape of the Nano ber . . . . . . . . . . 39
3.2 Decay Dynamics of the Thermal Cloud . . . . . . . . . . . . . . . . . 41
3.2.1 Time-Resolved Atom Loss . . . . . . . . . . . . . . . . . . . . 41
3.2.2 Inelastic Scattering Rates . . . . . . . . . . . . . . . . . . . . 47
3.3 Decay Dynamics of the Bose-Einstein Condensate . . . . . . . . . . . 48
3.3.1 Time-Resolved Atom Loss . . . . . . . . . . . . . . . . . . . . 49
3.3.2 Inelastic Scattering Rates . . . . . . . . . . . . . . . . . . . . 50
4 Theory, Evaluation, and Discussion 53
4.1 Casimir-Polder Potential of a Nano ber . . . . . . . . . . . . . . . . . 53
4.1.1 Dielectric Properties . . . . . . . . . . . . . . . . . . . . . . . 54
vCONTENTS
4.1.2 Hybrid Hamaker - Casimir-Polder Approach . . . . . . . . . . 55
4.2 Atom Loss From a Magnetic Trap . . . . . . . . . . . . . . . . . . . . 56
4.2.1 Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.2.2 Atom Loss and Nanowires / Nanocylinders . . . . . . . . . . . 57
4.3 Loss of Thermal Atoms on a Nano ber:
Theory and Application to Data . . . . . . . . . . . . . . . . . . . . . 58
4.3.1 Capture Radius of a (Nano)-Cylinder . . . . . . . . . . . . . . 58
4.3.2 E ective Radius and Capture Rate . . . . . . . . . . . . . . . 60
4.3.3 Thermal Loss Data and Nano ber Casimir-Polder Potential . 66
4.4 Loss of BEC Atoms on a Nano ber:
Theory and Application to Data . . . . . . . . . . . . . . . . . . . . . 67
4.4.1 Atom Loss on a Nano ber
using the Gross-Pitaevskii Equation . . . . . . . . . . . . . . . 68
4.4.2 Flow of a BEC onto a Nano ber . . . . . . . . . . . . . . . . . 71
4.4.3 BEC Loss Data and
Casimir-Polder-Enhanced Nano ber Capture Radius . . . . . 72
4.5 Discussion of Thermal and BEC Results . . . . . . . . . . . . . . . . 74
5 Summary and Outlook 77
A Appendix 81
A.1 Electric Dipole Forces from Adhered Rubidium . . . . . . . . . . . . 81
A.1.1 Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
A.1.2 Experimental Investigation of Adhered Dipoles . . . . . . . . . 82
A.1.3 Electric Field of Atoms Adhered to a Cylinder Surface . . . . 84
A.2 Shutters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
Bibliography 91
viChapter 1
Introduction
The e ect of zero-point uctuations of the electromagnetic eld has many con-
sequences such as the non-relativistic Lamb shift, spontaneous emission, and the
anomalous magnetic moment of the electron [Mil94]. Another important vacuum
uctuation e ect is dispersion forces whose best known manifestation is the van-der-
Waals force, acting between individual atoms (molecules). This thesis presents the
rst experimental characterization of the dispersive properties of a single nano ber
using ultracold atoms.
Dispersion forces generally give rise to attractive or repulse potentials, acting on neu-
tral atoms, molecules, and macroscopic bodies. The topic has attracted considerable
attention in recent experimental and theoretical physics research [Kli09] for at least
two reasons: the understanding of dispersion forces touches on the foundations of
quantum electrodynamics. Moreover, as dispersion forces become signi cant for ob-
ject separations on the order of a few micrometers and below, they are a central e ect
in several scienti c elds, including cell and colloid physics, material science [Par06],
and the research and technology of nanoobjects [Bhu04].
1Exact measurements of Casimir- and Casimir-Polder (CP) forces are important to
test the di erent dispersion force theories. The rst experiments on the Casimir
force have been made in the 1950ies [Der57] using a glass plate and spherical lenses.
The CP force has been rst experimentally investigated by atomic beam de ection
experiments in the 1970ies [Shi75]. Measurements were extended and improved by
using Rydberg atoms (and their high polarizability) by Anderson et al. [And88].
The rst experimental evidence for retardation e ects in atom-surface interactions
was obtained by Sukenik et al. [Suk93].
The tremendous progress in ultracold atom physics in the recent years has let to
further improvements and extensions of high-precision measurements of CP forces.
Since the rst realization of a Bose-Einstein condensate (BEC) in 1995 [And95,
Dav95a], ultracold atom physics in the proximity of surfaces has developed to an
important branch within the eld. Monitoring the center-of-mass oscillations of a
1In the following, these are the forces originating from the dispersive interaction between two
macroscopic objects (Casimir force), and atoms and macroscopic objects (Casimir-Polder force).
However, there is no unique nomenclature of dispersion force phenomena in the literature.
1CHAPTER 1. INTRODUCTION
BEC in the proximity of a surface, the CP force could be measured with unpreceded
precision [Har05] and CP temperature e ects were demonstrated [Obr07a]. In the
intermediate distance range, the CP potential has been measured using a BEC
and an evanescent wave [Ben10]. A signi cant technological improvement in the
study of atom-surface interactions has been made by the development of microtraps
for atomic ensembles [Vul98, Den98, For98, Den99, M99a, Rei99, H01]. A break-
through in the eld was the creation of a Bose-Einstein condensate in a surface
microtrap [Ott01, H an01], advancing and extending ultracold su