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Laser cooling mechanisms and Brownian motors in optical lattices
Umeå University, Faculty of Science and Technology, Department of Physics.
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [sv]

Denna avhandling innefattar såväl experimentella som numeriska studier av laserkylda atomer i optiska kristallgitter. Bland annat har laserkylningsprocesser studerats, där atomers rörelser i optiska kristallgitter har uppvisat andra typer av bakomliggande mekanismer än de som tidigare förutsågs genom “Sisyfoskylningsmodellen”. Sedan atomer kylda till några mikrokelvin först realiserades (sent 60-tal) så har Sisyfoskylningsmodellen varit hörnstenen för förståelsen av laserkylda och lokaliserade atomer i dissipativa optiska kristallgitter. I dissipativa optiska kristallgitter finns det en balans mellan den uppvärmande diffusionen och den kylande friktionen. Studier i denna avhandling visar att laserkylningsprocesser är mer komplexa än vad denna modell innefattar. Både experimentella och numeriska resultat visar att atomer i optiska kristallgitter har två hastighetsfördelningar där en “kallare” och en “varmare” mod av atomer omfördelas mellan moderna. Speciellt så visar det sig att varma atomer dels värms och diffunderar ut ur gittret, men samtidigt populeras den kalla moden med en tidsutveckling som inte förändrar dess temperatur nämnvärt. I detta arbete presenteras också resultat från den första realiserade tredimensionella Brownska motorn baserad på ljus-atom-växelverkan. Det unika med denna Brownska motor är att den är kontrollerbar både vad gäller dess hastighet som dess riktning. Den underliggande principen för denna Brownska motor är tämligen generell och den kan därför vara applicerbar inom andra vetenskapliga discipliner såsom nanoteknik, biologi, kemi och elektronik. Generellt så är förståelsen av Brownska motorer viktigt eftersom de återfinns i vår omgivning, från exempelvis härkomsten av muskelsammandragningar och materialtransporter i levande celler till rörelsen hos bakterier och mindre organismer. Det flesta av de experimentella resultaten presenterade i denna avhandling har varit möjliga genom utveckling och förbättringar av den experimentella uppställningen. Framförallt så har kvaliten och reproducerbarheten vid de olika mätningar som gjorts blivit avsevärt förbättrade jämfört med tidigare vilket utgör en bra grund för framtida studier av ultrakalla atomer.

Abstract [en]

In this thesis, detailed experimental studies and numerical simulations are presented of laser cooling mechanisms in dissipative optical lattices and results of the first realized three dimensional Brownian motor in optical lattices. A dissipative optical lattice is a periodic light shift potential, created in the interference patterns of laser beams. In this, atoms can be both cooled and trapped, and the most important relaxation mechanism is generally considered to be “Sisyphus cooling”. However, careful experimental and theoretical investigations indicate the presence of other cooling processes as well. This is studied by varying different parameters such as irradiance and frequency of the lattice light. The time evolution of atoms in optical lattices show strong evidence of a bimodal velocity distribution, where a population transfer between one mode containing “hot” atoms and one mode containing “cold” atoms is evident. The normal diffusion of atoms in optical lattices is characterized by isotrop random fluctuations and exhibit the nature of Brownian motion. We have realized a technique where this motion is rectified and controlled. This is done in a three dimensional double optical lattice. This Brownian motor has control properties for both its speed and its direction in three dimensions. Our three dimensional double optical lattice is created by using laser light, exploiting two transitions, in the D2 line of cesium. Two three dimensional optical lattices are spatially overlapped; each optical lattice traps atoms in one of two hyperfine ground states. The controllability comes about by inducing phase shifts in the lattice laser beams, which displace the lattices relative to each other. This type of highly controlled Brownian motor is of fundamental interest since Brownian motion is present in almost all systems and for the role they play in protein motors and the function of living cells, and for the potential applications in nanotechnology. Brownian motors of this kind also open the way to possible studies of quantum Brownian motors and quantum resonances that are predicted for atomic ratchets. Optical lattices, and especially double optical lattices, have also been suggested as a platform for quantum state manipulations due to the good isolation from environment and ambient effects. Most of the work in this thesis is a first step towards the implementation of quantum manipulation schemes in a double optical lattice.

