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Ultracold atoms in optical potentials: from noise-induced transport to superfluidity
Umeå University, Faculty of Science and Technology, Department of Physics.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, both experimental studies and numerical simulations of ultracold atoms in optical potentials are presented in a collection of nine scientific papers. In particular, noise-induced transport in dissipative optical lattices and superfluid properties of Bose-Einstein condensates have been studied.

Noise is usually regarded as a complication to most systems and as something that needs to be minimized. However, in a series of experiments at Umeå University, noise has been shown to play a key role for laser-cooled cesium atoms trapped in dissipative optical lattices. By using a combination of two dissipative optical lattices, where the relative spatial phase between them can be controlled, a so-called Brownian motor can be realized, where energy can be extracted from the inherent noise. In the experiment, this energy is used to control the transport of the laser-cooled atoms in real time and along pre-designed paths. This thesis also presents a way to characterize this system in terms of energy conversion efficiency and coherence of the transport, which may allow for a more straightforward comparison with other systems that rely on noise rectification. In the studies, it is also shown that the noise triggers a downward drift due to gravity, even though the optical potential should support the atoms. Further investigation of this might help to understand the underlying principles of laser cooling, as well as showing that the system might be suitable as a flexible test bed for statistical physics. In close relation to the experimental system, two numerical simulations are also presented, one in which different ways to induce asymmetries between two periodic potentials are investigated, and one in which a proposal for detecting quantum walks is explored.

In the second part of the thesis, a work from the Joint Quantum Institute is presented, where a long-lived persistent current in a toroidal Bose-Einstein condensate, held in an all-optical trap, is created. The critical velocity of the superflow is measured in the presence of a tunable barrier. The system can be seen as a first realization of an elementary closed-loop atom circuit. Finally a theoretical study of the crossover between one- and two-dimensional systems is presented, in particular the transition between a two-dimensional superfluid to a one-dimensional Mott insulator is investigated.

Abstract [sv]

Medelst nio vetenskapliga artiklar presenteras i denna avhandling experimentella och teoretiska studier av ultrakalla atomer fångade i optiska potentialer. Framförallt har brusinducerade transporter och supraytande egenskaper hos Bose-Einstein-kondensat studerats.

    För de flesta system betraktas brus som något negativt som bör minimeras, men i en serie experiment som redovisas i denna avhandling spelar bruset istället en avgörande positiv roll. I ett system där laserkylda atomer genom växelverkan med laserstrålar fångas i två individuella optiska kristallgitter, kan atomernas kollektiva rörelse styras genom att energi utvinns ur det inneboende bruset. I denna avhandling, genom att kontrollera de optiska potentialerna i realtid, visas att atomernas kollektiva rörelse kan styras längs förutbestämda banor med en så kallade Brownska motor. I ett annat experiment mäts verkningsgraden i omvandligen mellan brus och arbete, samt koherensen i atomtransporten. En sådan karakterisering gör att systemet blir enklare att jämföra med andra system som bygger på liknande principer. I avhandlingen presenteras också en studie där det visas att det inneboende bruset i systemet, tillsammans med en svag kraft, i detta fall från gravitation, kan skapa drifter trots att de optiska potentialerna borde vara tillräckligt djupa för att atomerna ska vara fångade. Denna upptäckt kan leda till ökad grundläggande kundskap om laserkylning. Dessutom visar det att systemet kan beskrivas med modeller från statistisk fysik. I relation till det experimentella systemet i Umeå redovisas även två teoretiska studier, en för två symmetriska periodiska potentialer och deras sätt att möjliggöra inducerade drifter med olika typ av asymmetrier, samt en annan för möjligheten att genomföra och detektera kvantvandringar.

    I avhandlingen presenteras också ett experimentellt arbete utfört vid Joint Quantum Institute, där en långlivad ihållande ström i ett torusformat Bose-Einstein-kondensat har skapats i en optisk fälla. Den kritiska hastigheten på strömmen har mätts i närvaron av en ställbar optisk barriär. Detta system kan ses som en första realisation av en grundläggande atomkrets. Slutligen presenteras även en teoretisk studie av övergången mellan en- och tvådimensionella system, där fasövergången mellan superytande och Mottisolation studeras.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, Institutionen för fysik , 2011. , 72 p.
Keyword [en]
Brownian motors, noise-induced transport, superfluidity, ultracold atoms, optical potentials, optical lattices, laser coooling
Research subject
Physics
Identifiers
URN: urn:nbn:se:umu:diva-39541ISBN: 978-91-7459-140-8 (print)OAI: oai:DiVA.org:umu-39541DiVA: diva2:393611
Public defence
2011-02-25, N320, Umeå universitet, Umeå, 16:01 (English)
Opponent
Supervisors
Available from: 2011-02-04 Created: 2011-01-31 Last updated: 2011-05-16Bibliographically approved
List of papers
1. A three-dimensional Brownian motor, realised with symmetric optical lattices
Open this publication in new window or tab >>A three-dimensional Brownian motor, realised with symmetric optical lattices
2009 (English)In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 246, no 5, 999-1005 p.Article in journal (Refereed) Published
Abstract [en]

