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Coherent matter waves near surfaces [Elektronische Ressource] / presented by Peter Krüger

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166 Pages
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Dissertationsubmitted to theCombined Faculties for the Natural Sciencesand for Mathematicsof the Ruperto-Carola University of Heidelberg,Germanyfor the degree ofDoctor of Natural Sciencespresented byDipl.-Phys. Peter Krug¨ erborn in: Princeton/NJ (USA)thOral examination: July 7 , 2004Coherent matter waves near surfacesReferees: Prof. Dr. J¨org SchmiedmayerProf. Dr. Markus OberthalerZusammenfassungKoh¨arente Materiewellen in Oberfl¨achenn¨aheNeutrale Atome k¨onnen in der N¨ahe von Atomchips auf mikroskopischer Skalagefangen und manipuliert werden. Ein Teil dieser Arbeit besch¨aftigt sich mit derEntwicklung und dem Test mikroskopischer atomoptischer Elemente. Ein rich-tungsunabh¨angigerMateriewellenleiterundneuartigemagneto-elektrischePoten-tiale werden experimentell mit kalten thermischen Lithiumatomen untersucht.W¨ahrend des Aufbaus einer neuen Apparatur fur¨ ultrakalte Rubidiumatomewurde ein neuer Typ einer integrierten magneto-optischen Falle fur¨ die verein-fachte Produktion von Bose-Einstein Kondensaten (BEC) in Oberfl¨achenn¨aheimplementiert. Der Einfluss von Oberfl¨achen-Storp¨ otentialen auf BECs wird un-tersucht. Eine Verringerung st¨orender Effekte um zwei Gr¨oßenordnungen fur¨lithographische im Gegensatz zu galvanischer Chipfabrikation wurde gemessen.

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Published 01 January 2004
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Dissertation
submitted to the
Combined Faculties for the Natural Sciences
and for Mathematics
of the Ruperto-Carola University of Heidelberg,
Germany
for the degree of
Doctor of Natural Sciences
presented by
Dipl.-Phys. Peter Krug¨ er
born in: Princeton/NJ (USA)
thOral examination: July 7 , 2004Coherent matter waves near surfaces
Referees: Prof. Dr. J¨org Schmiedmayer
Prof. Dr. Markus OberthalerZusammenfassung
Koh¨arente Materiewellen in Oberfl¨achenn¨ahe
Neutrale Atome k¨onnen in der N¨ahe von Atomchips auf mikroskopischer Skala
gefangen und manipuliert werden. Ein Teil dieser Arbeit besch¨aftigt sich mit der
Entwicklung und dem Test mikroskopischer atomoptischer Elemente. Ein rich-
tungsunabh¨angigerMateriewellenleiterundneuartigemagneto-elektrischePoten-
tiale werden experimentell mit kalten thermischen Lithiumatomen untersucht.
W¨ahrend des Aufbaus einer neuen Apparatur fur¨ ultrakalte Rubidiumatome
wurde ein neuer Typ einer integrierten magneto-optischen Falle fur¨ die verein-
fachte Produktion von Bose-Einstein Kondensaten (BEC) in Oberfl¨achenn¨ahe
implementiert. Der Einfluss von Oberfl¨achen-Storp¨ otentialen auf BECs wird un-
tersucht. Eine Verringerung st¨orender Effekte um zwei Gr¨oßenordnungen fur¨
lithographische im Gegensatz zu galvanischer Chipfabrikation wurde gemessen.
Der Einfluss thermisch induzierten Stromrauschens auf die koh¨arente Evolution
von Materiewellen in der N¨ahe der Oberfl¨ache wird theoretisch untersucht und
erste experimentelle Ergebnisse werden pr¨asentiert.
Abstract
Coherent matter waves near surfaces
Neutral atoms can be trapped and manipulated on microscopic scales near sur-
faces of atom chips. The work covered by this thesis includes the development
and test of microscopic atom optical tools. An omni-directional magnetic matter
wave guide and novel types of magneto-electric trapping potentials are investi-
gated experimentally with cold thermal lithium atoms.
