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 A 100-year-old physics problem has been solved at EPFL
23.06.2017 | Ecole Polytechnique Fédérale de Lausanne

nachricht Quantum thermometer or optical refrigerator?
23.06.2017 | National Institute of Standards and Technology (NIST)

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: Can we see monkeys from space? Emerging technologies to map biodiversity

An international team of scientists has proposed a new multi-disciplinary approach in which an array of new technologies will allow us to map biodiversity and the risks that wildlife is facing at the scale of whole landscapes. The findings are published in Nature Ecology and Evolution. This international research is led by the Kunming Institute of Zoology from China, University of East Anglia, University of Leicester and the Leibniz Institute for Zoo and Wildlife Research.

Using a combination of satellite and ground data, the team proposes that it is now possible to map biodiversity with an accuracy that has not been previously...

Im Focus: Klima-Satellit: Mit robuster Lasertechnik Methan auf der Spur

Hitzewellen in der Arktis, längere Vegetationsperioden in Europa, schwere Überschwemmungen in Westafrika – mit Hilfe des deutsch-französischen Satelliten MERLIN wollen Wissenschaftler ab 2021 die Emissionen des Treibhausgases Methan auf der Erde erforschen. Möglich macht das ein neues robustes Lasersystem des Fraunhofer-Instituts für Lasertechnologie ILT in Aachen, das eine bisher unerreichte Messgenauigkeit erzielt.

Methan entsteht unter anderem bei Fäulnisprozessen. Es ist 25-mal wirksamer als das klimaschädliche Kohlendioxid, kommt in der Erdatmosphäre aber lange nicht...

Im Focus: Climate satellite: Tracking methane with robust laser technology

Heatwaves in the Arctic, longer periods of vegetation in Europe, severe floods in West Africa – starting in 2021, scientists want to explore the emissions of the greenhouse gas methane with the German-French satellite MERLIN. This is made possible by a new robust laser system of the Fraunhofer Institute for Laser Technology ILT in Aachen, which achieves unprecedented measurement accuracy.

Methane is primarily the result of the decomposition of organic matter. The gas has a 25 times greater warming potential than carbon dioxide, but is not as...

Im Focus: How protons move through a fuel cell

Hydrogen is regarded as the energy source of the future: It is produced with solar power and can be used to generate heat and electricity in fuel cells. Empa researchers have now succeeded in decoding the movement of hydrogen ions in crystals – a key step towards more efficient energy conversion in the hydrogen industry of tomorrow.

As charge carriers, electrons and ions play the leading role in electrochemical energy storage devices and converters such as batteries and fuel cells. Proton...

Im Focus: Die Schweiz in Pole-Position in der neuen ESA-Mission

Die Europäische Weltraumagentur ESA gab heute grünes Licht für die industrielle Produktion von PLATO, der grössten europäischen wissenschaftlichen Mission zu Exoplaneten. Partner dieser Mission sind die Universitäten Bern und Genf.

Die Europäische Weltraumagentur ESA lanciert heute PLATO (PLAnetary Transits and Oscillation of stars), die grösste europäische wissenschaftliche Mission zur...

Alle Focus-News des Innovations-reports >>>

Anzeige

Anzeige

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

Von Batterieforschung bis Optoelektronik

23.06.2017 | Veranstaltungen

10. HDT-Tagung: Elektrische Antriebstechnologie für Hybrid- und Elektrofahrzeuge

22.06.2017 | Veranstaltungen

„Fit für die Industrie 4.0“ – Tagung von Hochschule Darmstadt und Schader-Stiftung am 27. Juni

22.06.2017 | Veranstaltungen

 
VideoLinks
B2B-VideoLinks
Weitere VideoLinks >>>
Aktuelle Beiträge

Radioaktive Elemente in Cassiopeia A liefern Hinweise auf Neutrinos als Ursache der Supernova-Explosion

23.06.2017 | Physik Astronomie

Dünenökosysteme modellieren

23.06.2017 | Ökologie Umwelt- Naturschutz

Makro-Mikrowelle macht Leichtbau für Luft- und Raumfahrt effizienter

23.06.2017 | Materialwissenschaften