A three-dimensional Brownian motor is realised using lasercooled caesium atoms trapped in a system of two static, and individually symmetric, optical lattices; a so-called double optical lattice. Isotropic fluctuations, emanating from light scattering, are rectified, and the diffusion of the ensemble of atoms is biased, with a resulting constant velocity that is controllable both in direction and magnitude. The working principle of the Brownian motor can be seen as a pulsation between two different potentials, both symmetric but around different points. The correlation between interferometric spatial offsets, and imbalance in optical pumping rates, leads to a spatio-temporal asymmetry sufficient for generating a controlled, directed motion.

National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-30839 (URN)10.1002/pssb.200881559 (DOI)
Available from: 2010-01-19 Created: 2010-01-19 Last updated: 2017-12-12Bibliographically approved
2. A set-up for flexible geometry optical lattices
Open this publication in new window or tab >>A set-up for flexible geometry optical lattices
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2008 (English)In: European Physical Journal: Applied physics, ISSN 1286-0042, E-ISSN 1286-0050, Vol. 42, 269-273 p.Article in journal (Refereed) Published
Abstract [en]

We have developed an alignment tool for optical lattices with arbitrary beam angles, allowing for optical potentials with a range of geometries and topographies. A calibration procedure has been introduced, giving a precision of 10 mrad in the chosen beam angle, and a high degree of flexibility. The tool has been tested in simplified experiments on the confinement and expansion of cold atoms in one-dimensional and two-dimensional optical lattices, and has proved to work well.

Identifiers
urn:nbn:se:umu:diva-10492 (URN)doi:10.1051/epjap:2008086 (DOI)
Available from: 2008-09-17 Created: 2008-09-17 Last updated: 2011-02-04Bibliographically approved
3. Fluctuation-induced drift in a gravitationally tilted optical lattice
Open this publication in new window or tab >>Fluctuation-induced drift in a gravitationally tilted optical lattice
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2010 (English)In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 82, no 3, 031136- p.Article in journal (Refereed) Published
Abstract [en]

Experimental and theoretical studies are made of Brownian particles trapped in a periodic potential, which is very slightly tilted due to gravity. In the presence of fluctuations, these will trigger a measurable average drift along the direction of the tilt. The magnitude of the drift varies with the ratio between the bias force and the trapping potential. This can be closely compared to a theoretical model system, based on a Fokker-Planck equation formalism. We show that the level of control and measurement precision we have in our system, which is based on cold atoms trapped in a three-dimensional dissipative optical lattice, makes the experimental setup suitable as a testbed for fundamental statistical physics. We simulate the system with a very simplified and general classical model, as well as with an elaborate semiclassical Monte Carlo simulation. In both cases, we achieve good qualitative agreement with experimental data.

National Category
Other Physics Topics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-36914 (URN)10.1103/PhysRevE.82.031136 (DOI)000282134100005 ()
Available from: 2010-10-21 Created: 2010-10-13 Last updated: 2017-12-12Bibliographically approved
4. Directed transport with real-time external steering and drifts along pre-designed paths using a Brownian motor
Open this publication in new window or tab >>Directed transport with real-time external steering and drifts along pre-designed paths using a Brownian motor
2011 (English)In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 83, no 2Article in journal (Refereed) Published
Abstract [en]

We have realized real-time steering of the directed transport in a Brownian motor based on cold atoms in optical lattices and demonstrate drifts along predesigned paths. The transport is induced by spatiotemporal asymmetries in the system, where we can control the spatial part, and we show that the response to changes in asymmetry is very fast. In addition to directional steering, a real-time control of the magnitude of the average drift velocity and an on-off switching of the motor are also demonstrated. We use a noninvasive real-time detection of the transport, enabling feedback control of the system.

