Bose-Einstein condensation in a robust microtrap [Elektronische Ressource] : the combination of wire traps and atom chips / presented by Alexander Kasper

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• Physik

Informations

• Publié par: ruprecht-karls-universitat_heidelberg
• Publié le: 01 janvier 2004
• Nombre de lectures: 23
• Langue: Deutsch
• Poids de l'ouvrage: 38 Mo

Extrait

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
Magister rer.nat. Alexander Kasper
born in: Innsbruck, Austria
Oral examination: 17th December 2003Bose-Einstein condensation in a
robust microtrap – the
combination of wire traps and
atom chips
Referees: Prof.Dr. J¨org Schmiedmayer
Prof.Dr. Andreas WolfZusammenfassung
Bose-Einstein condensation in a robust microtrap –
the combination of wire traps and atom chips
In der hier vorgelegten Arbeit wird die erfolgreiche Erzeugung eines Bose-
EinsteinKondensatsvonRubidiumAtomenineinermagnetischenMikrofalle
beschrieben. Die verwendeten Mikrofallen, sogenanne Drahtfallen, werden
durch einfache Dr¨ahte und homogene Magnetfelder erzeugt. Diese Draht-
fallen erlauben es Magnetfallen zu erzeugen, die sich besonders gut zur Bose-
Einstein Kondensation eignen. Desweiteren konnen¨ sehr komplexe Fallenge-
ometrienerzeugtwerden,diediemagnetischemanipulationerm¨oglichen. Wir
kombinieren makroskopische mit mikroskopischen Drahtfallen. Die mikro-
skopischen Drahtfallen werden durch den sogenannten Atom-chip realisiert
(vergleichbarmiteinerLeiterplatteausderElektronik), diemakroskopischen
Drahtfallen werden durch eine massive Kupferstruktur erzeugt. Die Leiter-
strukturen auf dem Atom-chip realisieren in Zukunft die eigentlichen Ex-
perimente. Diese Kombination erlaubte es uns, einen Kondensationszyklus
in der makroskopischen Drahtfalle zu entwickeln, der unabh¨angig von den
Strukturen des Atom Chips ist.
Desweiteren werden erste Experimente mit einem Bose-Einstein Konden-
sat vorgestellt.
Abstract
Bose-Einstein condensation in a robust microtrap –
the combination of wire traps and atom chips
In the presented work, we report about the successful creation of a Rubid-
ium Bose-Einstein condensate. We use so called magnetic wire traps, which
are especially simple, as they consist out of a wire and a homogeneous bias
field. These wire traps are especially suited for Bose-Einstein condensation.
FurthermorecomplextrappingpotentialstomanipulateaBose-Einsteincon-
densate can be realized. We combine ’large’ and small scale wire traps. The
’large’ scale is realized with a massive copper structure, while for the small
wire traps we use the so called atom chip. This combination is promising,
because it allowed us to develop a condensation process in the copper struc-
ture, which is independent of the structures on the atom chip, and thus the
exchange of the ’physics’ area.
First experiments with the Bose-Einstein condensate are presented and
discussed in detail.2Introduction
In the beginning of the last century quantum theory was developed. The
development of the new theory had a massive impact, not only onto the view
of the world in a physical sense, but it also effected the daily life view. Over
a relatively short period, quantum physics was well established, due to the
innumerable examples of experimental verifications.
One major discovery (among others) was made in 1924. In this year the
Indian physicist Satyendra Nath Bose established a new way of ’counting’
photons (the Bose statistics) [1]. Einstein realized, that this statistics could
also be applied to atoms [2, 3]. When he studied the new statistics for low
temperatures, the thermodynamical limit gives a wrong result - it seemed,
that below a certain temperature the density decreased. To overcome this
problem, he had to tread the lowest energy term separately, and with this
approach he got the right result. This actually means, that the ground state
is macroscopically populated!
Intheseearlydays, therewasnowaytoinvestigatethisnewphenomenon
experimentally. Ittookoverseventyyearsuntilthisnewstateofmattercould
be experimentally realized. The laser had to be developed and laser cool-
ing, starting from slowing atomic beams to the development of the magneto
optical trap, had to be established [4, 5, 6]. In 1997 Steven Chu, Claude
N. Cohen-Tannoudji and William D. Phillips got the Nobel Prize for their
contribution to this field. Further on magnetic traps, to confine atoms had
to be constructed until it was possible to reach temperatures where this phe-
nomenon became experimentally visible [7]. The development of the evapo-
ration technique [8, 9] was the last step which made it possible to increase
3the phase space density over the critical value nλ ≈ 1 (n is the particle
dB