During the construction of a new setup working with ultracold rubidium atoms,
a novel type of integrated magneto-optical trap for a simplified production of
Bose-Einstein condensates (BEC) near surfaces was implemented. The influence
of surface disorder potentials on BECs is studied. A reduction of disturbing
effects by two orders of magnitude for a lithographic fabrication process over
electroplating is found. The impact of thermal current noise on the coherent
evolution of matter waves near surfaces is investigated theoretically and initial
experimental results are presented.Contents
1 Introduction 1
2 Tools for matter wave manipulation near surfaces 5
2.1 Cold atoms in microtraps. . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Fabrication of atom chips . . . . . . . . . . . . . . . . . . . . . . 7
2.3 Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.4 Magnetic fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.4.1 Wire guides . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4.2 Single wire guides and scaling laws . . . . . . . . . . . . . 17
2.4.3 Two wire guides . . . . . . . . . . . . . . . . . . . . . . . . 19
2.4.4 Multi-wire . . . . . . . . . . . . . . . . . . . . . . . 21
2.4.5 Curved wire guides . . . . . . . . . . . . . . . . . . . . . . 23
2.4.6 Demonstration experiment: the spiral guide . . . . . . . . 24
2.4.7 Guiding BECs around curves: time orbiting potentials . . 28
2.4.8 Trapping geometries . . . . . . . . . . . . . . . . . . . . . 30
2.4.9 Splitting geometries . . . . . . . . . . . . . . . . . . . . . . 36
2.4.10 Rectangular wires . . . . . . . . . . . . . . . . . . . . . . . 40
2.5 Electric fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
2.5.1 Modulated wire guides: combined magneto-electric traps . 45
2.5.2 Controlled transport . . . . . . . . . . . . . . . . . . . . . 48
2.5.3 An electric beamsplitter . . . . . . . . . . . . . . . . . . . 49
2.5.4 State dependent operation . . . . . . . . . . . . . . . . . . 50
3 Surface disorder potentials 53
3.1 Atom cooling and BEC production near surfaces . . . . . . . . . . 54
3.1.1 Integrated U-MOT . . . . . . . . . . . . . . . . . . . . . . 54
3.1.2 Magnetic Cu-Z wire trap . . . . . . . . . . . . . . . . . . . 59
3.1.3 Ultracold thermal clouds and BEC in chip potentials . . . 63
3.2 Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.2.1 Previous experiments . . . . . . . . . . . . . . . . . . . . . 67
3.2.2 Edge versus bulk effects . . . . . . . . . . . . . . . . . . . 69
3.2.3 Height calibration . . . . . . . . . . . . . . . . . . . . . . . 70
3.2.4 Thermal atoms near surfaces . . . . . . . . . . . . . . . . . 73ii Contents
3.2.5 BEC near surfaces . . . . . . . . . . . . . . . . . . . . . . 75
3.2.6 BEC as ultra-sensitive magnetic surface microscope . . . . 78
4 Noisy potentials 81
4.1 Rate equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4.1.1 Negligible effects . . . . . . . . . . . . . . . . . . . . . . . 82
4.1.2 Majorana spin flips . . . . . . . . . . . . . . . . . . . . . . 84
4.1.3 Current fluctuations in conducting surfaces . . . . . . . . . 86
4.1.4 Experimental confirmation of predicted loss rates . . . . . 92
4.2 Wire size scaling . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
4.3 Lifetimes and heating rates: first experiments . . . . . . . . . . . 96
4.3.1 Lifetimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
4.3.2 Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
4.4 Interferometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
4.4.1 Guided matter wave interferometer . . . . . . . . . . . . . 100
4.4.2 Implementation and potential testing with thermal atoms . 104
4.4.3 BEC in a guided matter wave interferometer . . . . . . . . 109
4.4.4 Other geometries . . . . . . . . . . . . . . . . . . . . . . . 110
5 Outlook: controlled single and multi-particle quantum states 115
5.1 Entering the one-dimensional regime . . . . . . . . . . . . . . . . 116
5.2 Single atoms and qubits . . . . . . . . . . . . . . . . . . . . . . . 120
5.3 Next generation atom chips . . . . . . . . . . . . . . . . . . . . . 122
7A Li apparatus 125
87B Rb apparatus 127
B.1 Laser system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
B.2 Vacuum system . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
B.3 Atom chip assembly . . . . . . . . . . . . . . . . . . . . . . . . . 129
B.4 Experimental control . . . . . . . . . . . . . . . . . . . . . . . . . 130
B.5 Atom detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
C Chip design 133
D List of publications 137
E Acknowledgement 139
Bibliography 1431 Introduction
Modern physics has come a long way since Max Planck introduced his quantum
hypothesis in 1900 [161]. Today, the preparations for the centennial of the annus
mirabilis1905areinfullbloom. Duringthissingleyear, AlbertEinsteininitiated
andpromotedthedevelopmentofmodernphysicsinthediverseareasofBrownian
motion [55], special relativity [57], and quantum theory [56].