Identifiers
urn:nbn:se:umu:diva-38521 (URN)10.1103/PhysRevE.83.020101 (DOI)
Note
Received 26 September 2010; published 2 February 2011Available from: 2010-12-16 Created: 2010-12-16 Last updated: 2017-12-11Bibliographically approved
5. Experimental measurement of the efficiency and the transport coherence of a cold-atom Brownian motor realized with cold atoms in optical lattices
Open this publication in new window or tab >>Experimental measurement of the efficiency and the transport coherence of a cold-atom Brownian motor realized with cold atoms in optical lattices
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2011 (English)In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 83, no 2, 020102Article in journal (Refereed) Published
Abstract [en]

The rectification of noise into directed movement or useful energy is utilized by many different systems. The peculiar nature of the energy source and conceptual differences between such Brownian motor systems makes a characterization of the performance far from straightforward. In this work, where the Brownian motor consists of atoms interacting with dissipative optical lattices, we adopt existing theory and present experimental measurements for both the efficiency and the transport coherence. We achieve up to 0.3% for the efficiency and 0.01 for the Péclet number.

Place, publisher, year, edition, pages
American Physical Society, 2011
National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-38520 (URN)10.1103/PhysRevE.83.020102 (DOI)
Note

Received 16 September 2010; published 3 February 2011

Available from: 2010-12-16 Created: 2010-12-16 Last updated: 2017-12-11
6. Breaking the symmetry of a Brownian motor with symmetric potentials
Open this publication in new window or tab >>Breaking the symmetry of a Brownian motor with symmetric potentials
2011 (English)In: Journal of Physics A: Mathematical and Theoretical, ISSN 1751-8113, Vol. 44, no 15, 155002- p.Article in journal (Refereed) Published
Abstract [en]

The directed transport of Brownian particles requires a system with an asymmetry and with non-equilibrium noise. Here we investigate numerically alternative ways of fulfilling these requirements for a two-state Brownian motor, realized with Brownian particles alternating between two phase-shifted, symmetric potentials. We show that, besides the previously known spatio-temporal asymmetry based on unequal transfer rates between the potentials, inequalities in the potential depths, the frictions, or the equilibrium temperatures of the two potentials also generate the required asymmetry. We also show that the effects of the thermal noise and the noise of the transfer's randomness depend on the way the asymmetry is induced.

 

Place, publisher, year, edition, pages
Institute of Physics, 2011
National Category
Other Physics Topics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-42554 (URN)10.1088/1751-8113/44/15/155002 (DOI)
Available from: 2011-04-14 Created: 2011-04-08 Last updated: 2011-05-16Bibliographically approved
7. Theoretical investigation of quantum walks by cold atoms in a double optical lattice
Open this publication in new window or tab >>Theoretical investigation of quantum walks by cold atoms in a double optical lattice
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2009 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 80, no 1, 012302- p.Article in journal (Refereed) Published
Abstract [en]

We investigate the feasibility of carrying out quantum walks with cold atoms in a double optical lattice. Monte Carlo simulations of time-of-flight (TOF) detection and absorption imaging were carried out, focusing on a specific experimental implementation. These indicate that absorption imaging would be best suited for detection of quantum walks. With typical experimental parameters a few hundred quantum walk steps will be needed for an unambiguous detection of the quantum walk signature. We show that in special cases, few-step quantum walks can also be detected in our system if one measures the relative population of the atoms in their internal states rather than their displacement in space, that is, measurements are made in the space of the coin operator rather than in that of the displacement operator.

Place, publisher, year, edition, pages
American Physical Society, 2009
Identifiers
urn:nbn:se:umu:diva-38522 (URN)10.1103/PhysRevA.80.012302 (DOI)
Available from: 2010-12-16 Created: 2010-12-16 Last updated: 2017-12-11Bibliographically approved
8. One-dimensional phase transitions in a two-dimensional optical lattice
Open this publication in new window or tab >>One-dimensional phase transitions in a two-dimensional optical lattice
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2008 (English)In: European Physical Journal D: Atomic, Molecular and Optical Physics, ISSN 1434-6060, E-ISSN 1434-6079, Vol. 49, no 2, 223-230 p.Article in journal (Refereed) Published
Abstract [en]

A phase transition for bosonic atoms in a two-dimensional anisotropic optical lattice is considered. If the tunnelling rates in two directions are different, the system can undergo a transition between a two-dimensional superfluid and a one-dimensional Mott insulating array of strongly coupled tubes. The connection to other lattice models is exploited in order to better understand the phase transition. Critical properties are obtained using quantum Monte Carlo calculations. These critical properties are related to correlation properties of the bosons and a criterion for commensurate filling is established.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-10486 (URN)10.1140/epjd/e2008-00152-9 (DOI)
Available from: 2008-09-16 Created: 2008-09-16 Last updated: 2011-02-04Bibliographically approved

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