density and λ is the de Broglie wave length of the particles). Above thisdB
critical value Bose-Einstein condensation sets in. The macroscopic popula-
87 23tion of the ground state was first observed in Rb [10], followed by Na and
7 ∗Li [11, 12]. Over the years, experiments succeeded in condensing H, He , K,
Cs and Yb [13, 14, 15, 16, 17, 18, 19].
The condensation process was studied in detail, and the theoretical un-
derstanding was improved. The Bose-Einstein condensate was subsequently
manipulated with magnetic and optical fields, and experiments showing the
matter wave nature were performed. In 2001 Eric Cornell, Carl Wieman and
Wolfgang Ketterle earned the Nobel Price for their achievements in the field
of Bose-Einstein condensation.
In 1995 an experiment was performed, which used a wire [20, 21] for
trapping a neutral atoms. Then in 1998/99 groups [22, 23, 24] succeeded in
thedevelopmentofmorecomplextrappingstructures,whichwereremarkableii
simple. Thesewiretrapsusedjustasinglewireandahomogeneousbiasfield
forrealizingamagnetictrap. Soonitbecameclear,thatthisapproachopened
a new door to manipulate atoms. The first experiments were performed with
thermal atoms. Different trapping potentials were realized in order to guide
atoms(see[25]andreferencestherein). Thisdevelopmentwasfollowedbythe
firstcreationofaBose-Einsteincondensatesinsuchamagneticmicrotrap[26,
27]. The creation of a Bose-Einstein condensate strengthened the position of
wiretraps. Sincethen, severalgroupsworkinthisfieldanddevelopedsetups
with this new kind of traps.
In the presented work [28, 29], we developed a setup, which combines
’large’ and microscopic scale wire traps. This combination consists of a mas-
sive copper structure and the atom chip, which houses small wire structures.
With such a combination we established a robust condensation procedure
in the copper wire, independent of the atom chip. This has the advantage,
that the change of the atom chip, containing wires to realize more complex
experiments, is easy and fast.
Theaimofthepresentedworkwasthedevelopmentandtheinvestigation
of magnetic wire traps to establish a Bose-Einstein condensate as well as to
prove the flexibility of the atom chip concept.Contents
1 Bose-Einstein condensation 1
1.1 Basic considerations . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Bose-Einstein condensation at finite temperatures . . . . . . . 3
1.3 Finite particle number . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Interacting atoms - the Gross-Pitaevskii equation . . . . . . . 7
1.4.1 Thomas-Fermi approximation . . . . . . . . . . . . . . 10
1.5 Basic properties of a BEC . . . . . . . . . . . . . . . . . . . . 12
1.5.1 Free expansion of a Bose-Einstein condensate . . . . . 12
1.5.2 Exciting oscillations . . . . . . . . . . . . . . . . . . . 16
2 Laser cooling 23
2.1 Doppler cooling . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.2 Magneto-optical trap . . . . . . . . . . . . . . . . . . . . . . . 26
2.3 Polarization gradient cooling . . . . . . . . . . . . . . . . . . . 27
3 Magnetic trapping 31
3.1 Atom - magnetic/electric field interaction . . . . . . . . . . . . 31
3.2 Magnetic microtraps . . . . . . . . . . . . . . . . . . . . . . . 33
3.2.1 A straight wire and a bias field . . . . . . . . . . . . . 34
3.2.2 Creating three dimensional traps with wires . . . . . . 36
3.2.3 Finite size effects of real wires . . . . . . . . . . . . . . 41
3.2.4 Trap losses and heating. . . . . . . . . . . . . . . . . . 45
3.3 Evaporative cooling . . . . . . . . . . . . . . . . . . . . . . . . 48
4 Experimental realization 53
4.1 Trapping, transferring and pre-cooling
atoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.1.1 The laser system . . . . . . . . . . . . . . . . . . . . . 54
4.1.2 The double MOT apparatus . . . . . . . . . . . . . . . 66
4.1.3 The atom chip holder . . . . . . . . . . . . . . . . . . . 68
4.1.4 The atom chip - basic concepts . . . . . . . . . . . . . 72
iiiiv CONTENTS
4.1.5 The mirror MOT . . . . . . . . . . . . . . . . . . . . . 73
4.1.6 The atom transfer. . . . . . . . . . . . . . . . . . . . . 75
4.2 Measuring atom numbers and temperature . . . . . . . . . . . 77
4.2.1 The fluorescence method . . . . . . . . . . . . . . . . . 77
4.2.2 The absorptiond . . . . . . . . . . . . . . . . . . 79
4.2.3 Temperature measurement . . . . . . . . . . . . . . . . 81
4.2.4 The imaging system . . . . . . . . . . . . . . . . . . . 82
4.2.5 Interpretation of the pictures . . . . . . . . . . . . . . 84
4.3 Creating a Bose-Einstein condensate . . . . . . . . . . . . . . 86
4.3.1 Generating the magnetic fields . . . . . . . . . . . . . . 87
4.3.2 The U mirror MOT . . . . . . . . . . . . . . . . . . . . 89
4.3.3 The Molasses