The consequences of relativity are spectacular and in the public Einstein is prob-
ably best known for this conception. But the implications and applications of
quantum mechanics are driving today’s technology and research more than any
other modern scientific theory.
Soon after its mathematical formulation [87, 176], the interpretation problems
of quantum theory were expounded, in particular after the famous paper by
Einstein,Podolsky,andRosenwaspublishedin1935[58]. Theroleofexperiments
– at that time still gedanken-experiments – elucidating the nature of the new
theory was crucial. The most noted protagonists of the debate on issues like
locality of actions and completeness of the theory were Albert Einstein and Niels
Bohr [194].
It was not until 1964 that John Bell realized that some of the unresolved aca-
demic questions regarding the interpretation of quantum mechanics can actually
be put to a decisive experimental test [16, 17]. Soon after, experiments – now in
the laboratories –showed that Bell’s inequalities can indeed be violated. The im-
plicationofthenewresults, namelythefalsificationoflocalrealistictheories, was
so severe and so much against the intuition of classical physics that experimental
‘loopholes’ are continued to be eliminated until today [103].
The availability of sophisticated laser technology and the fast development of
quantum optics during the 1980s and early 1990s led to a whole sequence of
physical implementations of former gedanken-experiments. The often cited tele-
portation of photons is a typical example [18, 23].
Again it were the rapid advances in technology and experimental research that
initiated the drive towards actually applying quantum mechanisms to the devel-
opment of usable devices rather than the mere understanding of the theory. This
does not mean a discontinuation of investigations in fundamental issues. On the
contrary, fundamentalresearchandappliedsciencemutuallyprofitfromthisnew
trend.
Today, a high level of control over complex quantum systems in very different
physical systems is possible. This is true for condensed matter systems as well
as in quantum optics. Examples are molecules in liquid solutions that are used2 Introduction
for basic quantum computing by nuclear magnetic resonance (NMR) techniques
[39] and commercially available quantum cryptography devices based on single
photons [135].
Neutral atoms have specific properties that make them interesting objects for a
controlled manipulation on the quantum level. On the one hand, the natural
coupling to the uncontrolled environment is weak, on the other hand, quantum
optics techniques are well developed in a way that provides precise manipulation
handles. In particular, the invention of the magneto-optical trap (MOT) [163]
has made it possible to cool atoms to ultra-low temperatures. After only a few
years this led to the first demonstration of Bose-Einstein condensation (BEC) [1,
41, 24], a phenomenon that had been predicted 70 years before [22]. The regime
of quantum degenerate gases when defined quantum states are macroscopically
populated is one of the major research field in today’s quantum optics. In this
domain, it is even possible to manipulate the inter-particle interaction at will,
for example by means of Feshbach resonances [105]. The controlled formation
of cold molecules, even molecular BECs [104, 72] and the currently intensively
studied crossover between the superfluid BCS (Bardeen-Cooper-Schrieffer) state
and BEC [10, 99, 14] are an outcome of these fascinating possibilities.
The degree of control achievable in cold atomic systems allows to consider the
ideas of quantum information processing (QIP). Here, individual quantum sys-
tems carry information in their quantum state. The information is processed
by entangling these systems in a controlled way and by finally observing the
outcome of the quantum evolution. Originally, the discovery of specific efficient
quantum algorithms impossible to implement on classical computers initiated fo-
cussed research in this direction [50, 178, 76]. However, the true significance of
QIP, at least in the near future, will be the possibility of the simulation of one
(undecoded) quantum system by another. The observation of the Mott insulator
transition in a quantum gas [71] and the indications of a successful formation of
a degenerate Fermi gas in the superfluid BCS-phase [99, 14] are two examples.
The approach of this thesis is to combine the well established and still advancing
techniques of atom optics with today’s highly developed microfabrication tech-
niques to form matter wave devices that are capable to control complex atomic
quantum systems on a microscopic level. Such atom chips have the potential to
beassuccessfulasintegratedmicroscopicdevicesareinelectronicsandphotonics.
The goal of the atom chip concept is to form a single integrated device based on
various types of manipulation handles that would provide control on the single
particle level. This can then be used to build up controlled complex systems by
gradually increasing the number of particles. On the other hand, one can start
with complex quantum systems like ensembles of (fermionic or bosonic) quantum
gases and gradually gain control over ever smaller subsystems. We have reviewed
the concepts and the status of research in this new field in [62, 121].
Based on earlier experiments with simple magnetic wire guides for cold thermal
atoms [48, 46], we started to build and experimentally test the first atom chips