Place, publisher, year, edition, pages
Umeå: Fysik , 2007. , 93 p.
National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:umu:diva-1127ISBN: 978-91-7264-321-5 (print)OAI: oai:DiVA.org:umu-1127DiVA: diva2:140292
Public defence
2007-05-31, MA121, MIT Huset, Umeå Universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2007-05-10 Created: 2007-05-10 Last updated: 2012-01-26Bibliographically approved
List of papers
1. Generation of multiple power-balanced laser beams for quantum-state manipulation experiments with phase-stable double optical lattices
Open this publication in new window or tab >>Generation of multiple power-balanced laser beams for quantum-state manipulation experiments with phase-stable double optical lattices
2006 (English)In: Journal of Optics. A, Pure and applied optics, ISSN 1464-4258, E-ISSN 1741-3567, Vol. 8, no 5, 381-385 p.Article in journal (Refereed) Published
Abstract [en]

We present a method to obtain power-balanced laser beams for quantum-state manipulation experiments with phase-stable double optical lattices. Double optical lattices are constructed using four pairs of overlapped laser beams with different frequencies. Our optical scheme provides a phase stability between the optical lattices of 5 mrad s−1, and laser beams with a very clean polarization state resulting in a power imbalance in the individual laser beams of less than 1%.

Place, publisher, year, edition, pages
Bristol: IOP, 2006
Keyword
optical system design, optical lattices, cold atoms, quantum-state manipulation
Identifiers
urn:nbn:se:umu:diva-2330 (URN)10.1088/1464-4258/8/5/003 (DOI)
Available from: 2007-05-10 Created: 2007-05-10 Last updated: 2017-12-14Bibliographically approved
2. Time dependence of laser cooling in optical lattices
Open this publication in new window or tab >>Time dependence of laser cooling in optical lattices
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2005 (English)In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 72, no 3, 369-375 p.Article in journal (Refereed) Published
Abstract [en]

We study the dynamics of the cooling of a gas of caesium atoms in an optical lattice, both experimentally and with 1D full-quantum Monte Carlo simulations. We find that, contrary to the standard interpretation of the Sisyphus model, the cooling process does not work by a continuous decrease of the average kinetic energy of the atoms in the lattice. Instead, we show that the momentum of the atoms follows a bimodal distribution, the atoms being gradually transferred from a hot to a cold mode. We suggest that the cooling mechanism should be depicted in terms of a rate model, describing the transfer between the two modes along with the processes occurring within each mode.

Place, publisher, year, edition, pages
Les Ulis: Éditions de physique, 2005
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-2331 (URN)10.1209/epl/i2005-10244-6 (DOI)
Available from: 2007-05-10 Created: 2007-05-10 Last updated: 2017-12-14Bibliographically approved
3. A nonadiabatic semi-classical method for dynamics of atoms in optical lattices
Open this publication in new window or tab >>A nonadiabatic semi-classical method for dynamics of atoms in optical lattices
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2006 (English)In: European Physical Journal D: Atomic, Molecular and Optical Physics, ISSN 1434-6060, E-ISSN 1434-6079, Vol. 39, no 1, 67-74 p.Article in journal (Refereed) Published
Abstract [en]

We develop a semi-classical method to simulate the motion of atoms in a dissipative optical lattice. Our method treats the internal states of the atom quantum mechanically, including all nonadiabatic couplings, while position and momentum are treated as classical variables. We test our method in the one-dimensional case. Excellent agreement with fully quantum mechanical simulations is found. Our results are much more accurate than those of earlier semi-classical methods based on the adiabatic approximation.

