Forum für Wissenschaft, Industrie und Wirtschaft

Hauptsponsoren:     3M 
Datenbankrecherche:

 

A Sisyphean Task for Polar Molecules

15.11.2012
A new cooling method for polyatomic molecules paves the way for the investigation of molecular gases near absolute zero temperature.

The investigation of ultracold molecules is of great interest for a number of problems. It could lead to a better understanding of chemical reactions in astrophysics. Ensembles of ultracold molecules could be used as quantum simulators, single molecules as quantum bits for storage of quantum information.


Figure 1: Scheme of the experimental apparatus.
Graphik: Rosa Glöckner, MPQ


Figure 2: An artist's depiction of optoelectrical Sisyphus cooling.
Graphic: Alexander Prehn, MPQ

Whereas efficient cooling methods have already been demonstrated for the cooling of atoms down to the nano-Kelvin regime, these methods fail for molecules due to their rich internal structure.

A team of scientists in the Quantum Dynamics Division of Prof. Gerhard Rempe at the Max-Planck-Institute of Quantum Optics has now developed a cooling procedure – the so-called optoelectrical Sisyphus cooling – which for the first time offers the potential to reach these ultralow temperatures even for complex polyatomic molecules (Nature, AOP, 14 November 2012).

The essential progress in the cooling of atomic gases came with the development of laser cooling techniques. Here, atoms are irradiated with laser light whose energy is slightly below the excitation energy of an electronic transition. Atoms propagating towards the laser beams come into resonance as a result of the Doppler-effect, causing them to become excited and experience a slowing force in the direction of the laser. This method is the basis for the application of subsequent cooling techniques that bring the temperatures down to the nano-Kelvin regime where the atomic gases can form new and exotic phases of matter.

For polyatomic molecules, the principle of laser cooling can no longer work due to the much greater number of excited states: each electronic state is composed of a large number of vibrational and rotational substates. However, a majority of molecules have an alternative property which can be efficiently used for cooling: as the electrons inside a molecule show different affinities towards the various atomic nuclei, the electric charge is not equally distributed. For example, as is widely known, the electrons inside water (H2O) feel more strongly attracted to the oxygen atom than to the hydrogen atoms. As a result the molecules show a negatively and a positively charged pole – they exhibit a strong dipole moment. In a static electric field this leads to a splitting of energy levels – depending on whether the dipole is oriented parallel or anti-parallel with respect to the field direction. This Stark effect (named after the German physicist Johannes Stark) is the key to the optoelectrical Sisyphus cooling technique.

In the experiment described here, the new cooling method has been tested for an ensemble of about a million polar CH3F molecules. The particles are pre-cooled to a temperature of around 400 milli-Kelvin and are trapped inside a special electric trap composed to a large part of a pair of microstructured capacitor plates. The field in the trap centre is homogeneous whereas it is strongly increasing near the boundary due to the microstructures. As the molecular dipoles interact with the electric fields, the Stark effect evokes a splitting of the molecules’ energy levels. A cooling cycle now starts by pumping molecules which are in the centre of the trap to an excited vibrational state using infrared laser light. Shortly thereafter, the excited molecules decay spontaneously back to the ground state by emitting photons. Of particular importance: during this process the alignment of the dipole with respect to the electric field can change.

“For the successful cooling of the molecules two events must take place,” explains Martin Zeppenfeld, who conceived and together with coworkers built the experiment in the course of his doctoral thesis. “First, it is necessary for the molecule to end up in the more strongly aligned of the two Stark levels after the spontaneous decay. Subsequently, the molecule must move into the boundary region of the trap where the electric field is strongly increasing.” When the molecule moves up this ‘hill’ a large amount of its kinetic energy is transformed into potential energy. At this point the orientation of the dipole moment of the molecule is deliberately changed using radiofrequency radiation such that the molecule makes a transition back into the more weakly aligned Stark level. As the interaction with the electric field is now much smaller than before the molecule rolling back into the trap centre gains much less energy than it had lost by mounting the ‘energy hill’. “This is the analogy to the tedious work of the ancient hero Sisyphus,” Zeppenfeld says. “In our scheme the entropy in the system is very efficiently removed by the photons emitted during the spontaneous decay. However, the energy reduction itself is caused by the strong interaction between the molecular dipoles and the electric fields induced by the trap electrodes.”

By repeating the cooling cycle several times the molecules have been cooled down from 390 milli-Kelvin to 29 milli-Kelvin. “The new technique can be applied to a large variety of molecules as long as they are not too big in size and exhibit a large dipole moment,” Barbara Englert points out who works on this experiment as a doctoral student. As for possible applications, she envisions developing molecular circuits in particular in combination with superconducting materials. Rosa Glöckner, another doctoral student, is fascinated by the quantum many body aspects. “Our method offers the potential of subsequently applying other cooling techniques such as evaporative cooling. This should allow the nano-Kelvin regime to be reached which is necessary for the formation of a Bose Einstein Condensate.” It would be of particular interest to look at the behaviour of molecules in optical lattices because the long range of their dipole-dipole interactions would extend over several lattice sites.
There is still a long way to go until such applications become feasible. However, “we have quite a few possibilities to optimize the current experimental set-up, from improving the electric trap or the detection method to using a different species of molecules,” Martin Zeppenfeld points out. “Therefore we should be able to reach much lower temperatures in the near future. But even now our technique provides new ways of investigating polar molecules, for example with high resolution spectroscopy or by investigating collisions between trapped molecules in tuneable homogeneous electric fields.”
[Olivia Meyer-Streng]

