Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

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 Hope to discover sure signs of life on Mars? New research says look for the element vanadium
22.09.2017 | University of Kansas

nachricht Calculating quietness
22.09.2017 | Forschungszentrum MATHEON ECMath

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: The pyrenoid is a carbon-fixing liquid droplet

Plants and algae use the enzyme Rubisco to fix carbon dioxide, removing it from the atmosphere and converting it into biomass. Algae have figured out a way to increase the efficiency of carbon fixation. They gather most of their Rubisco into a ball-shaped microcompartment called the pyrenoid, which they flood with a high local concentration of carbon dioxide. A team of scientists at Princeton University, the Carnegie Institution for Science, Stanford University and the Max Plank Institute of Biochemistry have unravelled the mysteries of how the pyrenoid is assembled. These insights can help to engineer crops that remove more carbon dioxide from the atmosphere while producing more food.

A warming planet

Im Focus: Highly precise wiring in the Cerebral Cortex

Our brains house extremely complex neuronal circuits, whose detailed structures are still largely unknown. This is especially true for the so-called cerebral cortex of mammals, where among other things vision, thoughts or spatial orientation are being computed. Here the rules by which nerve cells are connected to each other are only partly understood. A team of scientists around Moritz Helmstaedter at the Frankfiurt Max Planck Institute for Brain Research and Helene Schmidt (Humboldt University in Berlin) have now discovered a surprisingly precise nerve cell connectivity pattern in the part of the cerebral cortex that is responsible for orienting the individual animal or human in space.

The researchers report online in Nature (Schmidt et al., 2017. Axonal synapse sorting in medial entorhinal cortex, DOI: 10.1038/nature24005) that synapses in...

Im Focus: Tiny lasers from a gallery of whispers

New technique promises tunable laser devices

Whispering gallery mode (WGM) resonators are used to make tiny micro-lasers, sensors, switches, routers and other devices. These tiny structures rely on a...

Im Focus: Ultrafast snapshots of relaxing electrons in solids

Using ultrafast flashes of laser and x-ray radiation, scientists at the Max Planck Institute of Quantum Optics (Garching, Germany) took snapshots of the briefest electron motion inside a solid material to date. The electron motion lasted only 750 billionths of the billionth of a second before it fainted, setting a new record of human capability to capture ultrafast processes inside solids!

When x-rays shine onto solid materials or large molecules, an electron is pushed away from its original place near the nucleus of the atom, leaving a hole...

Im Focus: Quantum Sensors Decipher Magnetic Ordering in a New Semiconducting Material

For the first time, physicists have successfully imaged spiral magnetic ordering in a multiferroic material. These materials are considered highly promising candidates for future data storage media. The researchers were able to prove their findings using unique quantum sensors that were developed at Basel University and that can analyze electromagnetic fields on the nanometer scale. The results – obtained by scientists from the University of Basel’s Department of Physics, the Swiss Nanoscience Institute, the University of Montpellier and several laboratories from University Paris-Saclay – were recently published in the journal Nature.

Multiferroics are materials that simultaneously react to electric and magnetic fields. These two properties are rarely found together, and their combined...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

“Lasers in Composites Symposium” in Aachen – from Science to Application

19.09.2017 | Event News

I-ESA 2018 – Call for Papers

12.09.2017 | Event News

EMBO at Basel Life, a new conference on current and emerging life science research

06.09.2017 | Event News

 
Latest News

Rainbow colors reveal cell history: Uncovering β-cell heterogeneity

22.09.2017 | Life Sciences

Penn first in world to treat patient with new radiation technology

22.09.2017 | Medical Engineering

Calculating quietness

22.09.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>