Place, publisher, year, edition, pages
Les Ulis: EDP Sciences, 2006
National Category
Other Physics Topics
Identifiers
urn:nbn:se:umu:diva-11022 (URN)doi:10.1140/epjd/e2006-00082-6 (DOI)
Available from: 2007-10-18 Created: 2007-10-18 Last updated: 2017-12-14Bibliographically approved
4. Comment on "Tunable Tsallis Distributions in Dissipative Optical Lattices"
Open this publication in new window or tab >>Comment on "Tunable Tsallis Distributions in Dissipative Optical Lattices"
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(English)In: Physical Review LettersArticle in journal (Refereed) Submitted
Identifiers
urn:nbn:se:umu:diva-2333 (URN)
Available from: 2007-05-10 Created: 2007-05-10 Last updated: 2016-02-25Bibliographically approved
5. Bimodal distribution of laser-cooled atoms in optical lattices
Open this publication in new window or tab >>Bimodal distribution of laser-cooled atoms in optical lattices
(English)Manuscript (preprint) (Other academic)
National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-2334 (URN)
Available from: 2007-05-10 Created: 2007-05-10 Last updated: 2016-02-25Bibliographically approved
6. Demonstration of a controllable three-dimensional Brownian motor in symmetric potentials
Open this publication in new window or tab >>Demonstration of a controllable three-dimensional Brownian motor in symmetric potentials
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2006 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 96, no 19, 190602- p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate a Brownian motor, based on cold atoms in optical lattices, where isotropic random fluctuations are rectified in order to induce controlled atomic motion in arbitrary directions. In contrast to earlier demonstrations of ratchet effects, our Brownian motor operates in potentials that are spatially and temporally symmetric, but where spatiotemporal symmetry is broken by a phase shift between the potentials and asymmetric transfer rates between them. The Brownian motor is demonstrated in three dimensions and the noise-induced drift is controllable in our system.

Place, publisher, year, edition, pages
New York: American Physical Society, 2006
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-11001 (URN)doi:10.1103/PhysRevLett.96.190602 (DOI)
Available from: 2007-05-15 Created: 2007-05-15 Last updated: 2017-12-14Bibliographically approved
7. Characterisation of a three-dimensional Brownian motor in optical lattices
Open this publication in new window or tab >>Characterisation of a three-dimensional Brownian motor in optical lattices
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2007 (English)In: European Physical Journal D: Atomic, Molecular and Optical Physics, ISSN 1434-6060, E-ISSN 1434-6079, Vol. 44, no 2, 381-388 p.Article in journal (Refereed) Published
Abstract [en]

We present here a detailed study of the behaviour of a three dimensional Brownian motor based on cold atoms in a double optical lattice [P. Sjölund et al., Phys. Rev. Lett. 96, 190602 (2006)]. This includes both experiments and numerical simulations of a Brownian particle. The potentials used are spatially and temporally symmetric, but combined spatiotemporal symmetry is broken by phase shifts and asymmetric transfer rates between potentials. The diffusion of atoms in the optical lattices is rectified and controlled both in direction and speed along three dimensions. We explore a large range of experimental parameters, where irradiances and detunings of the optical lattice lights are varied within the dissipative regime. Induced drift velocities in the order of one atomic recoil velocity have been achieved.

Place, publisher, year, edition, pages
Springer Verlag, 2007
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-16377 (URN)10.1140/epjd/e2007-00233-3 (DOI)
Available from: 2010-03-08 Created: 2007-09-18 Last updated: 2017-12-14Bibliographically approved
8. Controllable 3D atomic Brownian motor in optical lattices
Open this publication in new window or tab >>Controllable 3D atomic Brownian motor in optical lattices
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2008 (English)In: The European Physical Journal - Special Topics, ISSN 1951-6355, Vol. 159, no 1, 11-17 p.Article in journal (Refereed) Published
Abstract [en]

We study a Brownian motor, based on cold atoms in optical lattices, where atomic motion can be induced in a controlled manner in an arbitrary direction, by rectification of isotropic random fluctuations. In contrast with ratchet mechanisms, our Brownian motor operates in a potential that is spatially and temporally symmetric, in apparent contradiction to the Curie principle. Simulations, based on the Fokker-Planck equation, allow us to gain knowledge on the qualitative behaviour of our Brownian motor. Studies of Brownian motors, and in particular ones with unique control properties, are of fundamental interest because of the role they play in protein motors and their potential applications in nanotechnology. In particular, our system opens the way to the study of quantum Brownian motors.

National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-11033 (URN)10.1140/epjst/e2008-00688-8 (DOI)
Available from: 2010-02-24 Created: 2007-03-05 Last updated: 2012-01-26Bibliographically approved
9. Multidimensional coupling in a controllable three-dimensional Brownian motor in optical lattices
Open this publication in new window or tab >>Multidimensional coupling in a controllable three-dimensional Brownian motor in optical lattices
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(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:umu:diva-2338 (URN)
Note
Submitted for Europhysics letters.Available from: 2007-05-10 Created: 2007-05-10 Last updated: 2010-12-20Bibliographically approved

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  • modern-language-association-8th-edition
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