Original publication:
M. Zeppenfeld, B.G.U. Englert, R. Glöckner, A. Prehn, M. Mielenz, C. Sommer, L.D. van Buuren, M. Motsch, and G. Rempe
Sisyphus Cooling of Electrically Trapped Polyatomic Molecules
Nature, AOP, 14 November 2012, DOI:10.1038/nature11595
Contact:

Prof. Dr. Gerhard Rempe
Max-Planck-Institute of Quantum Optics
Hans-Kopfermann-Straße 1
85748 Garching
Phone.: +49 - 89 / 32905 -701
Fax: +49 - 89 / 32905 -311
E-mail: gerhard.rempe@mpq.mpg.de

Dipl. Phys. Martin Zeppenfeld
Max-Planck-Institute of Quantum Optics
Hans-Kopfermann-Straße 1
85748 Garching
Phone: +49 - 89 / 32905 -726
Fax: +49 - 89 / 32905 -311
E-mail: martin.zeppenfeld@mpq.mpg.de

Dr. Olivia Meyer-Streng
Press and Public Relations
Max-Planck-Institute of Quantum Optics
Phone: +49 - 89 / 32905 -213
E-mail: olivia.meyer-streng@mpq.mpg.de

Dr. Olivia Meyer-Streng | Max-Planck-Institut
Further information:
http://www.mpq.mpg.de

More articles from Physics and Astronomy:

nachricht Cassiopeia's hidden gem: The closest rocky, transiting planet
04.08.2015 | Harvard-Smithsonian Center for Astrophysics

nachricht Quantum States in a Nano-object Manipulated using a Mechanical System
04.08.2015 | Universität Basel

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Kleine Löcher, große Wirkung: Aktives Elastomerlager reduziert Schwingungen

Wo große Bewegungen ausgeglichen werden müssen, sind Elastomere in ihrem Element. Sie federn passiv Stöße bei Fahrzeugen ab und reduzieren Schwingungen in Maschinen. Aber sie können noch mehr als das, wie Forscher des Fraunhofer LBF zeigen konnten. Sie haben diese elastischen Komponenten smarter gemacht und ihnen beigebracht, sich aktiv zu verformen. Dazu nutzt das Institut dielektrische Elastomere (DE). Das sind weiche Materialien, die sich unter hohen elektrischen Spannungen verformen, und das prädestiniert sie für den Aufbau von Aktoren. Gegenüber Piezoaktoren haben sie den Vorteil, vergleichsweise große Dehnungen bei geringeren Kräften zu erreichen.

Diese Fähigkeit haben die Wissenschaftler des Fraunhofer-Instituts für Betriebsfestigkeit und Systemzuverlässigkeit LBF genutzt und ein Konzept für DE-Aktoren...

Im Focus: Greenhouse gases' millennia-long ocean legacy

Continuing current carbon dioxide (CO2) emission trends throughout this century and beyond would leave a legacy of heat and acidity in the deep ocean. These...

Im Focus: Kosten sparen beim Bau von Flugzeugturbinen

Verdichterscheiben für Flugzeugturbinen werden aus einem Materialstück herausgefräst. Bei der Bearbeitung fangen die Schaufeln an zu schwingen. Ein neuartiges Spannsystem steigert die Dämpfung der Schaufeln nun auf mehr als das 400-fache. Es lassen sich bis zu 5000 Euro Kosten bei der Fertigung einsparen.

Mal eben schnell in den Urlaub jetten oder für ein langes Wochenende nach Rom, Paris oder Madrid fliegen? Der Flugverkehr steigt, insbesondere der...

Im Focus: Gletscher verlieren mehr Eis als je zuvor

Der Gletscherschwund im ersten Jahrzehnt des 21. Jahrhunderts erreicht einen historischen Rekordwert seit Messbeginn. Das Schmelzen der Gletscher ist ein globales Phänomen und selbst ohne weiteren Klimawandel werden sie zusätzlich an Eis verlieren. Dies belegt die neueste Studie des World Glacier Monitoring Services unter der Leitung der Universität Zürich.

Seit über 120 Jahren sammelt der World Glacier Monitoring Service, mit heutigem Sitz an der Universität Zürich, weltweite Daten zu Gletscherveränderungen....

Im Focus: Glaciers melt faster than ever

Glacier decline in the first decade of the 21st century has reached a historical record, since the onset of direct observations. Glacier melt is a global phenomenon and will continue even without further climate change. This is shown in the latest study by the World Glacier Monitoring Service under the lead of the University of Zurich, Switzerland.

The World Glacier Monitoring Service, domiciled at the University of Zurich, has compiled worldwide data on glacier changes for more than 120 years. Together...

Alle Focus-News des Innovations-reports >>>

Anzeige

Anzeige

IHR
JOB & KARRIERE
SERVICE
im innovations-report
in Kooperation mit academics
Veranstaltungen

Managementkonferenz

04.08.2015 | Veranstaltungen

Tagung "Intelligente Beschichtungen für Außenanwendungen" in Dresden

03.08.2015 | Veranstaltungen

MS Wissenschaft in Stuttgart: Fraunhofer zeigt Chancen im Ländle auf

03.08.2015 | Veranstaltungen

 
B2B-VideoLinks
Weitere VideoLinks >>>
Aktuelle Beiträge

Siemens modernisiert Großteil des belgischen Eisenbahnnetzes

04.08.2015 | Verkehr Logistik

Wegweisende Konzepte rund ums Automatisierte Fahren und innovative Fahrzeuge gesucht

04.08.2015 | Verkehr Logistik

Mit Hightech und Honigtöpfen gegen Hacker

04.08.2015 | Informationstechnologie