James Millen
Ph. D. (Durham), MSci & ARCS (Imperial College London)

Room 146 / Lab 34
Fax +44 (0)191 33 43538
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April 2011: I have passed my Ph. D. viva voce! Many thanks to Team Strontium for getting me to this point. I have now started work as a PDRA in Peter Barker's group at UCL. Please contact me on j.millen@ucl.ac.uk or james.millen@gmail.com.

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What has James been up to?

All the time! Writing my thesis and papers. Please check the news page for, er, news!

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Graph of the week

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March 2011: A laser at 689nm is shone through a hot atomic beam of strontium, and excites atoms on the narrow singlet-triplet intercombination line to the 5s5p 3P1 triplet state. The resultant fluorescence is recorded on a CCD camera. This image shows that image, and a contour plot. The laser is moving into the page, and the hot atomic beam right to left. The atoms fluoresce strongly where they interact with the laser, and the population of the triplet state is seen to decay as the atoms move right to left. From this image we can gather information about the thermal distribution of the atoms, and the lifetime of the triplet state. We can use this triplet state transition to cool strontium from milliKelvin to hundreds of nanoKelvin.

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Research

I work with Matt Jones on the strontium project in the AtMol group, and started my Ph.D. in September 2007. I also work alongside Graham, who started his Ph.D. in September 2008. I started my Ph.D. by looking at Electromagnetically Induced Transparency (EIT) in a thermal beam, and have moved onto studying trapped strontium and clouds of highly excited Rydberg atoms. For more information see my research page.

Below is a "layman's synopsis"!

At the heart of the strontium project at Durham is the study of inter-atomic interactions. To study the forces between atoms we exploit the interaction of light with matter to create novel systems where the interactions are greatly enhanced.

Strontium is heated in an oven and the hot collimated beam of atoms is slowed from about 500mph to 2mph using a Zeeman slower, where a beam of laser light pushes against the atoms to slow them down, and a magnetic field keeps the energy structure of the strontium atoms resonant with the beam.

The centrepiece of the experiment is a chamber held at a vacuum comparable to the emptiness of space. As the slowed atoms reach the centre of this chamber they are illuminated from all directions by blue laser light, which creates a frictional "molasses" to further slow the atoms. The addition of a magnetic field turns this frictional force into a trap, and we trap about a million strontium atoms at a temperature of a few thousandths of a degree above absolute zero.

Once we have this ultra-cold, trapped gas of atoms we can really start to study them. By illuminating the atoms with purple light we can impart them with a lot of energy, one affect of which is to drastically increase their size, by thousands of times. These large, highly excited atoms are called "Rydberg" atoms. The interaction between Rydberg atoms can be thousands of billions of times larger than between unexcited "ground-state" atoms.

These extreme interactions will often cause the atoms to ionize, where an electron is ejected from the atom, leaving it charged. This process can cause a plasma to form, a plasma being a cloud of charged particles (in our case ions and electrons) with looks electrically neutral from a distance. Plasmas are usually extremely hot, since it takes a lot of energy to remove an electron, but because of the unique property of Rydberg atoms this plasma remains as cold as the original gas. Ultra-cold plasmas also exhibit very strong interactions, and collective effects.

The reason we use the atom strontium is that it has two electrons in its outer shell. Many cold atom experiments use "alkali" elements, where there is only one electron in the outer shell. Once an electron is removed through ionization this second electron remains, and we can still control the ion with light, providing a unique diagnostic. The same is true of the highly excited Rydberg atoms, there is an extra electron allowing direct imaging and control.

Finally, we can exploit another weapon is the armoury of cold atom physics, and create a deep, strong lattice of traps using an intense laser beam. In our case we would trap atoms in a line with regular spacing. However, because of the exaggerated size of the Rydberg atoms, we can actually bring the atoms so close in this trap that they overlap. This is a unique experiment whose outcome in unknown, but could lead to the charge being shared amongst the atoms effectively creating a metal.

Content © James Millen, Durham University 2009