Imperial College London

DrThomasWall

Faculty of Natural SciencesDepartment of Physics

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Contact

 

t.wall

 
 
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Location

 

Blackett LaboratorySouth Kensington Campus

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Summary

 

A cryogenic source of cold complex molecules

Techniques to cool and control molecules have developed over the last two decades, but so far these techniques have been limited to small molecules. We are now turning our attention to large, complex molecules with which we can answer questions from fundamental physics to biology. I am developing a source of cold chiral molecules with which we can hunt for signatures of parity violation, and investigate the phenomenon of biological homochirality: many of the biological building blocks of life on Earth are chiral molecules – those which can be left- or right-handed. Why is it that one handedness is always chosen for these molecules? By addressing this problem we will learn about much about the nature of these chiral molecules, and perhaps about the origins of life on Earth.

What’s more, the cryogenic molecular sources that I am developing will be able to produce cold sources of a very wide range of complex molecules for many applications, such as studying the nature of chemical reactions, developing sensitive spectroscopy techniques for trace gas analysis, and investigating and interpreting astronomical observations of inter-stellar gas clouds.

Laser cooling molecules

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The technique of laser cooling revolutionized atomic physics, expanding its scope to include previously unimaginable scientific endeavours. In the Centre for Cold Matter we are pioneering the development of this powerful technique to produce slow, ultra-cold molecules. I have worked on the development of this experiment since its inception in 2010, from the first hints of radiation pressure slowing to the successful production of laser cooled molecules slow enough to trapped in a magneto-optical trap.

Laser cooled molecules can be used to test fundamental physics (such as the precise measurement of fundamental constants, and tests of the long-term stability of these constants), for experiments with cold, controlled chemistry, and they hold much promise for quantum information processing.

Developing Type II MOTs of Rubidium

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Magneto-optical trapping is a hugely successful technique for cooling and confining cold and ultra-cold atoms. With the advent of laser cooling molecules, we turn now to creating magneto-optical traps (MOTs) for molecular systems. However,  the level structure of the molecules that have been successfully laser cooled is not amenable to trapping with a conventional (“Type I”) MOT. Working with a PhD student I have built a versatile optical set-up with which we can investigate alternative (“Type II”) MOTs. With this system we can trap atoms in a Type I MOT, and then rapidly load these cooled atoms into a Type II MOT. We have recently demonstrated the first Type II MOT operated with blue-detuned light. We are using this novel trapping mechanism to learn about methods of trapping and cooling atomic and molecular systems that have hitherto not been considered possible to confine in a MOT. These developments can greatly extend the scope of magneto-optical trapping.

In addition, we have developed radio-frequency (RF) magnetic field coils that can produce magnetic fields modulated at around 10 MHz. These field coils will be used to create “RF MOTs”, another method of trapping cold atoms and molecules whose level structure would normally preclude magneto-optical trapping.

Building an ultra-cold refrigerator

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After almost two decades of development into slowing and cooling molecules, there is now a range of techniques for producing trapped, cold ensembles of molecules (with temperature typically around 0.1 - 1 K). However, with the recent exception of laser cooling, there has been no significant development in the production of ultra-cold (< 1 mK) molecules. We are building an experiment that will use laser cooled atoms as a refrigerant for molecules. By co-trapping an ensemble of cold molecules with a cloud of ultra-cold atoms we can sympathetically cool the molecules. This will allow us to cool a wide range of molecules to unprecedentedly low temperatures. With these ultra-cold molecules we will learn much about fundamental physics and chemistry, including a wealth of information about the interactions of atoms and molecules.

We are building a dense source of ultra-cold lithium atoms, using a combination of laser cooling in a Zeeman slower, trapping in a magneto-optical trap, and cooling below the Doppler limit in a “grey molasses”. We will then use a travelling magnetic trap to load these molecules into a novel microwave trap that was developed in our group. This trap can be used to co-confine the ultra-cold lithium with a wide range of cold molecules, and to create the world’s first “ultra-cold refrigerator”!